Wednesday, August 26, 2009

Help astronomers understand the weird star epsilon Aurigae

An artistic envisioning of what epsilon Aurigae may look like up close
By Nico Comargo and courtesy www.citizensky.org
The star epsilon Aurigae is one of the most mysterious objects that you can see without the need for a telescope. With your eye, it looks like a pretty normal star in the constellation Auriga. But every 27 years, it gets noticeably fainter for almost two years, then it returns to its normal brightness for another 25 years. And the cool thing is, nobody is really sure why (read an older post of mine for a little more info, or read articles on this star in the May 2009 issue of Sky and Telescope (click here for the PDF version of the article) and in the October 2009 issue of Astronomy magazine.

Epsilon Aurigae is just beginning its first eclipse since the early 1980s. In order to better understand this system, a large team has been assembled by the American Association of Variable Star Observers, Denver University, Adler Planetarium and Astronomy Museum, Johns Hopkins University, and the California Academy of Sciences.

Best of all, this team wants and needs your help to study this weird star! As part of the International Year of Astronomy, the CitizenSky project has been created to recruit, train, and coordinate public participation in the study of epsilon Aurigae. It doesn't matter whether you have a PhD in astronomy or whether you wouldn't know which end of a telescope to look through, you are heartily welcome to help. (If you have a PhD in astronomy and still don't know which end of the telescope to look through, that's okay, too! It means you're a theorist, and you can probably come up with 30 new explanations for epsilon Aurigae for observers to test in the coming year.)

CitizenSky got a big boost earlier this week when it received three years of funding from the National Science Foundation for the project. So, instead of worrying how to pay for everything, the organizers can focus on getting the best science, instead.

Some professional astronomers will be studying epsilon Aurigae with big telescopes, too, but we can't look at the star 100% of the time for the next two years. In fact, the biggest telescopes can't even look at the star, because it is too bright. And, besides, there're other neat things going on in astronomy, too! So if epsilon Aurigae does anything unexpected, especially on day-by-day or even hour-by-hour basis, there's a good chance professional telescopes won't be looking. This gives you the chance to be the one making observations of epsilon Aurigae when something cool happens.

In order to succeed, CitizenSky needs your participation and help. If you have a nice telescope with some digital imaging equipment, great! If all you have is two eyeballs and a scrap of paper, that will work too! Just click on over to CitizenSky.org to read more about the project and how you can contribute valuable data to help solve the mystery of epsilon Aurigae.

And, if you know anyone else who might be interested, spread the word!

Tuesday, August 25, 2009

More thoughts on Pluto and the scientific method

While ruminating on my post on Pluto and the definition of planets yesterday, I thought of a couple other points on the topic that I wanted to make, though not necessarily related.

1. The story of the status of Pluto is a great illustration of the scientific method. The discussion of the discovery of Pluto, the much later discovery of the Kuiper Belt, and Pluto's subsequent demotion is (with one big exception I'll get to in a couple of paragraphs) an excellent illustration of the way that science actually works. When new data comes to light, we should never be afraid to re-examine even the most dear of scientific ideas. No matter how much we love Pluto (and it's okay to love Pluto, to study Pluto, to spend multiple careers working on Pluto, and to revere Clyde Tombaugh and the amazing amount of work he did to discover Pluto), we have to be willing to reconsider its status. To say, "Nope, it's a planet, end of story" or to say, "Nope, it's not a planet, end of story" is to be unscientific.

Further, science rarely gives us clear-cut answers, especially in the short term. Different teams of excellent scientists can examine the same evidence and come to different conclusions. Only through further study and analysis and debate can the deeper, underlying truths of science be brought out. The discussion of what Pluto is continues (though not always in the public eye), and the vast majority of scientists involved, deep down inside want to know the truth as to what is going on. If you want to learn how science really progresses, keep watching the unfolding saga of Pluto. In the end, the truth will be discovered. It just takes time and lots of work.

2. The International Astronomical Union's demotion of Pluto was far more a decision to solve a bureaucratic nightmare than a decision on the underlying science. Three years ago, the International Astronomical Union found itself with a problem. The IAU has, by consensus of the astronomical community, the final say in the naming of objects. And the IAU has devised specific rules for how to name objects, from planets to moons to asteroids. The rules differ greatly -- planets are named after Roman gods, moons of planets have restrictions also based on mythology, but asteroids ("minor planets") can have many other names, such as the asteroid Misterrogers.

