Professor Astronomy's Astronomy Blog

All I ask is a dung ball and a star to steer her by

2013-01-25T16:38:00.004-06:00

Image Credit: Dacke et al. / Current Biology via NBC News

Humans are not the only animals to navigate using the sky. Numerous birds and other animals use the sun and moon as navigational cues, and birds and seals also use stars. Evidentially so do dung beetles, which also use the Milky Way, according to a new study in Current Biology by Swedish scientist Marie Dacke and collaborators. (Insert your own dung-related joke here.)

Dung beetles are important insects who help to break apart the large mounds of dung produced by large animals. They go to a pile, make a ball, and then head for home as fast as possible to try and keep other dung beetles from stealing their hard-earned treasure. The best way to do that, of course, is to make a bee-line for home.

Being a small insect in a big world, dung beetles use visual clues to help go home. Previous researchers found that dung beetles use the sun, moon, and polarization of the light in the sky to orient themselves and move in a straight line. But the beetles still work if the sun and moon aren't in the sky.

Dacke and her collaborators tried to figure out what else the beetles might use to orient themselves. They made a little arena for the dung beetles and timed how long it took them to get their dung ball out of the stage area, first with visual clues and then without. The latter involved putting blinders on the beetles, as seen in the picture below. This part of the experiment made me glad I am not a dung beetle researcher.

Dung Beetle hats. Image credit: Marcus Byrne via livescience.com

Anyway, to cut to the heart of the matter, the researchers found that the beetles could steer their dung balls straight (meaning they can get away from potential dung thieves quickly) under a moonless, starry sky but only if the Milky Way was visible. Bright stars alone weren't enough. This was tested both in the wild and in a planetarium. Here's a drawing from their paper showing these paths. On the left are beetle tracks under a Milky Way-lit, moonless sky, and the right are tracks made when the stars were not visible. That's a significant difference!


Dung beetle tracks. Dacke et al, Current Biology, (c) 2013 Elsevier Ltd

In my opinion, this is really cool. We've known for a long time that animals can navigate long distances using all sorts of clever techniques, whether it be salmon finding their way back to the stream of their birth, arctic terns navigating from one polar sea to another, or Europeans off to exploit the wealth of the East Indies. So it is not surprising that even simple animals can use multiple cues to steer by. But still - beetles using a faint smear of light draped across the sky is a wild (and seemingly correct) idea.

We shouldn't be too surprised that this technique evolved. The moon is not visible in the night sky half of the time, so being able to steer by another celestial light is a useful adaptation. And while the Milky Way is not always visible in the starry sky, the Milky Way is often visible when the moon is not, and vice-versa.

Another neat finding is that the beetles cannot use bright stars to steer by. In the planetarium, researchers projected only bright stars, and the beetles were almost as lost as if it were a blank sky. I'd guess this is due to their poor eyesight being unable to detect the light of a single star.

I once had to navigate by the light of the Milky Way. I was at Las Campanas Observatory in Chile at the time of the New Moon. It was the night before I was to start using the telescope, so I was in their library working one evening and I lost track of time. When I realized that it was night, I decided to walk back to my dorm room. But it was so incredibly dark, I couldn't see the sidewalk when I first came out of the library. Knowing that there was a pretty sizable and dangerous drop on one side of the walk, I didn't want to risk moving. Being an observatory, there were no walkway lights or outdoor lights. It was pitch black except for the stars overhead.

As my eyes adjusted, I was happy to find that the light of the stars and the Milky Way was sufficient to light up the sidewalk, and I was able to walk easily and safely back to the dorms. Little did I know I was channeling my inner dung beetle!

Dung beetles can navigate by the Milky Way. That's so cool!

Dacke, M., Baird, E., Byrne, M., Scholtz, C., & Warrant, E. (2013). Dung Beetles Use the Milky Way for Orientation Current Biology DOI: 10.1016/j.cub.2012.12.034

The true dangers of December 21, 2012

2012-12-18T13:34:00.001-06:00

December 21, 2012. The end of the world. Mayan calendars. Rogue planets. Asteroids. Pole shifts. Magnetic madness. Fire and brimstone coming down from the skies. Rivers and seas boiling. Forty years of darkness. Earthquakes, volcanoes. The dead rising from the grave. Human sacrifice, dogs and cats living together. Mass hysteria! (Venkmann et al, 1984)

As a scientist, it is very tempting to dismiss these ideas out of hand and without a word (or with a few humorous statements before moving on to real science). There is absolutely no scientific basis to any of the over-the-top scenarios being touted as certain to happen this Friday. Granted, a few of the scenarios (like asteroid impacts or solar flares) have a needle or two of science deep in the hyperbolic haystack, but that science has been distorted and twisted to serve utterly non-scientific ends. The sun will rise on December 22, and no creature on this planet besides a few humans will be surprised by that development.

No, the true dangers of this Friday lie with us humans. NASA scientists have been getting emails and calls from genuinely worried children and teachers of those children. Many of us adults are comfortable joking about doomsday theories because we've seen many doomsdays come and go, a trend that dates back to Assyrian and Sumerian civilizations.

But children don't have that perspective. I remember when I was in kindergarten and I heard that Skylab was going to fall back to Earth. Despite my parents' assurances that it would most likely hit Jaws, I still nervously watched the skies. Why would the news talk about it if it wasn't a big danger? And the 2012 date has been in movies, on the news, discussed by "documentaries" on supposedly serious cable channels, and splashed all over the Internet. If I were that kindergartner today, I'd be freaked out. So if you know a child who expresses any concern about the end of the world, reassure them that the world is not ending, and that they and their friends will be safe. Schoolchildren have enough real news to worry and frighten them without the need for fictitious dangers.

Doomsayers are also preying on other vulnerable people. Millions of dollars of books, videos, survival kits and doom bunkers have been sold. (Which begs the question - if these charlatans truly know the world is ending, what's the point of collecting money for goods and services? Money won't be useful in their post-apocalypse scenarios). These snake-oil salesmen target many different groups of people: those who are already nervous about the economy, those who are deeply religious, those who are paranoid, those who are not well-educated. And on December 22, these swindlers will be laughing all the way to the bank with no legal repercussions, while their victims will have spent their life savings or even gone into bankruptcy over worthless fears.

Many people with mental health issues also suffer from well-publicized end-of-the-world scenarios. Many of these people are not capable enough of rational scientific thought to be assured that this 2012 hooey is just that.

"Popular" doomsday scenarios like 2012 are much more insidious than simply the deranged ravings of a few kooks. Some polls suggest 10% of Americans think the world will end this Friday. I suspect that is high, but even if it is just one out of a thousand people who is truly worried, that still ads up to over 7 million people worldwide. (And what number of them are stocking up on guns and ammunition with the sole aim of protecting themselves in a post-apocalyptic world? I shudder at the thought).

There are real threats to humanity, and the phantom threats about this Friday are not among them. Hoaxes like 2012 distract us from these very real concerns. To name just a few: violence, disease, nuclear proliferation, global warming, hunger, poverty, hatred -- all of these are very real threats. We must better educate ourselves and our children to be able to discern clear and present dangers from monsters that hide in our closets at night. Alas, my personal doomsday scenario is that we will fail in that crucial mission.

Here are some trustworthy and sound sources on the 2012 Doomsday Hoax:

NASA - Beyond 2012 - NASA scientists lay out some of the commonly cited agents of doom and the science disproving each.
2012hoax.org - Scientists and rational citizens have created a massive repository of the hoax, its roots, its proponents, its lies, and the scientific truth.
Resources for Responding to Doomsday 2012 - Scientist and educator Andrew Fraknoi has created a compendium of links with honest discussions and facts about 2012, especially useful for educators needing to address students' concerns

Even white dwarfs must obey Einstein

2012-08-28T16:38:00.000-05:00

The Earth-Moon system (top) and binary white dwarf system (bottom) to scale. Click to enlargify. Earth (right) and Moon (little brown spec on left) images from NASA/JPL/Galileo; artwork by yours truly.

3,100 light-years away in the direction of the constellation Gemini lurks one of the most extreme pair of stars that we know about. Two white dwarfs, the remains of ordinary stars similar to the sun, whirl around each other every 12 minutes and 45 seconds. As they orbit, Einstein's theory of general relativity predicts that their gravity distorts space and time itself, and these distortions (called gravitational waves) carry away some energy from the system, forcing the two white dwarfs to draw ever nearer. Locked by gravity in a slow death spiral, these white dwarfs are destined to collide and merge in two million years. At least that was the prediction, and today it was confirmed by an international team of astronomers (including many friends and colleagues of mine, though I wasn't involved).

At least that was the prediction when the the pair of white dwarfs, with the ungainly name of SDSS J065133.338+284423.37 (we'll just call it J0651), was discovered the spring of 2011. While most binary stars are separated by millions, if not billions, of kilometers, these two are separated by about 113,000 kilometers (70,000 miles) – less than 1/3 the average distance between the Earth and the Moon, or just 8% of the Sun's diameter.

Clearly these are not ordinary stars! White dwarfs are the burnt out embers of stars like the Sun. Whereas the Sun counteracts the inward pull of gravity with heat and pressure produced by nuclear fusion, white dwarfs have no fusion anymore, and gravity shrinks them to orbs roughly the size of the Earth. So a single white dwarf can have the mass of the Sun in the volume of the Earth. That's cool enough.

The close binary white dwarfs are so close together that the stars that made the white dwarfs must have been almost touching when they were normal stars. Usually, when a star runs out of fuel, it swells up into a red giant. The red giant sun will reach out to beyond Earth's orbit. But if there is another star right there, the companion star's gravity will pull the dying star's outer layers over, beefing itself up. Meanwhile, the dying star becomes a white dwarf.

Since beefier stars live shorter lives, the newly-hefty companion star will now finish its life cycle in a relatively short time and try to become a red giant. But as it swells up, it engulfs the already-existing white dwarf. The complex interplay of gravity, orbits, gas and magnetic fields tosses off the outer layers of they dying star and causes the old and new white dwarfs to spiral close together, just like J0651. The picture at the top of this post is a scale composite drawing of the Earth-Moon system and the binary white dwarf system.

While the Earth and Moon take 27 days to complete one orbit, these stars take just 13 minutes. This extreme situation means that Einstein's Theory of General Relativity rears its ugly head. Einstein's theory states (among other things) that if two large masses are orbiting each other, they will create ripples in the fabric of space and time. These ripples, called gravitational waves, move out at the speed of light and carry away some of the stars' energy. With less energy, the stars must move closer together.

This effect of gravitational radiation has been detected before. Russell Hulse and Joseph Taylor shared the 1993 Nobel Prize in Physics for their discovery of two neutron stars in an orbit that is slowly decaying at exactly the rate predicted by Einstein.

Today, a team of astronomers led by University of Texas Astronomy graduate student J.J. Hermes announced that they had detected this same decay in J0651. From our vantage on the Earth, the two white dwarfs in J0651 pass in front of each other during their orbital dance. This causes an eclipse, which we see as a drop in light from the star, that lasts for less than a minute. These eclipses recur every 13 minutes.

If the stars are spiraling together, the eclipses should occur more and more frequently. Over the course of a year, J.J. and collaborators found that the eclipses are now happening about 6 seconds sooner than they should if we ignore general relativity. That's shown in the figure below. If there were no general relativity, then the middle of the dips should all lie along the dotted red line, though you see that the April 2012 eclipse is a little early.