With the discovery of Kuiper Belt objects larger than Pluto, the IAU needed to decide what set of rules the naming conventions should follow. What if someone found a Mars-sized Kuiper Belt Object and wanted to name it Bartsimpson? And which committee in the IAU would get to choose the name? Would the discoverer get a say in the name?

So the IAU decided to create the classification of "dwarf planet" to include all the smaller things that were generally round in shape (the biggest asteroids and Kuiper Belt objects), but were not moons and were not one of the classical planets. This designation allowed the IAU to come up with new naming rules, and everyone could be happy.

This turned into a disaster. What should have been simply a rule on how to name bigger round things turned into a vote on what constitutes a planet. And, as I pointed out yesterday, there are many ways of drawing lines, none of which seems inherently obvious. And this definition went through revisions, and astronomers voted on it. Yet this is not how science works. Scientific truths and laws are not decided by vote. The laws of nature are what they are, and it is our job as scientists to figure out what those laws are over time. This can take years, decades, or even centuries, yet the IAU definition was debated, altered, and approved in a couple of weeks.

I think the IAU should probably just have said something like, "We're making a new class of objects for the purposes of naming conventions. Objects orbiting the sun and large enough to be made round by their own gravity shall be named after mythological gods, and not just Greco-Roman gods, and Committee X has the right to decide how such names are to be selected." That would have solved the naming crisis yet allowed the scientific community to continue to debate exactly what makes a planet, and whether there is a fundamental physical difference between different types of rocky bodies.

Alas, this isn't what happened. They shoulda asked me.

3. What should the role of public opinion and historical precedence be, if any? I'm not going to do much more than open this can of worms, but to what extent can or should public opinion matter? After all, even if 100% of the world's people felt that the moon was made out of green cheese, the moon wouldn't magically transform into a giant Limburger; it would stay a round body made of metal and rock. And if 95% of Americans wrote letters to Congress demanding that Saturn be declared imaginary, and even if Congress passed a law declaring Saturn imaginary, it wouldn't suddenly vanish into the (non-existent) ether. Saturn would continue to circle the sun as always.

So, what should be scientists do about the millions of people still angry about Pluto? We can give in and say, "you're all right, Pluto's a planet." That solution would make people happy, but it completely ignores the scientific process. We could come to a scientific consensus, announce the result, and tell people to "deal with it." That solution would reach the scientific truth (whatever that result might be), but it clearly doesn't work (see point 2 above), and it would really tick off the people who pay our salaries. Maybe we should acknowledge the public's interest in the topic while doing our research, reach a scientific conclusion, and all the while try and teach y'all what we are doing, why, and how we reach the conclusions we do? Nah -- that would be hard. And make too much sense.

Monday, August 24, 2009

The never-ending saga of Pluto

Pluto and its moons
Image Credit: NASA / ESA /H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team

I get back from a fun week of supernova discussions, and the top astronomy news story I spy is on CNN, which says, "Debate over Pluto rages on." Alas. It's the three-year anniversary of the International Astronomical Union's vote to revoke Pluto's planetary membership card, so I guess these stories are bound to come up. The problem is, there's really nothing new, it's just the Earth has gone around the sun three times since the vote was taken.

So, I thought I'd sum up my opinion on the matter. And this opinion starts with one crucial bit of information: I'm taking as pure of a scientific stand as I can, which means setting aside history, public opinion, and as much human emotion as possible. That's part of my job as a scientist. I am supposed to be able to look at factual evidence and see if that evidence supports a hypothesis, no matter where that evidence or hypothesis came from.

So, let's look at the Solar System and start with something easy. Our Solar System is dominated by the sun. Roughly 99.987% of the mass in the solar system is found in the sun. The sun is almost at the dynamical center of the Solar System. The sun is the only thing in our Solar System that produces its own energy by fusion reactions. The sun is clearly different from everything else in our Solar System. So we can set it apart.

Alrighty, let's look at what is left. Jupiter has about 3/4 of the remaining matter in the Solar System. It is a big, almost spherical collection of gas (with maybe some rocks in its core, but we can't see those). It's mostly made out of hydrogen and helium, but has never performed nuclear fusion. But Jupiter has a smaller sibling, Saturn, and a couple of similar cousins, Uranus and Neptune. So, although there are some differences between these four objects, they are all similar enough that we'll lump them together for the time being. For the lack of a better term, let's call them "gas giant planets".

Okay, now we are getting to the hard stuff. There remains about two or three Earth masses of material left in the Solar System, and this material is distributed among a bunch of rocky things. Ordered by decreasing diameter, Earth is the biggest, Venus a close second, and Mars a distant third. These rocky bodies are able to hold on to an atmosphere. Next, in radius, comes Ganymede, Jupiter's largest moon, which doesn't have an atmosphere. Then we have Titan, Saturn's largest moon, which has an atmosphere as thick as Earth's. Now we get to Mercury. Next come a bunch of moons: Callisto, Io (Jupiter), the Moon (Earth), Europa (Jupiter), and Triton (Neptune). Then comes Eris, the largest known member of the Kuiper Belt, a collection of icy bodies further from the Sun than Neptune. Finally, in 13th place among the rocky/icy things in our Solar System, we get to Pluto.

If we keep going down in size, we quickly pick up many more icy objects in the Kuiper Belt and the largest asteroids in the asteroid belt between Mars and Jupiter. Ceres, the largest asteroid, is less than half the diameter of Pluto, and only about 1/6000th the mass of the Earth. As we keep looking smaller, we find more and more asteroids down to the smallest sizes we can see -- objects only a hundred meters across or even less!

There's an important point here. Once we start looking at rocky things in our Solar System, there's no obvious demarcation between types of objects. The bigger "terrestrial planets" all have atmospheres, but so do some moons. A couple of moons are larger and more massive than the classical planet Mercury. Some things are icy, some are rocky. Things the size of the largest asteroids and bigger are all round in shape. So, from considering the size, mass, shape and compositions, there are some lines that could be drawn, but none of these classifications seem obvious.

Okay, so let's look at something else. Let's look at the structure of the Solar System. Sun near the middle, all by itself. Then comes some of the small rocky things: Mercury, Venus, Earth (with its moon) and Mars, with a few tiny asteroids scattered about in there. In this case, the asteroids in this region are so small compared to the big five rocky things that they look fundamentally different. Continuing outward, we next find the asteroid belt, the collection of most asteroids, including the biggest ones: Ceres, Pallas, and Vesta. Next come the four big gas giant planets, each of which has a bevy of much smaller moons, and with a few asteroids and other small icy things swimming about. Last comes the Kuiper Belt, which has a lot of icy things of all sizes. Eris and Pluto are both clearly in this Kuiper Belt -- they're the biggest things in the Kuiper Belt, but otherwise they share similar orbits at similar distances from the Sun.

In this structural view of the Solar System, the four gas giants and four of the rocky bodies (Mercury, Venus, Earth and Mars) all stand out as unique, dominating their respective parts of the Solar System. The asteroid belt is its own thing, and the Kuiper Belt is its own thing. In this view, we can call the eight unique things "planets", and everything else "non-planets". It seems a natural division, and we can define mathematical and physics-related definitions that would include the eight planets in the "planet" category and relegate everything else to the "other" category.

But this definition is also not satisfactory. If we were to take the Earth, stick it on a circular orbit in the middle of the Kuiper Belt, and let billions of years go by, the result would be a round ball of ice the size of the Earth in the middle of a bunch of other smaller balls of ice. An astronomer looking at this Solar System would probably just include Earth as a giant member of the Kuiper Belt. Mars or Mercury certainly would just be considered larger Kuiper Belt objects. But if we moved Jupiter into the Kuiper Belt, it would quickly fling every tiny ice ball out into deep space or down into the sun, and the Kuiper Belt would quickly cease to exist, other than the colder version of Jupiter happily orbiting the sun. In similar thought experiments, Jupiter, Saturn, Uranus and Neptune would still be unique objects, but the smaller rocky planets would not be.