Eclipses of J0651, happening just a bit faster than they should. "Phase" is like "fraction of an orbit", so a phase of 0.1 is 1.3 minutes. Image credit: Hermes et al. / arXiv

Confirming general relativity yet again is cool, but more detailed science waits down the road. After all, we already knew that Einstein's theory was right in situations like this. But look again at the picture at the top of this post. Notice that the distance between the two white dwarfs is only a few times the size of the white dwarfs themselves. This small distance means that the gravity of the white dwarfs is distorting their shapes - they aren't perfect spheres. The larger-sized white dwarf (which actually has less matter, because white dwarfs are contrary in that way) is stretched by about 3% away from a perfect sphere and toward an egg shape.

The exact distortions can be measured by carefully studying the eclipses and other information from the stars' brightnesses and spectra. The material that makes up white dwarfs is squeezed by gravity to a state called "electron degeneracy", which is a weird effect from quantum physics. By studying the distortions, J.J. and his collaborators hope to learn more about the exact structure and degeneracy in these white dwarfs, which in turn tests a lot of theories about extreme physics. We still need to wait a few years for that information, because the degeneracy physics should interact with the changing orbit to produce measurable deviations from Einstein's predictions in just a few years' time.

So, in short, another proof of Einstein's theory of general relativity will allow astronomers to look for future deviations from that theory to probe our understanding of the insides of stars 3100 light years away and our understanding of the physics of atoms. How cool is that!

J. J. Hermes, Mukremin Kilic, Warren R. Brown, D. E. Winget, Carlos Allende Prieto, A. Gianninas, Anjum S. Mukadam, Antonio Cabrera-Lavers, & Scott J. Kenyon (2012). Rapid Orbital Decay in the 12.75-minute WD+WD Binary J0651+2844 The Astrophysical Journal Letters : 1208.5051

What professional astronomers know about telescopes (often, not much!)

2012-07-18T11:39:00.000-05:00

Yours truly at Kitt Peak in 2002 (I think).

Most people assume that we astronomers know everything there is to know about telescopes. After all, we use giant ones for much of our work! So it shouldn't be surprising that one of the most common questions we get asked (after the black holes and aliens have been addressed) is something like: "I've been thinking about getting a telescope. What should I get?"

Many of these people are then quite disappointed to find out that I cannot help them much. Sure, I give them the standard (and excellent, IMHO) advice that they should avoid $49 specials at Walmart, start with binoculars and then, if still interested, progress to something like an Astroscan. (Full disclosure - I own an Astroscan and love it, but I don't get any compensation whatsoever to talk them up).

But if you ask me which is better: a Celestron NexStar or an Orion StarMax, and I will give you a blank star. I have no clue. Or if you ask me why your iOptron SmartStar Maksutov has this weird coma when you put a certain filter in but not with a different filter by the same manufacturer, and I'll only be able to blurt out the obvious "maybe there is something wrong with the filter?"

Most professional astronomers probably know less about telescopes and the nitty-gritty details about how they work than many amateur and semi-pro astronomers. Certainly I understand optics, and I know the basics of telescope design and operation. When I am observing, I can tell if something is wrong and have helped diagnose the problem through testing. But if I'm using the Keck Telescope and determine that a metal plate in the spectrograph is blocking most of our image, I don't go out into the dome and start banging on things with a wrench. In fact, I wouldn't even know where the metal plates in the spectrograph are. I call the engineers who come and fix everything.

It doesn't take a rocket scientist to see that something is blocking this picture of the sky. Fixing it is altogether different. (Keck I Telescope, August 2001)

There are exceptions to this ignorance. For example, our planetarium director, Dr. Kent Montgomery, was building his own telescopes as a teenager. He directed the construction of our A&M - Commerce observatory, and has overseen its operation since. I would have struggled mightily with these tasks, but Dr. Montgomery and his staff have developed a nice system.

This week, Dr. Montgomery started a two week vacation. We have two undergraduate students here as part of our Research Experience for Undergraduates program, and they are using our observatory almost nightly to study asteroids and extrasolar planets. And weird things have started happening in their images - instead of nice round dots for stars, sometimes they are little trails. That's not supposed to happen.

So, Monday night I went with our assistant planetarium director, her staff, and the students to troubleshoot. We checked wire connections, telescope balance, camera connections, and everything we could think of. Yet at certain parts of the sky, the images were still wonky (a scientific term). I ran some tests that convinced me the problem was mechanical - something in our telescope was moving when it shouldn't - and I was even able to narrow it down to a list of three most likely things: the mirror was moving (but it seemed tight), our guide telescope was moving (but it seemed tight, too), or our camera was bending slightly (it seemed tight, but this was my personal guess). But at this point, I didn't know what to do.

Since our summer students are here only 10 weeks, I didn't want to wait for Dr. Montgomery to return to fix the telescope, but I don't trust myself to go mucking about in the telescope's guts. A quick Google search did not help, and my schedule doesn't have a lot of time for deep searches on Internet forums.

So I put out a call for help. Rather than contacting my colleagues at other PhD institutions (most of whom would not know any better than I, and those with the requisite knowledge are busy working on 2 to 30 meter telescopes and probably not likely to know much about modern 0.4-meter commercial telescopes), I emailed some contacts at the AAVSO and at the Central Texas Astronomical Society. Within a couple of hours, I had a very detailed and extraordinarily diagnosis and likely resolution from an expert citizen scientist named Tom Krajci. I still don't trust myself with the repairs, but at least we know what is going on and have some ideas how to minimize the problem until Dr. Montgomery returns.

I am very grateful to Tom and Mike Simonsen at the AAVSO as well as Brad, Willie, and Dean at the Central Texas Astronomical Society for their help, quick responses, and kindness not to roll their eyes when a bumbling novice like me comes along with a question.

Now, hopefully, you see why I can't give you detailed answers to your questions on commercial telescopes. If you need help, your best bet is to find a nearby amateur astronomy club and ask - you'll get a better and more accurate answer than I would be able to help you with! Now if you ever need help diagnosing problems with your 10 meter telescope, I may be able to help...

Pictures of the Venus Transit and Festivities

2012-06-07T10:49:00.002-05:00

Tuesday afternoon, the planet Venus passed in front of the Sun as seen by the Earth. (If you've been reading this blog, you already know that). The next transit of Venus will happen on December 10-11, 2117 (my birthday, though I don't expect to see it).

Here at Texas A&M University - Commerce, we had an open house at our observatory. Roughly 150 people came, which is absolutely amazing. We had to fight some clouds - the transit started behind a thunderstorm, and the last hour that the sun was still up it was hidden behind clouds, and there were enough clouds we closed down early and didn't stay up to view the night sky. But those who braved the heat and waited out the clouds were treated to quite the spectacle.

In case you missed the transit because of clouds, work, school, sleep or indifference, here are some pictures, both from us and from others. Picture sources are indicated in the caption. You may feel free to use any images labeled as being from NASA (here are their terms of use), and you may use any images labeled as from me as long as you attribute them and don't use them for commercial purposes (my terms of use).

All of my images were taken with an iPhone held up to an eyepiece. Most of these photos were lousy, but a few came out okay!

The beginning of the transit was hidden behind a cumulonimbus (thunderstorm cloud). Thankfully we didn't get rain. Image credit: Professor Astronomy

The early stages of the transit, as seen through a telescope with an appropriate solar filter. The mottled appearance of the sun is due to the top portions of the cumulonimbus we were looking through. Venus is the big dot; the smaller dots are sunspots. Image credit: Professor Astronomy

Some of the early crowd at Commerce Observatory. The sun was still playing hide-and-seek with the thunderstorms at this time. Image credit: Professor Astronomy

Finally, a good view! I took this picture through a pair of binoculars with solar filters. Image credit: Professor Astronomy

An image of the sun and Venus through a Coronado Solar Telescope. These telescopes only let light corresponding to hydrogen atoms come through, which allow the viewer to see activity on the sun. Around the edge of the sun you can see some prominences. The brighter regions are called faculae and are related to sunspots. Image credit: Dr. Kent Montgomery / Texas A&M University - Commerce

About 1 hour before sunset (and only two hours into the 7-hour transit), another thunderstorm formed and blocked Venus and the sun for the remainder of the day. This transit of clouds across the transit of Venus, while beautiful, has little scientific value. Image credit: Professor Astronomy

This image was taken by the sun-observing satellite Hinode, a joint NASA/Japanese Aerospace Exploration Agency mission. The picture is in a wavelength of ultraviolet light that highlights solar activity. The yellow ring around Venus is real and caused by Venus's atmosphere bending light. Image Credit: NASA/JAXA

This image was also taken by the sun-observing satellite Hinode, a joint NASA/Japanese Aerospace Exploration Agency mission. The picture is in visible light. The ring around Venus is real and caused by Venus's atmosphere bending light. Image Credit: NASA/JAXA

A movie of the Venus transit as seen by NASA's Solar Dynamics Observatory, an orbiting satellite. This movie is taken in ultraviolet light, which shows the Sun's activity. As the movie plays, watch for mini solar flares and other changes in the sun.

Venus has moved on; it is now visible in the early morning sky just before sunrise (it's moving fast!). I guess it is time to leave this fun and rare event behind, too, and move on to more astronomy!

Venus neareth

2012-06-05T13:13:00.001-05:00

Venus as seen by the sun-observing satellite SOHO. In this picture, the sun is blocked by a disk, and the white circle indicates the location of the Sun. The trails on either side of Venus are artifacts - the planet is so bright, it dazzles the camera. Image Credit: ESA/NASA

The planet Venus is only four hours away from its date crossing the face of the Sun. Here are some last minute links and reminders:

Do you like old radio shows? Relic Radio has posted an episode of a 1957 science fiction show called The Merchant of Venus in celebration of the transit.
Want to see the transit in person? NASA's Sun-Earth Day website has a link to events around the globe.
Cloudy or night where you are? Watch the transit online! Here are a handful of links to webcasts; I'm sure there are many, many more:

NASA EDGE broadcasting from Mauna Kea, Hawaii, starting at 9:45pm UTC (5:45pm Eastern Daylight Time)
Las Cumbres Observatory Global Telescope Network will broadcast from Haleakala in Hawaii
The SLOOH SpaceCamera with images from around the world

Live in northeast Texas? Come to our public viewing at Texas A&M University - Commerce.
In the DC area? Check out a Venus viewing at NASA Goddard Visitor Center. I know a couple of people there, so feel free to give them a hard time.
(Added later) SAFETY FIRST!! Do not, under any circumstances, look at the sun without appropriate safety equipment. Sunglasses (even multiple pairs) are NOT safe. Here is a list of safe and unsafe ways to view the transit from transitofvenus.org

Modern transit science

2012-06-04T21:14:00.000-05:00

How the Kepler Mission detects planets around other stars. Image Credit: NASA

The last transit of Venus across the sun for 105.5 years begins in less than 24 hours. As I wrote last time, past transits were a scientific bonanza, allowing astronomers to determine the size of the solar system and, eventually, distances to other stars. But in a time when we have spacecraft orbiting Venus, can a transit still provide a scientific return?

The short answer is yes. We don't expect to learn any groundbreaking new facts about Venus, the Sun, or the Solar System. But we can use the fact that we already know so much about these objects to make use of this transit by testing our techniques to study planets around other stars.

One of the main techniques astronomers use to study planets around other stars is called the transit method. A tiny fraction of planets around other stars have their orbits oriented just right so that they pass between their parent star and our view from Earth. We can't see the dark spot of the planet crossing the star from dozens of light years away, but we can measure a tiny drop in the amount of light from a star. NASA's Kepler Mission uses this technique in its search for Earth-sized planets around other stars.