So, from a scientific-motivated standpoint, I think it is unclear if even the Earth should qualify as a planet. Certainly the Earth is unique given its current place in the Solar System, but if it were a moon of Jupiter or out in the far reaches of the Solar System, it would be just another ball of ice. In short, I would argue that there is no good definition for a planet, at least one that would include the Earth and Mars and Mercury while excluding Jupiter's larger moons or giant ice balls in the Kuiper Belt. The rocky things in our Solar System form a continuum of sizes and relevance, and any lines we draw are, to some extent, arbitrary. I'd argue that this is true even if these distinctions are motivated by physics (such as whether an object's gravity dominates part of the Solar System, or if its gravity is strong enough to make it round)!

So, back to poor Pluto. What should we call it? I think that, had we not known about Pluto and we were to discover it today, we would not call it a planet. We'd call it a member of the Kuiper Belt. But, as I've described in far too many words, even a scientifically-motivated definition is, in the end, arbitrary, at least among the rocky and icy things in our Solar System. So I'm perfectly happy to allow history and sociology to help us define what we'll call a planet in our Solar System, and I won't argue with anyone who wants to say that there are nine planets, or even thirteen (let's make Eris, Ceres, Pallas and Vesta planets, too!)

More important than which little objects are planets is that we realize there is a hierarchy of objects in the Solar System, and that the Earth is not in the first or second tiers of that hierarchy. So, if we are looking for other Earth-like planets in other Solar Systems, we will have to look past a lot of big balls of gas before we find the insignificant rocks that may be home to other astronomers studying the four planets of our Solar System.

One last note, for those who have read more about these arguments at other places or even been involved in the discussions of definitions of the word "planet" themselves. I'm NOT arguing that concepts like Hill spheres and formation mechanisms are useless; I actually find them very appealing. But, in the end, those are definitions that leave as much to chance as they do to deeper truths. No matter where a Jovian planet is in a solar system, we would call it a planet. The same is not true for the Earth. This bothers me. Maybe I shouldn't think so hard about it.

Saturday, August 22, 2009

Weird things in the night

The weird transient event SCP 06F6
Image Credit: NASA / ESA / K. Barbary

In spite of everything we know about the Universe, there are still some things out there that we cannot yet even provide a basic explanation for. At the supernova conference I attended this last week, we heard a talk about a new type of object that is not yet explained.

In 2006, the Hubble Telescope was watching a patch of sky for supernovae in very distant galaxies. Hubble finds supernovae by watching for "new" sources of light that get brighter and then fade away; different types of supernovae brighten and fade in specific ways, and so the most interesting ones could be flagged for additional observations.

One such event with the boring name of SCP 06F6 (pictured above) was very curious. It got brighter over a period of 100 days, and then faded away over a similar length of time. That time scale is very long for supernovae. Also, SCP 06F6 did not take place in a detectable galaxy, so if it is not in our own Milky Way galaxy, it took place either on the far edge of the Universe, or it took place in a very wimpy galaxy. But we don't know.

Astronomers took a spectrum of SCP 06F6. Spectra, or the splitting of light into component colors, are a very powerful tool for studying astronomical objects. We can learn the composition of stars and the distances of other galaxies by studying spectra. Different types of supernovae all have different types of spectra. But the spectrum of SCP 06F6 is like no other supernova spectrum. It shows what looks like carbon molecules in a galaxy at a distance of 1.8 billion light years (z=0.14, for those of you who speak redshift).

1.8 billion light years is a big distance, but it's not that big for today's large telescopes and the Hubble Telescope. We should be able to see most small galaxies at that distance, and, again, we don't see a galaxy here. Also, supernovae are very powerful explosions, and carbon molecules are easily broken apart in such extreme conditions. This explanation doesn't add up. It could also be possible to explain the lines as calcium at a distance of 6 billion light years (z=0.5), but even that isn't all that far for supernovae.

Since the discovery of SCP 06F6, supernova searches have started to find many similar events, with the same long rise times, the same weird spectra, and the same lack of a host galaxy. Odd. Very odd.

So, what else could these events be? Astronomer Andy Howell showed the spectrum of a couple of these things, and my first thought was that they look like a rare type of white dwarf called "peculiar DQ" white dwarfs. These white dwarfs show what looks like carbon in their spectra, but at slightly the wrong wavelengths. This could be due to magnetic fields on the white dwarfs, or maybe due to hydrogen adding itself to the carbon molecules (making hydrocarbon white dwarfs!). White dwarfs would be in our own galaxy, so you wouldn't expect to see a host galaxy in Hubble pictures (we're in it!). These peculiar white dwarfs are also very faint, so if they are thousands of light-years away in our galaxy, we would not see the star before or after the event.

But the white dwarf explanation doesn't explain how these things get brighter and fainter. Gravitational lensing is one possibility. If another star passes between us and the distant white dwarf, the gravity of the interloping star can focus the light and make a star get much brighter than it was, even over a timescale of weeks and months. This would seem like a natural explanation to me, with one major problem. Gravitational lensing does not change the color of a star, only its brightness. But there is pretty good evidence that SCP 06F6 and its kin are changing color. If true, that would seem to rule out a gravitational lens. Maybe. (I can think of ad-hoc ways to save it, but then why should these ad-hoc ways happen more than once in different parts of the sky?).

There are many other ideas astronomers have proposed, from Texas-sized asteroids running into white dwarfs in our galaxy to carbon-rich stars halfway across the universe being shredded by distant black holes to exotic types of supernovae. We know so little about these events, it is hard to rule any weird things out yet. As we discover more of these, though, hopefully we can build up enough information to explain these rare and mysterious events.

Friday, August 21, 2009

Light echoes

Light echoes from old supernovae in the Large Magellanic Cloud
Image credit:
X-ray: NASA/CXC/Rutgers/J.Warren, J.Hughes; Optical (Light Echo): NOAO/AURA/NSF/Harvard/A.Rest et al.; Optical (LMC): NOAO/AURA/NSF/S.Points, C.Smith & MCELS team

In 1572, the astronomer Tycho Brahe noticed a "new" star in the sky that grew in brightness until it was visible in the daytime, and then slowly faded away. This star challenged ideas that the heavens were unchanging, and led Tycho to start studying the heavens. Thirty years later, in 1604, Tycho's protege Johannes Kepler noticed a new star in the constellation Ophiuchus; this "star" also became the brightest star in the sky for some time before fading away.

Today, we know these two events were the explosions of stars, known as supernovae. Using optical and X-ray telescopes, we can study the nebulae left from these explosions. Therefore, Kepler and Tycho's supernovae are important bits of data for studying exploding stars. Unfortunately, no supernovae have been observed in our galaxy since 1604. We would very much like to observe a nearby supernova with all of our modern astronomical instruments. Alas, the closest supernova we've gotten was in 1987, when a star exploded in the Milky Way's companion galaxy the Large Magellanic Cloud. And while we see supernovae in more distant galaxies fairly often, it is often impossible to study these events more than a year later.

About 5 years ago, astronomer Armin Rest and his collaborators discovered filaments of light in the Large Magellanic Cloud that appeared to be moving away from three former supernovae. Further analysis found that this light was an echo, or reflection, of light released in supernovae explosions hundreds of years ago. The light started off moving away from the explosion in random directions, and then it hit a cloud of dust, which reflected the light toward the Earth. The light ended up having to travel a few hundred light-years further than the original light from the supernova, so it is just now arriving at the Earth. What this means is that we can see the original light from those explosions and study it as if the supernova were happening now. The picture at the top of this post is from that study.

A couple of days ago, Dr. Rest presented some newer work on light echos. He and his collaborators have now found light echos from both Tycho's supernova and Kepler's supernova. Using telescopes, Rest has managed to use modern instruments to study light from the same explosions that Kepler and Tycho saw. He's been able to confirm what kind of supernova Kepler and Tycho saw -- Tycho saw a Type Ia supernova (an exploding white dwarf), and he saw a very normal Type Ia. Kepler saw a Type IIL supernova (a rare type of exploding massive star).

One of the exciting things about this research is that not only can we study the explosions themselves 400 years later, but we also do not have to wait to look at the final supernova remnant -- we already see those! We are just now starting to see the new nebula that Supernova 1987A is forming; for Kepler and Tycho, 400 years have already passed, so the nebula is already well-formed and has been well-studied by all kinds of telescopes.

One other great thing about light echoes is that they allow us to see the explosion from different directions, like mirrors held hundreds of light years on either side of the explosion. Tycho looks the same from different directions, but Supernova 1987A looks slightly different. With a little luck and a lot of hard work, we may be able to piece together an even better picture of these explosions.