When planets are discovered by the transit method, we have to infer a lot of the planet's properties. There are sources of error, including what we know about the parent star, how active the parent star may be, what the parent star looks like in detail, and so on. From our vantage point on Earth, the Venus transit will have similar properties to a Neptune-size planet transiting a sun-like star (since we are close to Venus, it appears larger in proportion to the more distant Sun than it really is). So, we can test all sorts of sources of error and uncertainty.

The Hubble Space Telescope will use the transit to test how we probe atmospheres of extrasolar planets. Venus has a thick and cloudy atmosphere, and we know what that atmosphere is made out of. Some of the light from the sun will pass through Venus's atmosphere and pick up some information about the atmosphere's composition and structure. Astronomers have used this same technique to probe the atmosphere of planets around other stars. Since we already know what Venus's atmosphere is like, we can use the Hubble observations to try and recover that answer, and see how good (or bad) our current studies are.

Astronomers, like Captain Cook 243 years ago, have set sail to study the transit of Venus from around the world (and even outside it). The science may have changed, but the opportunities afforded by this rare event are still launching expeditions centuries later. When the next transit happens in 2117, will there still be scientific value in the transit? Perhaps, though it is hard to imagine what. Yet during the transits of 1874 and 1882, could the astronomers of that time have imagined us measuring transits of other worlds around other stars? Of measuring atmospheres with orbiting telescopes and electronic equipment? I doubt it. So perhaps we should close with a quote from William Harkness, director of the U.S. Naval Observatory in 1882. Change the word "twenty-first" to "twenty-second", and these words still ring true:

"There will be no other transits of Venus till the twenty-first century of our era has dawned upon the earth. When the last transit occurred the intellectual world was awakening from the slumber of ages, and that wondrous scientific activity, which has led to our present advanced knowledge, was just beginning. What will be the state of science when the next transit season arrives God only knows. Not even our children's children will live to take part in the astronomy of that day." (gathered from space.com)

The past importance of transits of Venus

2012-05-31T19:52:00.000-05:00

A French cartoon showing a view of the 1761 transit of Venus. Note the devil in the background, which I assume is meant to remind the reader that viewing the transit through a telescope without proper eye protection is a deadly sin (at least for your retina)
Image Credit: NASA Goddard Space Flight Center Sun-Earth Day

Next week's transit of the planet Venus across the sun gives us the chance to learn a little history of transits and their scientific importance. In a couple days, I'll discuss the current scientific interest of transits.

The importance of Venus transits starts with the famed astronomer Johannes Kepler, who, in the early 1600s, was the first person to figure out the shape and properties of planetary orbits. His three laws of planetary motion allowed Kepler to figure out the relative size of the solar system. If we call the average distance between the Earth and the Sun as 1 Astronomical Unit, then Kepler knew the distances to other planets in terms of this unit. For example, he knew that Venus was about 0.7 Astronomical Units. The problem was, Kepler didn't know what an astronomical unit was in terms of familiar units like miles or kilometers.

In 1687, Sir Isaac Newton published his book Philosophae Naturalis Principia Mathematica (Mathematical Principals of Natural Philosophy). In this book, Newton introduced his now famous laws of motion and law of gravity, and he found that Kepler's Laws were a natural consequence of his laws of motion and gravity. In particular, Newton knew that the force of gravity between two objects depended on the mass of each object (how much "stuff" it is made out of) and the distance between the objects. So, if the distances were known, the masses of the objects could be figured out.

Thirty years later, British astronomer Edmund Halley (of Halley's Comet fame) realized that careful observations of a transit of Venus across the sun would allow scientists to determine the size of an astronomical unit, which would in turn allow scientists to calculate the mass of the Sun (as well as the actual sizes of all the known planets). The reason for this is geometry. People at different points on the Earth would see Venus take a slightly different path across the sun because they were viewing Venus from a slightly different angle. Because the size of the earth is known, a little geometry would give us the size of the astronomical unit in miles or kilometers (see picture below). The next transit of Venus wasn't until 1761, which gave expeditions nearly 50 years to prepare.

A not-to-scale image showing how observers on different parts of the Earth would see Venus cross the sun at slightly different places. In reality, the Earth and Venus are the same size, the sun is much, much larger and much, much further away, and the difference in paths much smaller. Image credit: Wikipedia Commons

A pair of Venus transits occurred during 1761 and 1769. Astronomers from across Europe observed these transits, and some traveled to far corners of the world (see Richard Pogge's excellent summary of the many expeditions). A particularly famous expedition was the 1768-1769 voyage of Captain Cook to Tahiti and the South Pacific. Captain Cook's data revealed the fly in the ointment: the now infamous black drop effect (picture below). Basically, because of Earth's atmosphere and imperfections in telescopes, when Venus is close to the edge of the sun, it is hard to determine exactly where the planet is, which makes the necessary measurements less precise. Even so, by 1770, astronomers calculated that the distance to the sun was 153 million km, less than 3% away from the precise value we know today.

An actual photograph of the black drop effect from the 2004 transit of Venus across the sun. The "bridge" that appears to connect Venus with the edge of the sun is an illusion caused by Earth's atmosphere and limitations of telescope optics. Image Credit: Stanislava Simberova, Ondrejov Observatory, and ESO

With that information, astronomers finally knew the actual size of the Solar System and all the objects in it. And, in 1838, astronomer Friedrich Bessel used that same information to measure the distance to another star for the first time.

These days, we don't need a planetary transit to measure distances in the solar system. We can use laser and radar ranging to determine distances. But the 1761 and 1769 transits of Venus were part of the transformation of astronomy from a descriptive science to the mathematical and physics-based science it is today.

Live in northeast Texas? Come see the transit of Venus with us!

2012-05-29T15:30:00.000-05:00

2004 transit of Venus, photograph (c) Scott Thompson

If you live in northeast Texas, you are invited to come watch the Transit of Venus at the Texas A&M University - Commerce Observatory in Commerce, TX. We will get underway at 5 p.m. on Tuesday, June 5 at the observatory, which is about 5 miles north of Interstate 30 exit 101 or 5 miles south of Commerce. Details and driving directions are here. The transit will last until sunset (awesome photo opportunity!). We will have all the safety equipment you will need to get great up-close (and safe) views of the transit.

After sunset, the party doesn't end! As it gets dark, we will turn the telescopes on the wonders of the spring and summer skies.

The 16" telescope dome

In case of poor weather, the observing will be cancelled. Check the observatory webpage for updates or watch my Twitter feed.

Coming June 5/6: The Last Transit of Venus You'll See

2012-05-29T10:01:00.002-05:00

It is rare that you can see something and know you will never see it again. A week from today (June 5 or 6, depending on where you live: June 5 in the western hemisphere including the U.S., June 6 in the eastern hemisphere) you have that chance when the planet Venus crosses in front of the sun as seen from the Earth. The next time this happens will be in December 2117. That's not a typo - you have to wait 105.5 years for another crossing.
A planet crossing in front of the star is called a transit. In our Solar System, the only planets that can transit the sun (as seen from Earth) are Venus and Mercury, because they are the only two planets that can come between the Earth and the Sun.

Venus orbits the sun once every 225 days. But since the Earth is moving, too, it takes Venus nearly 19 months to make a complete trip around the sun as seen from the Earth. So, every 19 months, Venus will pass between the Earth and the sun. But Venus's orbit and the Earth's orbit are tilted slightly, and the vast majority of the time, Venus will pass slightly above or slightly below the sun as seen from the Earth. Only when the Earth and Venus are at exactly the right points in their orbits will Venus pass directly in front of the sun. Due to the vagaries of planetary orbits, Venus transits come in a pattern: transit, wait 105.5 years, transit, wait 8 years, transit, wait 121.5 years, transit, wait 8 years, transit, wait 105.5 years, transit, and so on.

The previous transit of Venus was in June 2004. I went to see that transit from my mom's house near Lancaster, PA. The transit was underway at sunrise, and we watched from a Wal-Mart parking lot as the sun rose through fog and clouds - but we saw it.

Many web sites have lots of information on where and how to see the transit of Venus. Rather than reproduce all that information, I'll put a list of links below (including to a Sousa march!). You don't need to go anywhere special, but you do need to be extremely careful. Venus only covers a small part of the sun, and so you must be sure to protect your eyes. Sunglasses (or even multiple pairs of sunglasses) are not safe - infrared radiation can still get through and seriously damage your eyes in seconds, and you won't feel your retina being cooked (your retina has no pain sensors). There will be many groups and schools safely watching the transit, so if you don't have safe equipment, find one of those groups.

I know it is a pain to make a trek, but this is almost certainly the last chance any of us will ever have to see a transit of Venus. (I'm sure a few young'uns may live to see the next transit, but I bet they won't remember this one.)

In the next few days, we'll talk about some of the science behind past and future transits.

Links to Venus transit information and fun stuff:

Transitofvenus.org This website is very thorough and contains tons of information about where, when, and how to view the transit, plus lots of educational materials, links, history, etc. A very well-done site!
NASA's 2012 Transit of Venus page Fred Espenak's detailed information about the transit.
Sky and Telescope's transit information from the editors of the popular astronomical magazine
Astronomy Magazine's Venus Transit information from the editors of another popular astronomical magazine
The Transit of Venus phone app (free!) for iPhone and Android. Includes information on the transit as well as a global effort to determine the size of the Solar System from the transit
The Transit of Venus March by John Phillip Sousa, performed by the Virginia Grand Military Band. This march was inspired by the December 1882 transit of Venus. More information from Wikipedia.
Captain Cook and the Transit of Venus The story of how the June 1769 transit of Venus launched one of Captain Cook's voyages of exploration.

Sunday's "Ring of Fire" Solar Eclipse

2012-05-18T09:33:00.000-05:00

This Sunday, May 20 (in the U.S.; Monday morning on the 21st for Asia) there will be a spectacular eclipse of the sun. Residents of the western U.S. get a great show; those on the Eastern seaboard get to see nothing. Details on how to see the eclipse can be found here from Sky & Telescope and on many other websites. Rather than reproduce others' details on how/where/when to look, I thought I'd put a personal spin on the story and mention a few things I haven't seen on many other websites.

Solar eclipses are caused when the moon comes between the Sun and the Earth; basically we see the moon's shadow. The moon's orbit is tilted with respect to the Earth, so most months the moon passes well north or south of the sun in the sky, but every 6 months it has a chance of passing over part or all of the sun.

The moon and the sun are almost exactly the same size, as seen from the Earth. But the moon's orbit is not a circle, it is elliptical (oval), so sometimes it is a little closer to the Earth and sometimes a little further away. Remember a few weeks ago when the "Supermoon" was big news? I was in an ice cream shop when one of the other patrons saw the full moon rising and shouted "It's the supermoon! Look how big it is! Let's all go look! Supermoon!". You can see some pictures of the Supermoon here. The reason for the "supermoon" was that the full moon was almost exactly coincident with the moon's closest approach to Earth, so it was fully lit at the same time it appeared largest in the sky.

We are now 2 weeks after "Supermoon". The moon takes about 4 weeks do a complete orbit around the Earth, and if it was at its closest two weeks ago, it is at its furthest point from the sun this Sunday. This means the moon will appear the smallest it can in the sky, and, in fact, it will appear slightly smaller than the sun.

So, during this weekend's solar eclipse, the moon will try to fully cover the sun, but it will be too small to do so, and we Earthlings can see what is called an annular eclipse, where the sun will be visible as a ring around the dark shadow of the moon. Because the sun is not fully blocked, viewers must take care to protect their eyes. It may not hurt to look at the eclipsed sun, but harmful radiation from the sun will be very quick to permanently damage your eyes.