These light echoes are therefore letting us study the past and present of famous supernovae. We can use modern telescopes to study these famous supernovae, and get information almost as good as if Tycho had today's observatories at his disposal. And we don't have to wait decades or centuries for the next nearby supernova explosion.

The topic of light echoes also reminded me of the famous Diana Ross song Reflections. "Reflections of...the way light used to be." (Or something like that)

Wednesday, August 19, 2009

The continuing mystery of Type Ia supernova

After two days at a conference on stellar death and supernovae, it's become pretty obvious that the more we learn about Type Ia supernovae, the less we seem to know or understand. It's fairly amazing that after 40 years of intensive work on the problem, our understanding is still so muddy.

First, what do we know? Type Ia supernovae are a type of exploding star. In these explosions, we do not see any hydrogen or helium, which are the most common elements in the Universe. We also see lots of silicon and iron, which are the products of explosive nuclear reactions involving carbon and oxygen. These two facts make it seem likely that the exploding star is a white dwarf. White dwarfs are the nuclear ashes remaining after most stars finish their lives; they're made mainly of carbon and oxygen, and they have very little hydrogen and helium.

We also understand pretty well what happens after the explosion. Type Ia supernovae have a relationship between how bright they are and how long they take to fade away. This is very useful, because if we see Type Ia supernova in the distant universe, and if we can measure how long it takes to fade away, we know how bright it was. This allows us to figure out how far away the explosion was (because things appear fainter the further away they are). This relation allowed astronomers to find some of the first evidence for "dark energy."

But there are some mysteries. First, how do you get a white dwarf to explode? The typical white dwarf is very stable and resistant to explosion. If we can take a white dwarf and add more and more material to it, eventually it will get big enough that gravity will squeeze the white dwarf, cause it to heat up, and set off nuclear reactions. We think.

But how can you add material to a white dwarf? There are several ways. If you take two white dwarfs and merge them together, you might get a big enough white dwarf to explode. But colliding white dwarfs is hard, and we've never actually seen two white dwarfs that are going to collide and be massive enough to explode. This doesn't mean it can't happen, because it is hard to confirm that you have two white dwarfs in such a system, but until we find such a system, this is just an idea.

Another way to grow a white dwarf is to have it orbiting a normal star. If the white dwarf is close enough to the normal star, the white dwarf's gravity will siphon gas off of the normal star and onto the white dwarf. We actually do see several stars where this is happening; these are called cataclysmic variables. But in cataclysmic variable stars, there is usually too little material being transferred to build up the white dwarf to an explosive mass. There are some special types of these cataclysmic variables that may work, with names such as supersoft X-ray sources and recurrent novae, but we still can't be sure that they will explode.

Either of these methods, colliding white dwarfs or cataclysmic variables, take time to work, perhaps billions of years. But about 5 years ago, astronomers started discovering more and more evidence that the number of type Ia supernovae in a galaxy depends on how fast the galaxy is making new stars. That was a complete shock -- how can a process that takes billions of years to work know how fast a galaxy is making stars right now? Maybe there is a way of adding material to brand new white dwarfs in very short times.

Supernova scientists have just been coming to terms with that finding and developing explanations, when yesterday one of the teams counting Type Ia supernovae stirred the pot further. They said, essentially, that maybe they had made a mistake, and the rate of Type Ia supernovae in a galaxy doesn't depend on its current rate of making stars, but is just an artifact of how we were doing the observations. That claim caused a lot of controversy and discussion, and needs a lot of further exploration.

So, in short, we don't know exactly what causes Type Ia supernovae, and we have mixed signals as to how long it takes to start getting these supernovae. I'm cool with this, because it means there is a lot of work yet to be done, and lots of ways I can contribute to understanding these supernovae.

But there should be some concern, too. Scientists are using Type Ia supernovae to try and understand dark energy and to see whether it changes over time in our Universe. But can we believe the results that supernovae give us if we don't understand the explosions even moderately well? There are certainly other lines of evidence that dark energy exists, so the Type Ia supernovae are clearly pretty good for this purpose. But if Type Ias change at a slight level over time, that change in the supernovae could be interpreted as a change in dark energy instead. For this reason, we need to keep studying Type Ia supernovae and the stars that they come from, whatever those stars might be.