Solar eclipse aficionados are aware of something called the Saros cycle. Every 18 years and 10 or 11 days the Sun, moon, and Earth are in almost exactly the same geometry, so a very similar eclipse can be seen. 18 years and 10 days ago was May 10, 1994. That day there was another annular eclipse of the sun, this one visible from much of the United States as well (a bit of a coincidence, since the eclipse path moves 120 degrees west each 18 years). And on that day, the moon's shadow passed straight over my grandmother's house in northern Indiana. Since I had a good excuse to visit my grandmother, I did so. (I had just finished my sophomore year of college a few days previously).

We had some fantastic weather. I owned a small telescope but only a point and click camera, so my pictures aren't professional quality. But we still had a great view (click on the image for a larger view):

Here my grandmother is looking at the eclipsed sun through a mylar eclipse viewer. These are safe to observe the sun through. Grandma was just starting to suffer from the early stages of Alzheimer's, and so she tended to forget what we were doing. But she still enjoyed the experience and the views.

Here is my little refracting telescope. Because it is highly dangerous to look at the sun through a telescope, we projected the sun's image onto a screen. I actually had a really nice filter that went over the top of the telescope to block the sun's light for safe solar viewing, but another advantage of projecting the sun's image was everybody could see it all at once. From this view you can see that the moon is smaller than the sun and going to easily fit inside the sun.

This view is of the sun during the annular portion of the eclipse. You can see the sun fully surrounds the moon.

Another fun thing to do during solar eclipses is to use your hands to make pinhole cameras. In this picture, my mom is off to the left and holding her hands at right angles to make a cross-hatch pattern with her fingers. The gaps between her fingers let little bits of sunlight through, and in the shadows of her hands on the white board below you can see several little images of the sun. Since it is an annular eclipse, we thought it was fun to make lots of "olives" with just our hands.

Any small hole acts like a pinhole camera. After the annular part of the eclipse, I took this picture of the sidewalk near a flowering shrub. The shrub's leaves allow little patches of sunlight through, which you see here as a myriad of little crescent suns. This same phenomenon works when the sun isn't eclipsed; if you look closely at the dappled sunlight and shadows under a tree, you'll see that the sunlight is really a series of circles - hundreds of pictures of the sun projected on the ground.

If you live in the western U.S. and have clear skies on Sunday, go take a look! Just be safe - never look at the sun without protected eyes. Sunglasses aren't good enough! If you don't have mylar eclipse glasses, welder's glasses #13 or #14 will work, otherwise project the sun's image by putting a pinhole in a piece of paper and holding it above another surface (or just use your fingers). Again, don't look at the sun through the pinhole! Details on where to see the eclipse are in the Sky and Telescope article. The eclipse path goes through several national parks, and many of them will have special viewing events. If it is cloudy or you live too far east, you can also watch the eclipse online.

If you miss this eclipse, there will be more. Looming on the horizon is a total solar eclipse in August 2017, which crosses the entire continental U.S. I also saw portions of the last total eclipse of that Saros cycle from a hill on the French-German border in August 1999. Hopefully in 2017 I won't end up quite as wet as in 1999.

A diamond planet? I dunno..

2011-08-26T10:42:00.002-05:00

Image Credit: Swinburne Astronomy Productions, Swinburne University of Technology

Yesterday, the news wires were alive with excitement. Astronomers had a confirmed discovery of a planet made out of diamond! DeBeers better load up a rocket ship and blast off! But before we put the cart before the horse (and the cart is already halfway across the country while the horse is still munching oats in the barn), let's look a little deeper. The likely real story is, in my opinion, even more exotic than a planet-sized diamond.

First, let's go over what the astronomers observed. The team, headed by Professor Matthew Bailes of Swinburne University of Technology in Australia, was looking at a millisecond pulsar. Pulsar are the remains of very massive stars that exploded as supernovae at the end of the stars' lives. Pulsars typically contain about 1.3 times the mass of our sun squeezed into a sphere only a dozen miles across. This is so dense that ordinary atoms cannot exist, and most of the protons and electrons that made up the atoms in the original star merge to form neutrons. We therefore call these very dense remains of massive stars neutron stars.

Many neutron stars have very strong magnetic fields. These fields cause electrons and other particles in space to spiral around at speeds near the speed of light, a process that creates light and other radiation. As the star spins, the poles of the magnet sweep around. Every time a magnetic pole on the star sweeps across our line of sight, we see a flash, or pulse, of light. Neutron stars that are seen to flash are therefore called pulsars.

Just like an ice skater spins faster when he/she pulls her arms in close, stars that are squeezed down to the size of the sun spin faster and faster. New-born neutron stars often spin several times a second! The pulsar in the Crab Nebula (the remains of a star that exploded in 1054) spins at 30 times a second. Over time, as the pulsar's magnetic field interacts with gases in space, the pulsar slows its spinning. We can measure the Crab Pulsar slowing down, and we see pulsars that spin much more slowly than the Crab pulsar.

However, there are a class of very old pulsars which, though they should be spinning very slowly, are spinning at hundreds of times a second! Why are these pulsars spinning so fast? We think it is because these pulsars have companion stars. As the companion stars age, they swell up and get big enough that the pulsar's gravity can pull in the companion star's gas. As this stellar cannibalism continues, the pulsar's spin gets faster (because material is coming from far away and being moved very close to the center of the pulsar -- exactly what caused the pulsar to spin fast in the first place). Eventually, the pulsar will consume or blow away most of the companion star's mass, and we expect a small white dwarf to be left behind.

This idea of how to get pulsars spinning at hundreds of times a second predicts that these fast spinning pulsars should have companions, and most of those should be white dwarfs that have been whittled away to almost nothing. These companions should be detectable. Although they are small in both mass and radius, they still will have gravity that will pull on the pulsar. This causes the pulsar to move in a small orbit. This further means that the pulsar will be closer to Earth some times, and further away other times. When the pulsar is closer to Earth, its light reaches us a few seconds earlier than when it is further away. And since the pulsar spinning is often more reliable and constant than
atomic clocks, it is easy to see if pulses are arriving earlier, then later, then earlier again, then later again, as the pulsar traces out its tiny orbit.

Which brings us to yesterday's press release. Professor Bailes's group was studying one of these fast-spinning pulsars. They were looking for evidence of the companion star that sped up the pulsar's spin by looking for the changing arrival times of the pulsar's radio flashes. And they indeed saw that change. By measuring how early and late the pulses arrived and using some basic laws of gravity and orbits, the team was able to estimate how massive the companion is. The answer: it could be only as massive as our planet Jupiter!

Moreover, this pulsar companion orbits the pulsar every 2.2 hours, which means that it is only about half a million miles away from the pulsar. If we replaced our sun by the pulsar and its companion, the companion's orbit would be about the same size as the sun. This is shown in the artist's impression of the system at the top of this post, where you can see the pulsar (with squiggly lines representing the light coming from its magnetic poles), the companion, its orbit (the dotted oval), and our sun's size on the same scale (the golden circle).

If a companion is that close to a pulsar, the pulsar's gravity will try to steal material from the companion. If the companion is small enough, its gravity will be strong enough to hold on to its material. If it is too big, its gravity will be weak and material will get pulled onto the pulsar. We don't see any signs of any transfer of matter, so it must be pretty small. Remember that this companion could have about the same mass as the planet Jupiter. If we were to put Jupiter in the same orbit as this object, Jupiter's atmosphere would get pulled onto the pulsar. So the pulsar companion has to be smaller than Jupiter. Much smaller, in fact.

Some more calculations show that the largest the companion can be without transferring matter onto the pulsar is about 50,000 miles across. To still have the same mass as Jupiter, it would need to be twice as dense as lead, and even denser than platinum. And remember, this is the minimum density; if the companion is smaller it could be even denser.

So you can see why many people have called this companion a planet. It has about the same mass as Jupiter, it must be smaller in radius than Jupiter, and it is denser than our densest metals. But remember that, whatever this companion is, it had to transfer enough stuff onto the pulsar to make the pulsar spin hundreds of times a second. The amount of matter it takes to do that is far, far more matter than a planet that got too close would have. This companion used to be a star.

As I said above, fast-spinning pulsars are thought to form when the companion star is in the process of dying, because then it can swell up enough for the pulsar's gravity to begin pulling matter over. Most stars in the process of dying end up forming white dwarfs. White dwarfs are very dense (squeezing the sun into something the size of the Earth), and can be made out of either helium or carbon. Carbon white dwarfs are the most common kind. But white dwarfs tend to be about half the mass of the sun or more, not the mass of Jupiter. However, if the pulsar swallowed enough material and nearly swallowed the entire star, it is possible to whittle what was once a star like the sun down to a pile of ashes no bigger than Jupiter. And this object would be far more dense than platinum -- about 40,000 times more dense. So it seems reasonable to guess that the companion to the pulsar is a white dwarf made out of carbon. There are other, more complex arguments, too, and it is far from certain that this white dwarf is not made out of helium. But we cannot see the white dwarf directly, so we can't confirm what it is made out of.

Let's assume we indeed have a white dwarf made out of carbon that is 40,000 times the density of lead but only the mass of Jupiter. What would it be like?

When a white dwarf is first formed, it is hot. Hundreds of millions of degrees hot – this used to be the central fusion reactor of a star. Over time, it will cool off, and in general less massive white dwarfs cool off the fastest. So a Jupiter-mass white dwarf, only 1/500th the mass of a normal white dwarf. should cool off relatively quickly, astronomically speaking.

As a white dwarf gets cooler and cooler, it can begin to crystallize (think of it "freezing"). On earth, one form of crystallized carbon is what we call a diamond. So, we astronomers often say that crystallized white dwarfs are Earth-sized diamonds. In reality, it is not a diamond. Crystallized white dwarfs are a hundred thousand times denser than diamond, and the detailed atomic structure is very different from diamond, too. A ring with a gemstone of white dwarf "diamond" would weigh several hundred pounds. So, the "diamond" term is an analogy, used to explain a white dwarf in a way that paints a picture most people can understand. But is it really a diamond? No.

So, is it fair to call the whittled down, crystallized white dwarf that used to be a full-sized star but now is about the mass of Jupiter a "diamond planet". I think not. The term "diamond planet", to me, suggests something that was always planet sized but made out of diamond. It is catchy, though.

So, DeBeers doesn't need to invest in a rocket to protect their diamond monopoly, and there is no "diamond planet". Too bad. But reality, that we are seeing a whittled down chunk of superdense material, the mass of Jupiter but smaller in diameter, and that used to be a full-blown star perhaps similar to the sun, is just as cool as a diamond planet. Perhaps even cooler.

26 Aug 2011 11:37am CDT: Edited to correct broken link in citation.

M. Bailes, S. D. Bates, V. Bhalerao, N. D. R. Bhat, M. Burgay, S. Burke-Spolaor, N. D’Amico, S. Johnston, M. J. Keith, M. Kramer, S. R. Kulkarni, L. Levin, A. G. Lyne, S. Milia, A. Possenti, L. Spitler1, B. Stappers, & W. van Straten (2011). Transformation of a Star into a Planet in a Millisecond Pulsar Binary Science Express : doi 10.1126/science.1208890

Y Dwarfs? Because they're cool.

2011-08-24T11:00:00.000-05:00

Image Credit: NASA/JPL-Caltech/UCLA

That little green dot in the center of the picture above may not look like much, but it is, in fact, one of the first absolutely definitive members of a predicted type of brown dwarf, the "spectral class Y" dwarfs. It was discovered by astronomers using data from the Wide-field Infrared Survey Explorer, a satellite mission that scanned the entire sky in the infrared wavelengths of light during 2010. The star above has a temperature of about 25 degrees Celcius, or roughly 80 degrees Fahrenheit -- measurably cooler than the endless summer heat here in Texas. The discovery was announced yesterday by the WISE team, and an official journal article announcing the discovery has been accepted for publication in the Astrophysical Journal. Even cooler than the brown dwarf is the fact that this paper was headed up by a good friend of mine, Michael Cushing (now a new faculty member at the University of Toledo).

First, some quick background and definitions. Stars like the sun shine because they are giant nuclear fusion reactors, with most fusing hydrogen into helium. This process releases a lot of energy and can last for a long time - the sun's lifetime is about 10 billion years. In stars that are smaller (less massive) than the sun, the fusion reactions still occur, but at a lesser rate, because the star is cooler and the hydrogen a bit more lethargic. This trend continues until you get to stars that are about 8% of the sun's mass. Since these stars fuse hydrogen so slowly, they may actually survive for thousands of billions of years (i.e., trillions of years), even though they don't have as much hydrogen fuel as the sun does.

Objects less massive than 8% the sun's mass simply are too cold to perform hydrogen fusion. There are a few uncommon nuclear reactions that can occur and keep the object warm for a few million years, but those reactions quickly flicker and die. Without energy from nuclear reactions, the "failed star" will begin to cool off and slowly fade away. These objects are called brown dwarfs.

Smaller yet, at about 1.5% the mass of the sun, objects can never perform fusion reactions at all. Many astronomers refer to these tiny objects as planets, although there is no physical law that says these things must orbit stars (a condition some astronomers think is necessary to call something a planet). Jupiter, although a whopping big planet in our Solar System, is only 0.1% the mass of the sun, 15 times less massive than this limit. Although some books and people may refer to Jupiter as a "failed star", it never came close to doing any type of nuclear fusion and so being a brown dwarf, let alone sustaining nuclear fusion like stars do. Calling Jupiter a "failed star" is, in my opinion, like calling my house cat a tiger. It may look and act like a little tiger, but it's a different animal. Anyway, I digress.

Stars and brown dwarfs are classified based on their surface temperatures. When we take a spectrum of a star, we split its light up into its component colors. The different elements and molecules present at the surface of a star each have a sort of bar code in the spectrum, so we can tell what a star is made out of by looking at the spectrum.

But the elements we see depend not just on how much of that element is present, but also how hot the star is. For example, about 90% of the sun's atoms are hydrogen atoms, but the sun's spectrum is dominated by bar codes from elements like iron, magnesium and calcium, elements that make up less than 1% of the sun's atoms.

For stars hotter than the sun, like the bright star Sirius, we see mostly hydrogen in the spectrum. for stars even hotter yet, we see mostly helium. But all stars are nearly identical to the sun in composition -- it's the temperature that makes the huge differences in the spectrum.

About 100 years ago, a group of women astronomers at Harvard College Observatory devised a system for classifying the spectra of stars by using a letter for each type of spectrum, starting with "A", "B", and so on up to "Q". Each letter stood for a spectrum that was dominated by different elements. The system was developed before astronomers understood that the star's temperature was important. Once the temperature effects were figured out, the spectral types were simplified and re-arranged to the now-familiar "O B A F G K M" sequence. O stars are very hot, while M stars are the coolest true stars around.

Brown dwarfs were first discovered in the 1980s and 1990s. Young brown dwarfs are still hot from their formation and the tiny amounts of nuclear fusion that they can perform, and so have temperatures that give them M type spectra. But as astronomers discovered more and more brown dwarfs, they began to find some with cooler spectra that looked quite different from the spectra of M stars. In 1999, Caltech astronomer Davy Kirkpatrick organized these brown dwarfs into two new spectral types.
After careful consideration of possible names for these new spectral types, he and his collaborators settled on the letters "L" and "T", being two of the only three letters left that would not be confusing. L-type brown dwarfs, the next class cooler than M stars, show strong features of molecules like iron hydride and atoms such as sodium and potassium in their spectra.

As a brown dwarf cools further, methane will form in its atmosphere and become a strong feature in the spectrum. Since these spectra look much different, they are a different spectral type, the "T"-type brown dwarfs. Until recently, all known brown dwarfs were spectral type "L" and "T", which covered temperatures down to about 500 Kelvin (about 230 Celcius, or about 450 degrees Fahrenheit) -- much cooler than stars, but still fairly toasty by human standards. As a comparison, the planet Jupiter has a temperature of about -150 Celcius, or what my grandmother would call "boo cold".

The third remaining spectral type letter, "Y", Kirkpatrick suggested to be used for the coolest possible brown dwarfs, those with temperatures below about 500 K. At these temperatures, ammonia can form and become strong in the spectrum. While a few cool brown dwarfs that might have been "Y"-type had been discovered and described, there has been vigorous debate over whether these really were a different spectral class.

Then along came WISE. WISE is a telescope that looks in the infrared and is optimized to find things with temperatures close to room temperature (or colder). WISE has therefore found numerous asteroids and comets, and it was expected to find many brown dwarfs. Brown dwarfs don't give off much visible light, but shine in the infrared. Once the WISE team identified candidate cold brown dwarfs, they went to some of the largest existing ground-based telescopes like Magellan and Keck to get spectra. After all, if you are going to be claiming to find a new spectral type, you better have a spectrum to prove it.

The spectra tell the tale. There does indeed appear to be some ammonia visible in the spectra of these WISE brown dwarfs, and there are other substantial differences with the coolest T dwarfs. So, it seems very likely that these cool brown dwarfs are indeed "Y" dwarfs, fulfilling a prediction made over a decade ago. And the temperatures of these stars are around 300 to 500 Kelvin, or from room temperature up to a few hundred degrees. The coolest of the confirmed Y dwarfs, with the typical astronomically inscrutable name of WISE 1828+2650 (the green dot in the photo at the top of this post), has a surface temperature of about 80 degrees Fahrenheit, very comfortable for humans!

Don't start making vacation plans to visit WISE 1828+2650 yet, though. First, like all stars and brown dwarfs, WISE 1828+2650 is made entirely of gas and has no "surface" on which you could stand. And even if there were some sort of platform on which you could stand in the 80-degree atmosphere, the force of gravity would be 10 to 100 times that on Earth -- not exactly pleasant. And, as if that weren't bad enough, like all stars, WISE 1828+2650 is composed almost entirely of hydrogen and helium, so there wouldn't be enough oxygen to breathe.

Could there be cooler brown dwarfs out there? Quite possibly. We don't know exactly how fast brown dwarfs cool off, but small brown dwarfs can cool to 300 Kelvin in 5 to 10 billion years, less than the age of the Universe. "Planets" obviously can get much colder yet, like Jupiter at its frigid 125 Kelvin. But the oldest, largest brown dwarfs may not have had enough time to get this cold yet. So, the green dot in the picutre at the top of this post could be one of the coolest stars in the sky, in more ways than one.

Michael C. Cushing, J. Davy Kirkpatrick, Christopher R. Gelino, Roger L. Griffith, Michael F. Skrutskie, Amanda K. Mainzer, Kenneth A. Marsh, Charles A. Beichman, Adam J. Burgasser, Lisa A. Prato, Robert A. Simcoe, Mark S. Marley, D. Saumon, Richard S. Freedman, Peter R. Eisenhardt, & Edward L. Wright (2011). The Discovery of Y Dwarfs Using Data from the Wide-field Infrared Survey Explorer (WISE), accepted for publication in The Astrophysical Journal, arXiv: 1108.4678v1

The last shuttle launch

2011-07-08T11:49:00.000-05:00

I just watched the space shuttle Atlantis safely launch on the final mission of the Space Shuttle program, 30 years after I watched the first launch of the program. 30 years. Wow.

On April 10, 1981, I was a first-grader, and my parents kept me home from school to watch the launch. I had never seen a rocket launch. The final Apollo moon landing happened a year before I was born, the final Skylab mission was halfway over when I was born, and I was a toddler when Apollo-Soyuz was launched. So space travel was a foreign idea to me, and I didn't understand the fascination. I remember seeing this ungainly machine sitting on the launch pad and telling my parents that it would never work, and that I wanted to go to school. When the countdown clock stopped 31 seconds before launch, I laughed, said, "See, I told you", and saw a pained look on my parents' face.

Two days later, the STS-1 did launch, and again, I was forced to watch. But as soon as the engines fired and the shuttle lifted off the pad, I was hooked. Although commander John Young had been in space as part of both Gemini and Apollo, those missions meant nothing to me, and for years I associated him solely with the space shuttle.

Over the coming years, I read everything I could about the space shuttle and astronauts, and even subscribed to a magazine and ordered a science encyclopedia without my parents' permission. (Kids, don't do that!) Thankfully, they paid more or less willingly for my enthusiasm.

I grew up with the shuttle program, entering my turbulent teens at the same time as the Challenger disaster. I went to Space Camp (technically Space Academy) in 1988 and saw models of the great Space Station Freedom and second-generation shuttle-like vehicles that would be operating in just the next few years. Then I watched as these programs were cancelled, reconstituted and rescoped, cancelled again, and yet again reborn. I watched the shuttle launch and repeatedly save the Hubble Space Telescope, perhaps its finest hour. I took my first job as a professional astronomer at the same time as the Columbia disaster. And now, 30 years after it began, we're finishing the space station and putting the shuttle into retirement.

Many people question whether the money we spend on space exploration is worth the cost. Without hesitation, I say that it is. The space program is less expensive than many people realize; out of every $100 in federal spending, 47 cents goes to NASA. And it's not like we are launching bales of money into space; most of that costs pays American workers and American companies for labor and products, so that money goes directly back into our economy. Our modern economy relies on space, from satellite communications, weather satellites, and through GPS navigation, space exploration impacts our everyday lives.

And space exploration serves another purpose. It is inspiring. How many hundreds of thousands or millions of children are like me, inspired to study and advance technology and science by watching big, lumbering rockets atop a thin spindle of smoke and flame? Our technology-driven economy relies on that spark of interest.

Are sunspots going away?

2011-06-14T19:10:00.000-05:00

A press conference today announced three research projects that suggest that our sun's familiar sunspot cycle might be heading toward a major change or even a pause. You can read good summaries of the findings in this post from space.com and this article on Universe Today; I'll wait while you do that and then give you my first thoughts on this news.

Done reading? Good. First, let me state that I am not a solar physicist, and I do not claim to be an authority on the research being done. So feel free to take my opinions with a grain of salt.

I am skeptical that the prediction of a pause in the solar cycle is indeed coming. This is for many reasons, including:

Past history suggests sunspot predictions more than a few years out are very hard to do (to put it nicely). Sunspot cycle 23, which peaked around 2000, was predicted in 1997 to be a fairly high cycle, but it ended up being somewhat ordinary and lower than predicted.
The current sunspot cycle took a lot longer than normal to get started than any major predictions that I saw (such as this one, which predicted a stronger-than-normal cycle), and it seems like it will be pretty darn weak.

My personal takeaway: predicting sunspots involves really complex physics that we don't fully understand (and that may not be predictable!), and I will remain highly dubious of any model until someone gets it right a few times in a row.

Also, the new studies extrapolate currently-observed trends into the future. Extrapolation is always dangerous. Will the trends hold? Maybe. Maybe not. Past performance is no guarantee of future results. When we don't really understand the physical process behind the trends, extrapolating is fraught with peril.