Monday, August 17, 2009

Stellar Death and Supernovae

This week I am conferencing. I'm in Santa Barbara, California, at a conference called "Stellar Death and Supernovae". As you might guess from the name of the conference, we're spending the week talking about the ends of the lives of stars (the tiny points of light in the sky, not the stars of the silver screen that live about 60 miles down the coastal highway in Hollywood).

The conference is being held at the Kavli Institute for Theoretical Physics (KITP) on the campus of the University of California, Santa Barbara. Not only is Santa Barbara a gorgeous place, with the campus right on the coast, but the KITP staff really excel at putting on these conferences.

This week we will be learning about all aspects of the ends of the lives of stars. Today involved a lot of talk about white dwarfs. I led off the day talking about the relationship between the mass of white dwarfs and the mass of their parent star (not surprisingly, bigger stars make bigger white dwarfs). And I talked about the new type of variable star my collaborators and I found last year. I think the talk went well. I'm still digesting a lot of the other talks, I made some good contacts, and I reconnected with several people. So, all in all, a good day.

Tomorrow, we talk about the explosions of white dwarfs, also known as Type Ia supernovae.

Thursday, August 13, 2009

Report says NASA doesn't have money to look for asteroids. Kinda.

Artist concept of the asteroid impact that killed off the dinosaurs
Image Credit: Don Davis / NASA

Run for the hills! NASA doesn't have enough money to keep up with killer space rocks! We're all doomed! Whatever can we do to avoid going the way of the dinosaurs now??

Well, first, we can stop and take a deep breath, because we are not going to be killed by an asteroid tonight. Or tomorrow. Or, most likely, any time in the next several million years. So, since we have some time to spare, let's look at the real story behind yesterday and today's flurry of stories on NASA's beleaguered asteroid hunt.

In 2005, Congress mandated that NASA discover 90% of all asteroids larger than 140 meters (150 yards) in diameter that could threaten the Earth, and they set a deadline of 2020. Since then, several major surveys have been looking for asteroids, including the Catalina Sky Survey, Spacewatch, and the Lincoln Near Earth Asteroid Research program (more are listed here). These surveys have discovered over 6200 potentially dangerous asteroids. Out of all of those, only one has been found to be on a collision course with Earth. That tiny asteroid, called 2008 TC3, hit the Sudanese desert last October, causing no damage and dropping several pounds of meteorites on the desert.

Last year, Congress asked for an update on NASA's search for potentially dangerous asteroids. In a bit of, um, interesting timing, that report came out this week, just after the impact of an unknown rock into the planet Jupiter and the impact of some other unknown thing with Saturn's rings. You can read the report here; it's great for insomnia.

The gist of the report is that the near-Earth asteroid searches are going well, but that there isn't enough money to finish the search by the Congress-mandated date of 2020. The report also suggests that some money be invested in a new telescope or two, some additional staffing, and maybe even a space mission. Again, that is if we want to finish the search by 2020. At current funding levels, the search will proceed, but at a slower pace. But, since Congress mandated the 2020 deadline, and astronomers won't be able to meet them with current funding, there's a problem.

The one thing that almost certainly isn't a problem is whether a delay in completing the search could result in a disaster for humans, with an unseen asteroid creeping up and ending civilization as we know it. This is highly unlikely. We think we've found most of the large asteroids (that can do the most damage) -- these are rare, and relatively easy to see because they are big. It's the smaller ones capable of wiping out several hundred square miles (city-sized areas) that are hard to find, and we probably have a long way to go before we find all of these -- they are hard to see, and there are probably tens of thousands of them yet to be found. But despite there being so many of these small asteroids, the chances of one of these asteroids hitting the Earth in the next few decades is very tiny -- outer space is much bigger than most people realize, and the Earth much smaller than we think.

In short, we are not endangering ourselves if it takes longer than 2020 to find all the potentially-hazardous asteroids larger than 140 meters in diameter. The facts that we are funding the search, even if at a minimal level and that we are even looking for potentially hazardous asteroids (no other country is!) are very encouraging. All the report says is that more money is needed, and if we want to finish in 11 years, we need more funding than has been provided. It's up to NASA, the Obama Administration, and Congress to decide if more funding will be forthcoming. I'm not holding my breath.