Don't take this to mean that I think these predictions are worthless. Quite the opposite; they are crucial. Bold predictions allow us to test new ideas. If these scientists are correct and the sunspot cycle comes to a temporary halt, then perhaps we've experienced a breakthrough in our understanding of how the sun's magnetic field works. And if the predictions are wrong, then we will know that these ideas were not correct (or misinterpreted; there's a slight difference).

However, if these predictions do hold up and the sunspot cycle pauses for some extended time, we do have some idea of what could happen. In the late 1600s, sunspots all but disappeared for nearly 50 years, in what we now call the Maunder Minimum. During the Maunder Minimum, global temperatures cooled by about 1 degree Celcius (2 degrees Fahrenheit).

Should this happen again, we would expect to see a temporary pause in global warming. Most climate predictions expect a 1 to 2 degree Celcius rise in global temperatures over the next 50 years due to the greenhouse effect. A new Maunder Minimum on the sun might almost exactly cancel that out, and so average global yearly temperatures might stay about the same for the next half century. (Year-to-year fluctuations and local fluctuations would certainly remain!)

This would be a mixed blessing. We might get some breathing room to reduce greenhouse gases while avoiding some of the worst effects of global warming, but this would probably also lessen an already weak societal desire to do something. And, if over the next half century, we did not reduce greenhouse gas concentrations, when the solar cycle restarted that few degrees of warming would occur in just a few years instead of over several decades, which could be an economic, ecological, and societal disaster.

So, in short, don't panic. I am dubious that these predictions of a pause or major change to the sunspot cycle will come to pass. Even if they do, sunspot cycle pauses have happened in the past and we all came through just fine, so there's no reason to worry, let alone panic. Making these predictions is important to the advancement of science, and even if the predictions are wrong, we will learn something about how the sun works.

A new type of supernova?

2011-06-14T18:16:00.000-05:00

Image Credit: Caltech / Robert Quimby / Nature

Last week, a group of astronomers led by Caltech astronomer Robert Quimby announced that they had learned a few crucial pieces of information about these enigmatic sources. This new evidence suggests that we are seeing a new type of stellar explosion, though we still don't know exactly what we are seeing.

Two years ago, I attended a conference on supernovae (exploding stars), and I blogged about weird objects that we could not explain. In apparently blank parts of the sky, a couple of "new stars" had appeared and slowly faded away, just like supernovae. Only these new objects changed their brightness on much longer time scales than normal supernovae, they did not appear to be located inside another galaxy, and their spectra showed weird features that could not be identified with certitude. Many different explanations were proposed, from white dwarf stars in our own Milky Way Galaxy to carbon stars being shredded by black holes halfway across the Universe.

These new types of supernovae were discovered as part of the Palomar Transient Factory program. This program continuously searches several patches of the night sky, looking at these same patches over and over and over again, night after night, week after week. The program uses an automated telescope and camera that looks for things changing in brightness and tries to figure out what they are. Automated routines try and figure out what the objects might be, such as a variable star or a supernova, and then flags the most interesting ones for further investigation. Several similar searches are underway and are discovering ever-increasing numbers of "transient" events, or objects changing their brightness on short time scales. These searches have turned up new types of supernova explosions before.

So, while these weird new types of supernovae are rare, the automated searches never tire and found many instances. When Quimby's team finally had several examples to go off of, they looked for similarities between each of the supernovae. We'll look at a few of them.

First, and most importantly, Quimby and his collaborators were able to figure out where these objects were. (As I said above, as of two years ago, we didn't know if they were right next door to our Solar System or halfway across the Universe, billions of light-years away). Each of these "new stars" was associated with a different, extremely faint galaxy. These faint galaxies have less than 1/100th the number of stars that exist in our Milky Way, which makes them hard to see. It also means that astronomers have to explain why these explosions only happen in very tiny galaxies. You'd expect that this type of explosion should be much more common in galaxies that have hundreds of times more stars, but they don't seem to be.

Second, the team was able to measure distances to the weird supernovae. These distances ranged from a few billion light-years (relatively nearby, in astronomical terms) to ten billion light-years (a good fraction of the size of the visible universe). Once we know how far away something is, and since we know how bright it appears to be, we can calculate how bright it actually is. Doing so, the team found that these objects are much brighter than most normal supernovae.

Third, we know that the Universe is expanding, and that expansion changes what we can see in the spectrum of an object due to the Doppler shift. The spectrum of an object contains information about what it is made out of, its temperature, and even how fast it is moving. Most astronomical objects have fairly recognizable spectra, and so astronomers can calculate the amount of the Doppler shift and account for it in their analysis. But these weird supernovae had spectra that astronomers could not recognize. Quimby's team finally cracked the code by finding a couple of recognizable features in the spectra, which allowed them to measure the Doppler shift and correct for it. When they did so, they found that all of the weird supernovae had very similar spectra (showing that they are the same type of object), and that these supernovae have very different chemical makeups than normal supernovae.

These three new bits of information are crucial. When we discover new things in astronomy, we always want to know how far away they are, how bright they are, and what they are made out of. And we now have that information. But we still don't know exactly what we are seeing!

Think of it this way: suppose you see smoke on the horizon, and after some measurement you figure out that the smoke is from a wood fire 5 miles away with flames leaping 50 feet in the air. That's all well and good, but you probably would like to know what is burning. Is it a tree? Is it a barn? Is it a bonfire? Is it your house? Or is it something else entirely? The answer to this question could be very important to you!

Likewise, here we see something that in some ways looks like an exploding star, but yet it is brighter and has a different chemical makeup. Also, this explosion seems to only happen in dwarf galaxies and not in big galaxies like our Milky Way. What could it be?

One idea that Quimby's team proposed is a hypothetical kind of supernova called a "pair-instability supernova". As stars go through their life cycles, they convert hydrogen into helium, and helium into heavier elements like oxygen, calcium, silicon, iron, and every other element known in nature. The more stars a galaxy has, the more of these heavy elements that are made. So, big galaxies like the Milky Way have a lot of these heavy elements, while dwarf galaxies (like the host galaxies of these weird supernovae) have a lot less.

It turns out that if you try and make really big stars, say stars that are a hundred or more times the mass of the sun, it gets really hard if the star is polluted by a lot of heavy elements. The most massive stars in the Milky Way seem to tip the scales at about 100 times the mass of the sun. But in dwarf galaxies, with few metals around, you can make even more massive stars.

Not only that, but metal pollution changes the life cycle of a star. Stars with lots of heavy elements tend to shed a lot of weight as they get older. A heavily-polluted star that starts out with 100 times the mass of the sun might end its life with only 1/10th that amount. But a star with very little heavy element pollution can keep all of that mass.

Computer models of extraordinarily massive stars that keep all of their material as they approach their death show that these stars can get ridiculously hot in their centers. They can get so hot that they begin to produce antimatter. Producing this antimatter makes the star unstable, and it blows itself apart in a supernova, and likely one that is very bright with different chemical composition than the more-common, typical supernovae we see coming from metal-polluted stars in big galaxies.

So, are these weird supernovae really the hypothetical pair instability supernovae? Maybe. Maybe not. Quimby's team mentions another possibility (involving a magnatar, a star with a super-strong magnetic field). There may also be other, even more exotic explanations. And I've seen other, different types of supernovae explained using pair-instability supernovae -- since it is a hypothetical kind of explosion, they make good scapegoats for things we just don't understand well. But this is how science often works. These new, weird supernovae and pair instability supernovae are two pieces of the giant jigsaw puzzle of the Universe. Maybe they fit together, maybe not, but we might as well try it and see!

A supernova in the Whirlpool Galaxy

2011-06-10T17:32:00.001-05:00


Supernova 2011dh in the Whirlpool Galaxy. Image Credit: Peter Edwards

Supernovae, the explosive end of the life of some stars, are among the most powerful and most spectacular events in the universe. They are also very rare. Our Milky Way galaxy, with tens of billions of stars, sees one of them explode every 100 years or so. The last known supernova in our galaxy was seen in 1604 and was studied by the famous astronomer Johannes Kepler. Since that time, we think that at least two stars may have exploded in the Milky Way, with the explosions veiled by some of the Milky Way's many thick, opaque clouds of dust and gas. But none have been seen.

Thankfully, there are lots of galaxies in the universe. So, when astronomers want to study supernovae, they look at a lot of galaxies. Such surveys for supernovae are turning up new explosions in distant galaxies all the time. Still, many of these galaxies are fairly far away, and it is rare to find a supernova in our neck of the woods.

Enter the Whirlpool Galaxy, also called Messier 51. The Whirlpool is nearby, as far as galaxies go – "only" 26 million light-years away. It is also a favorite target of amateur astronomers, because it is a beautiful face-on spiral galaxy, and its spiral arms can be glimpsed by modest-sized telescopes in dark places. Last week, sometime before the evening of May 31, a star exploded in one of the spiral arms. The picture at the top of this post shows a picture with the supernova (the "new star" marked by white lines on the left picture) and a picture of the galaxy taken a couple of months ago, before the star exploded.

Although the new star may look faint to you, remember it is 26 million light-years away. The other stars in the picture are stars in our own galaxy and a few hundred light-years away. The supernova is 100,000 times more distant than these stars, but yet appears the same brightness! If one of the nearby Milky Way stars in this picture were to explode as a supernova, it might get as bright as the Full Moon, casting shadows at night and being easily visible at noon.

Because this supernova is nearby, it is fairly easy to study with modern equipment. We also are lucky that the Hubble Space Telescope took a detailed picture of the Whirlpool in January 2005. Astronomers at the University of California Berkeley and the California Institute of Technology were able to locate the star that exploded in the Hubble's image. This means that we know some details about the star before it exploded, which allows astronomers to determine some important parameters about the star. For example, before it exploded, the star was probably about 18 to 24 times the mass of our Sun.   We expect stars with this much matter should end their lives in supernova explosions. (Some of my work is related to this very question – How big does a star have to be before it will die as a supernova instead of more gently becoming a white dwarf?)

With this new supernova, now christened with the unspectacular name of SN2011dh, the Whirlpool appears to be a veritable supernova factory. Since 1994, at least three stars have exploded in the Whirlpool: one in 1994, another one in 2005, and now this one in 2011.   Prior to 1994, no supernovae were known to have occurred in the Whirlpool Galaxy, though astronomers had only been watching the galaxy for a century or so, and it's possible they missed one. But why does the Whirlpool get three in 17 years while our own Milky Way has not produced one visible from Earth for over 400 years?

It could be luck. Random, rare events, like supernovae, often seem to come clustered together. This is a property of statistics.   Also, the Whirlpool's gorgeous spiral arms are visible because they are furiously producing new stars, perhaps at 5 times the rate that our Milky Way does. If the Whirlpool is producing stars five times faster than the Milky Way, then we should expect it to have supernova explosions five times faster (averaging one every 20 years). At this higher supernova rate, it is completely probable from statistics to see three explosions in 17 years after decades of not seeing any.

One thing we can be certain of: these three supernovae are not related. The stars that exploded are thousands of light-years apart. Each star exploded before any light from the other explosion reached them, and even if they were within a dozen light-years of each other, we know of no way for stars to cause one another to explode. (If the stars are really close, like a few light minutes apart, that could be different). Any aliens living on a planet near the 2011 supernova could have looked elsewhere in their sky and still have seen the stars that we on Earth saw explode in 1994 and 2005, and would have had to wait thousands of years to see those explosions. Of course, this all happened 28 million years ago by their reckoning. Time, space, and the order of events can be quite mind-blowing.

So, when will we see the next Milky Way supernova? It could be tomorrow. It could be a decade. It could be centuries. But I can guarantee that the light from that explosion (and many others) is already on its way – we just have to wait for it to get here.

Oh, deer

2011-05-31T04:47:00.001-05:00

One of the nice things about going observing is that the trip involves visiting a secluded portion of wilderness, albeit one with hot meals, indoor plumbing, high-speed internet, and soft beds. It's the best of both worlds!

With the short nights of summer, I have some extra time to kill in the afternoon. Yes, I could be a good astronomer and work on papers and data analysis. But instead, I've been taking some walks in the great outdoors. Here are some pictures from the past few days here at McDonald Observatory. Click on each one to see a larger view. And, be warned, I'm not and never will be a great photographer.

The mountain is full of wildlife, including larger animals like deer and javelina. It's a bit scary to run into the latter at night.

This is a 180-degree panorama taken near the 36-inch telescope (in the dome on the right). The center of the view faces southeast, toward the town of Fort Davis. I made this panorama using a demo version of some stitching software, which explains the random watermarks sprinkled liberally over the image.

On a clear day, you can see for miles, sometimes well over a hundred miles. The last couple of evenings, there have been gorgeous views of building thunderstorms. This picture was taken after sunset, so the base of the cloud is in shadow, but the top of the cloud is still bathed in sunlight. This storm was off to the northeast, in the direction of Pecos, but I never checked the radar to see exactly where it was. The distant dome on the left is the Hobby-Eberly Telescope, and the dome on the right is the Harlan J. Smith Telescope.

Finally back at the telescope

2011-05-27T05:05:00.000-05:00

Tonight I've been observing at the McDonald Observatory. It's been 13 months since I've last sat at the controls of a telescope to do some research. I think this is the longest interval I've gone without using a telescope since I started graduate school in the late 1990s. I can feel a bit of the rust.

A lot has changed since last I was sitting in front of the telescope controls. Last April, I was negotiating terms of a new job as a university professor. Today, I've finished that first academic year. Or perhaps "survived" is a better term. I was warned by many colleagues that this was going to be a busy year, and I believed them, but I was still shocked how busy the year was. The transition from astronomy research to educator is not easy.

So now that the summer is here, I'll be working hard on my research projects, and hopefully getting to write a lot more here than I have over the past several months.

New discoveries on odd stellar explosions

2011-04-08T11:03:00.000-05:00

Image Credit: NASA/Swift/Penn State/J. Kennea

Over the last week and a half, there have been a couple of news releases about stellar explosions. In the first story, astronomers have spotted a puzzling blast of gamma ray and X-ray emission that could be a star being ripped apart by a massive black hole. In the other story, astronomers have made substantial progress in understanding the brightest supernovae ever observed.

First, the puzzling gamma rays. For decades, astronomers have seen sudden, short bursts of gamma rays coming from all over the sky. About ten years ago, after a lot of hard work (and a little luck) by many different researchers, most astronomers came to believe that many of these "gamma-ray bursts" are the birthing cry of new black holes formed at the centers of massive, exploding stars.

Several space missions have been studying these gamma-ray bursts, including the currently-operating Swift satellite. These satellites automatically detect the few-second long burst of gamma rays, locate where in the sky they are coming from, and send emails and instant messages to astronomers around the globe alerting them to the event. Especially interesting events can get rapid observations from large telescopes and major satellites such as the Chandra X-ray Observatory and the Hubble Space Telescope.

On March 28, the Swift satellite detected a burst of gamma rays in the direction of the constellation Draco. Since gamma-ray bursts are seen every few days, this burst started the normal response. Automated messages went out, a team analyzed the data and put out some standard preliminary analysis. But just 43 minutes later, Swift detected another burst at exactly the same place. This is very rare, though not unheard of - but it is rare enough that additional resources started swinging into action. Over the next few days, many additional bursts of both gamma rays and X-rays were seen coming from the same object.

Finally, data from the Chandra X-ray Observatory and the Hubble Space Telescope came in. The source of the gamma rays and X-rays lies very close to the center of an otherwise normal-looking galaxy. In fact, as far as astronomers can tell, the source lies directly in the center of that galaxy.

This discovery, that the weird source lies at the center of a galaxy, casts suspicion squarely on the type of object that lives in the center of most galaxies: a supermassive black hole. Now unlike what many people think, a black hole is not some sort of cosmic vacuum cleaner, sucking in everything around it. A black hole can only eat anything that wanders too close.

How close is too close? The diameter of a black hole can be found by multiplying its mass (in terms of the sun's mass) by 3.7 miles. So, if the sun were to collapse into a black hole, the black hole would be 3.7 miles across. Typical black holes that form from dying stars are about 10 times the mass of the sun, and so are a few dozen miles in diameter. The black hole at the center of our Milky Way galaxy is about 4 million times the mass of the sun, and so it is about 15 million miles in diameter.

The really weird stuff that happens around black holes due to Einstein's general relativity (time slowing way down, space highly distorted, light being highly bent, and unfortunate space explorers being turned into spaghetti) only happens when you get closer than a few times this distance. So, if the sun were to be magically transformed into a black hole, really weird things would only happen if you happened to get within a dozen miles or so of the black hole. The Earth, 93 million miles away, would be unharmed.

The black holes at the centers of galaxies are much larger, but compared to the distances between stars, they are still tiny. The black hole at the center of the Milky Way has many stars orbiting it, including one star that gets within 10 billion miles (about three times the average Sun-Pluto distance) every 16 years. That star passed by the black hole in 2002 with no ill effects.

Still, if a star were to somehow wander within a hundred million miles or so of a supermassive black hole in the center of a galaxy, it would get ripped to shreds. This shredding would release a lot of energy in the form of gamma rays and X-rays. A press release from NASA suggests that this is precisely what caused the multiple gamma ray bursts from the otherwise normal galaxy in Draco last week.

This explanation makes sense, but it's important to emphasize that it is just a hypothesis right now. More data continues to come in, and as news of the discovery spreads, more astronomers will begin to compare these data to simulations of what happens when a star is shredded by a black hole. Perhaps they will agree, and perhaps they won't. Time will tell.

This leads us to the second story, which was announced this week by McDonald Observatory. This story is based on a journal article that has been published in the Astrophysical Journal, one of the main astronomy journals, so the science has already passed significant vetting by peer reviewers. It doesn't mean the science is absolutely, positively right, but it does mean the science has met some substantial level of quality control.

About four years ago, astronomers announced the discovery of what was then the most energetic supernova ever detected. The initial discovery was made by Robert Quimby, then a graduate student at the University of Texas in Austin, and now a postdoctoral researcher at Caltech.

Many people initially speculated that this supernovae, and a few others like it, was a new kind of exploding star. Some models of really massive stars suggest that, as the star ages, it becomes unstable, manages to create large amounts of antimatter, and rips itself apart in the ensuing explosion, called a pair instability supernova.

However, new studies by Emmanouil "Manos" Chatzopoulos, a graduate student at the University of Texas at Austin, and his advisor, Dr. Craig Wheeler, seem to show that these very luminous explosions are not a pair instability supernova. The stars are, alas, not being torn asunder by the explosive mixture of matter and antimatter. Instead, the evidence suggests that these are normal supernova explosions, but as the blast wave from the star travels outwards at high speeds, it rams into shells of matter thrown off by the star decades or centuries before the supernova. This violent collision releases tremendous amounts of energy in the form of visible light, and makes the supernova appear much more luminous than it otherwise would.

These shells of matter are known to exist around a type of star called a Luminous Blue Variable (LBV). These stars sometimes shed huge amounts of material into space via dramatic eruptions from the surface of the star. In our own Milky Way, the LBV Eta Carinae had just such an eruption back in the 1840s. The Hubble Space Telescope has taken amazing images of the material blown off the star during that eruption:

Image Credit: Jon Morse and NASA

If Eta Carina were to explode as a supernova now (and it almost certainly will explode within the next million years), the blast wave from the supernova would smash into those large lobes of material, brightening in a very similar way to the very luminous supernovae Manos has been studying.

So, it looks as if Manos's work may have changed the explanation of these ultra-bright supernovae from some exciting and exotic antimatter-driven explosion mechanism to a slightly more mundane "giant outer space train wreck" explanation. But this is so often how science works, and how it should work: explanations for observed phenomena must be tested, re-tested, and then scrutinized some more. Only then can we be reasonably sure we understand what is happening in the depths of space.

The nuclear crisis in Japan

2011-03-16T14:49:00.000-05:00

Like many of you, I've spent a lot of time this week watching and reading coverage of the earthquake, tsunami, and subsequent nuclear crisis in Japan. The news coverage I've seen of the earthquake and tsunami has been heart-wrenching, and I strongly encourage all of you to give generously to reputable relief charities.

However, the news coverage of the unfolding crisis at Japan's Fukushima Daiichi nuclear power plant has been, through most of what I've seen, poorly done. Two days ago I spent an agonizing 15 minutes watching a national news anchor and a meteorologist discussing what impact the wind might have on radiation levels in Japan. Both people freely admitted to not understanding why the wind would have an impact, or even where nuclear radiation comes from. Yet rather than bring on an expert who could explain these important issues, they admitted their ignorance and threw it to commercial.

News coverage like this leads to fear and panic, and fails to provide that one crucial item that the field of journalism is built upon: correct information. So today there are reports of panic purchasing of radiation-related health products on the West Coast of the U.S. when, to the best I've heard, there has not yet been a catastrophic release of radiation from the Fukushima Daiichi plant, and if there were, it would be many days before any radiation could reach the U.S.

Radiation leaks are scary. We need special equipment to detect radiation, so unlike many threats, the public can't see it coming. We have to rely on people "in the know" to be honest with us. And, in the worst radiation accident that we know of, the explosion at Chernobyl, those people tried to cover up the accident.

What we as the public need is sound journalism with expert scientific reporting to help us to understand what is happening at nuclear plants in Japan. We don't need anchors wildly speculating about radiation and constant reports on fear and panic. Perhaps major news organizations will rethink their attitudes toward science journalism – there should be more effort involved than incorrectly regurgitating a story from a marginally-reputable news outlet.

Thankfully, some expert sources of information are finally starting to be heard. This morning, the American Astronomical Society provided links to some detailed sources of information. As far as I can tell, these links are providing some of the most thorough reporting, including detailed explanations about what we do and do not know about the ongoing situation. So, without further ado, here they are for your consumption:

http://ansnuclearcafe.org/ - A blog from the American Nuclear Society, compiling information from several different sources.
http://www.world-nuclear-news.org/ - The World Nuclear News is provided by the World Nuclear Association; this content is produced by in-house journalists who have access to nuclear experts around the globe
http://mitnse.com/ - the MIT Nuclear Science and Engineering Nuclear Information Hub, maintained by students at MIT's department of nuclear science and engineering.

As in any crisis, the situation is fluid, could change rapidly, and is subject to interpretation. From all that I can tell right now, there is no need for panic. Fear is a natural response, but those of us outside of the immediate area surrounding the reactor are not in immediate danger.

Be concerned. Stay informed. Don't panic.

a visit from a famous scientist

2011-03-04T10:49:00.000-06:00


Image Credit: Department of Physics, University of Oxford

This week, our department was honored to host a visit by Professor Jocelyn Bell Burnell. Dr. Burnell is most famous as having discovered pulsars while a graduate student at Cambridge University. Her thesis work involved building a radio telescope (radio astronomy was still a young field at the time) and searching the sky for radio sources. She discovered multiple sources that emitted strong pulses of radio waves every second or so. These sources turned out to be neutron stars, or stars with the mass of the sun squeezed in to a ball about 10 miles across. Neutron stars are the remnants of many stars that end their lives as supernova explosions. The discovery of neutron stars revolutionized our understanding of how stars live and die. You can read her own description of her discovery here. Dr. Burnell's thesis advisor was awarded a share of the Nobel prize for this discovery, but Dr. Burnell did not receive that recognition. Her contributions to this discovery have been well-recognized and rewarded since, and Dr. Burnell has had a long and fruitful career in astrophysics.

Dr. Burnell visited our department through the efforts of one of our postdoctoral researchers, Will Newton. Dr. Newton came to know Dr. Burnell while he was studying at Oxford. When Dr. Newton learned that Dr. Burnell would be in the United States this spring, he was able to convince our department to invite and host Dr. Burnell for a week.

During this week, Dr. Burnell delivered several talks, including a talk on poetry and astronomy, a talk on the astronomical evidence against various "end of the world in 2012" hype, and a research talk on neutron stars. She also spoke with many of our classes, including a question-and-answer session with my introductory astrophysics class. The students seemed really enthusiastic to have her visit, and they asked some great questions. I was very happy.

I hope that some of the benefits of Dr. Burnell's visit will be more than memories of a visit by a great astronomer. First, I hope that many of the students saw, consciously or unconsciously, that women scientists arre absolute equals in research and teaching ability. Our department is male-dominated, and a very large fraction of our regular visitors to the department are male. I and many of my colleagues and I would like to see this change, and I think that having a visitor with the stature of Dr. Burnell is a big step in the right direction.

Second, I was glad that my students heard Dr. Burnell talk at length about the discovery of pulsars. While the end result, a new class of star, was tremendously important and transformative to the science, this discovery was much more involved than simply seeing a repeating blip on a piece of paper and then dancing around while popping champagne. The efforts that Dr. Burnell and her advisor, Dr. Anthony Hewish. went through to prove that they were seeing some new class of astronomical object are a classic example of how new discoveries should be made. They tested their equipment, they considered every possible existing explanation, they made multiple observations, and they finally found a second (and then third and fourth) radio source that was similar, but different enough to prove that these must be different sources of radio waves. Then they considered the most likely sources of those radio waves and wrote a journal article announcing their discovery, observations, and possible explanations. Other scientists were then able to duplicate their observations and expand upon them.

We all-too-often present scientific discoveries as a "Eureka!" moment, where finally there is understanding and it all makes sense. But good science doesn't work that way. Certainly, there are flashes of inspiration, and luck plays an important role. Yet the discovery of pulsars was a months-long process involving multiple people tracking down multiple possibilities, not an instant of seeing a blip on a radio telescope. Seeing some odd blips on radio data started the process of discovery.

We are very grateful that Dr. Burnell took the time to visit our university. I was pleased to have a good talk about white dwarfs and my research with her, as well as to learn a lot about neutron stars and the present state of various radio and gravitational wave observatories. I even heard some inside anecdotes about the International Astronomical Union's decision on the status of Pluto (maybe I'll write about that another day).

Watching the world change

2011-02-03T16:23:00.000-06:00

World-changing events come in many guises. Sometimes they are highly visible, like the launch of Sputnik, the fall of the Berlin Wall, or the current political upheaval in the Arab world. We rarely know what the eventual outcome and import of such events will be, but there is little doubt that what has happened is Important and will impact us all.

Other world-changing events are more subtle, with impacts that take a long time manifest themselves, but are no less dramatic. The invention of the telegraph and the creation of the Internet were not trumpeted by the forebears of Anderson Cooper swooping in with live reports, but events like these inexorably led to a changed world. (The first public demonstration of the telegraph by Samuel Morse was in 1838; his famous "What Hath God Wrought" message was not sent until 1844. Seven years later, Western Union was founded, and the rest is history.)

Yesterday, NASA and its Kepler Mission team announced a small landslide of new planets and planet candidates. In one announcement, the number of known/suspected planets went from just over 500 to over triple that number, at around 1700. Not only that, but the number of known Earth-sized planets went from a zero or a few (depending on your definition) to nearly 70. And this is certainly just the tip of the iceberg. In two years, Kepler has discovered more planets than all of humanity had discovered in all of history. Think about that for a second. Granted, we've been getting better at finding planets recently, but Kepler's pace is still far beyond all other efforts combined.

In many regards, yesterday's data avalanche was expected. We knew Kepler was working well, and there had been many hints and insider slips that the team was finding planets everywhere in Kepler's sight. But the fact remains that, before the Kepler Mission, we did not know how common Earth-sized planets are. And now we are starting to get the answer. They are very common.

One of the questions humans have been asking for ages is, are we alone in the Universe?   We don't have an answer to that yet, but Kepler has taken a giant step along the path to answering that question. There are profound implications in the Kepler team's work, not just for understanding how the Universe works, but for understanding our place within this Universe. There are potentially huge philosophical implications that extend far beyond the science of astronomy.

The Kepler Mission still has work to do. Its primary goal is to count how common Earth-sized planets are at distances from their parent stars where life similar to that on Earth might be possible.   Yesterday we got some initial hints, but the final answer is still years away.

Undoubtedly, yesterday's Kepler data release will fundamentally transform our study of planets around other stars, in our understanding, our future observing programs, in the challenges to our ideas about planet formation, and even in the basic questions we astronomers are asking.   But I also feel that there will be a societal impact extending far beyond the astronomy of planets. It may take years or decades for that impact to be felt, and this impact will be driven not just by the Kepler mission, but also by the hard work and discoveries of astronomers who will work (and have been working for nearly two decades) on planets around other stars.

The universe of astronomy has changed, even if that change was mostly anticipated. And I think these discoveries will change the rest of the world. How long we have to wait, and where will this change lead society I do not know. But I do know that this is Important.

Planets everywhere

2011-02-02T13:52:00.000-06:00

Image Credit: NASA/Wendy Stenzel

Today, NASA and the Kepler Mission team announced the most up-to-date results of the Kepler mission's search for planets around other stars. Today's haul was nothing short of astounding (though, dare I say, mostly expected): 1235 candidates, 68 of which are Earth-sized. 54 planet candidates (not necessarily the same ones that are Earth-sized) are the right distance from their parent star that they could have liquid water. 170 of these 1200 candidates also show some evidence of being in multiple-planet systems, and one has at least six planets!

Originally, I was going to miss the press conference, because I am supposed to be teaching a class at that time. Then our classes were canceled due to an ice storm, so I intended to watch the press conference. However, I was unable to watch the press conference when my apartment was hit by the 5th in an ongoing series of rolling blackouts to hit the Dallas area. So, I have to pick up the highlights from the web.

NASA's Kepler Mission's main goal is to find Earth-sized planets in Earth-like orbits around sun-like stars. Kepler works by watching for planets whose orbits carry them directly between their parent star and the Earth. This means that Kepler will not see most of the planets that are out there, as this alignment is rare.

Before today, we mostly had heard about planets from Kepler that were very close to their parent star and absolutely, positively, undoubtedly real. Today, roughly 1200 new planet candidates were announced, and perhaps 20% of them may not be real. More on that in a minute.

There are many reasons so many of the first planets were close to the parent star. First, Kepler has to see a planet go in front of its parent star at least three times (in other words, complete at least three orbits) to know that it is real. One passage alerts the mission that this star is interesting, a second passage gives astronomers a predicted orbital period, and a third pass confirms that period. The closer a planet is to its parent star, the more quickly it completes each orbit, and the sooner we've seen those magical three orbits.

But there are many other astronomical objects that can look like planets that really aren't planets. So much of the early work on candidate Kepler planets is to follow them up with ground-based telescopes. These telecopes can measure how fast the parent star moves in response to the planet's gravity, and confirms if we are seeing a planet, or if it is something else. These observations require a lot of time, so positive planet identifications are slow in coming. The closer a planet is to its parent star, the easier it is to make these measurements. So, the most certain planets in the early data tend to be big (their larger gravity tugs more on their parent star) and they tend to be very close to their home star.

As Kepler finds smaller planets further from their parent star, this follow-up work becomes prohibitive. The motion of the Sun due to the Earth's gravity cannot be measured with current telescopes, and each orbit of the Earth takes a year. So if we find a candidate Earth-sized planet in an Earth-like obit around a sun-like star, we have few options for proving it is real beyond all doubt.

However, all the work that goes in to proving the close-in planets also tells the Kepler team how likely any one detection is to be real or not. And it seems that at least 80% of the Kepler planet candidates that pass some initial straightforward tests are real. So, while some of those 1200 planet candidates are probably not real, most of them probably are. If we know what percentage of the planets are likely false detections, we can still analyze Kepler's treasure trove of planets using statistics. This has been the plan all along, and this is Kepler's main mission goal -- to see how common planets are. The results: they're everywhere. Which we already suspected.

One of the coolest planetary systems Kepler scientists announced today is that of Kepler-11. This star has at least six planets around it, and five of these are closer to the parent star than our planet Mercury, the closest planet to the Sun. There are so many planets close in to this star that they sometimes blend together, with more than one planet passing in front of the star at the same time. This is horribly difficult to disentangle! Here's an artist's conception of the Kepler-11 system:

Image Credit: NASA/Tim Pyle

The Kepler mission will continue for several more years. It needs at least another 18 months before that desired trophy of an Earth-sized plaet in an Earth-sized orbit around a Sun-like star will have had a chance to complete those all-important three orbits. We are getting closer all the time to knowing how common our Solar System is (or is not).

The last warning, which I've made before. Kepler only tells us the size (diameter) of the planets, not what they are made out of. So any Earth-sized planet in an Earth-sized orbit around a sun-like star that Kepler finds could be as desolate as the Moon, as hellish as Venus, or as pleasant as the Earth. We don't have the ability to see how livable these planets might be. Yet.

In Remembrance

2011-01-28T12:34:00.000-06:00

25 years ago today, the space shuttle Challenger was lost 73 seconds into her flight. Seven souls were lost. At the time, I was at home -- school had been canceled due to snow. Though a was a space nerd even then, I wasn't watching the launch. I was out sledding, and came inside for a break when my mom called to say she'd heard something on the radio about the space shuttle landing in the Atlantic Ocean. I was glued to CNN for the rest of the day.

Like many people, I felt that combination of shock, sadness, disbelief, even occasional hope that someone may have survived. At the time, I was angry that the majority of the coverage seemed to focus only on Christa McAuliffe, the "teacher in space" astronaut. As a kid, I didn't fully appreciate what it meant to have a civilian astronaut, and I felt that the rest of the crew was being shortchanged. I had never appreciated just how dangerous space travel is, and how much each of our astronauts risks every time they journey into the heavens.

This week always contains a day of remembrance at NASA. Yesterday was the 44th anniversary of the Apollo 1 fire, in which three souls were lost, and February 1 will be the 8th anniversary of the loss of the Space Shuttle Columbia and her crew of seven. Each of these tragedies was the result of cultural and technical failures that, in retrospect, probably should have been caught. But each also led to crucial improvements in safety, technology, and bureaucracy. Perhaps the Apollo 1 fire is the most obvious example, as improvements made to Apollo capsules following the fire likely saved the Apollo 13 crew.

It appears that our country's space program may be beginning a transition to one dominated by the private sector. When one of the many competing firms has a failure resulting in a loss of life, how will we respond? Will we ground all private space missions until the exact cause is identified? Will we end private investments in space travel? I fear these reactions. Instead, I hope we may be able to respond similar to how we have in the past -- be saddened, learn lessons and make necessary improvements, and then continue to push out bravely into the stars.