EDIT: While reading through more of the released report, I came across this graph showing the expected fraction of near-Earth asteroids that would be found without increased funding (lower dashed line) and with increased funding from various upcoming and proposed telescopes and space missions. In short, additional funding meets the goal, at current levels we don't come close but do make progress.

Fraction of Potentially-Hazardous Asteroids predicted to be found over time
Image Credit: Lindley Johnson, NASA, “Near Earth Object Program” presentation to the Committee to Review Near-EarthObject Surveys and Hazard Mitigation Strategies, December 9, 2008.

Tuesday, August 11, 2009

Congratulations Dr. Forestell!

Yesterday, Texas astronomy grad student Amy Forestell became the newest astronomy PhD when she successfully defended her doctoral dissertation. After a party tonight, she is packing up and moving to New Paltz, NY, where she will join the faculty at SUNY New Paltz. Classes start in just under two weeks, so it will be an abrupt transition, but Amy will do well. She's the only graduate student to have won the McDonald Observatory teaching award, so she will no doubt land on her feet and become an excellent professor.

Dr. Forestell's dissertation is on the search for dark matter in elliptical galaxies. Dark matter is generally thought to be some sort of particle that can only interact with normal matter by gravity (and maybe by the weak nuclear force, a force that works only on subatomic scales). There is a lot of evidence for dark matter in clusters of galaxies and in spiral galaxies, but the evidence for dark matter in elliptical galaxies has been more controversial.

A big reason for the controversy in elliptical galaxies is because elliptical galaxies are denser in normal matter than spiral galaxies. Normal matter likes to clump together (as evidenced by our own existence), while dark matter tends to be more spread out. So, in the centers of elliptical galaxies, the vast majority of the gravitational pull is provided by normal matter, not by dark matter.

In the outer parts of elliptical galaxies, dark matter should start to dominate the gravitational pull. So, why don't we look here? The problem is that elliptical galaxies get really faint really fast in their outer regions, so that by the time dark matter should dominate the gravity, there is much less light to study.

There are two approaches that people can take to deal with the faint galaxy. One choice is to look for brighter objects embedded in the galaxy, like globular clusters or planetary nebulae, and use them as tracers of the galaxy's gravitational pull. Individual clusters and planetary nebulae are bright enough to be studied. There are a couple of problems with using these tracers. First, there aren't that many of them. In an individual galaxy, astronomers are able to study maybe a few hundred globular clusters or planetary nebulae. That sounds like a lot, but these studies involve breaking the sample up into smaller samples based on their distance from the center of the galaxy, so you very quickly end up with groups of a couple of dozen, which result in big errors. There's also a question as to how these tracers are moving around in the galaxy. If they are swarming every which way, like bees in a swarm, then they are a good measure of the pull of gravity. But if most of the clusters or nebulae are plunging back and forth through the center of the galaxy, then it is much harder (though not impossible) to calculate the pull of gravity. Further, it's hard to tell how these tracers are moving around unless you have a whole bunch of them (more than a couple of dozen!)

The other choice to studying the faint outer regions of elliptical galaxies is the brute force method, which involves collecting as much light as possible with as large of telescope as possible. That's the approach that Dr. Forestell took. The great part about this approach is that, if you can get enough light, you are sampling the motions of tens of thousands of individual stars. So, with careful and clever work, you can figure out how the stars are moving around. There are also a lot of people who have put a lot of time into analyzing the motion of stars from light gathered in the central part of galaxies, so the analysis tools already exist.

Dr. Forestell used the Hobby-Eberly Telescope here at McDonald Observatory to gather her data. She also used a supercomputer at the Texas Advanced Computing Center to do hundreds of thousands of CPU hours worth of calculations necessary to explore many possible shapes and sizes of dark matter halos, as well as to explore the myriad of possible motions of stars. Tying all of this together, she found convincing evidence for dark matter halos in elliptical galaxies, which was not a surprise, but she also found that the dark matter halos aren't as dense in the center as most theorists would have predicted. This is mildly surprising, but other groups are finding the same result for dark matter halos around other kinds of galaxies. I think this is telling us that dark matter is more complicated than we thought. And that's cool -- astronomy would get boring quickly if we didn't have mysteries to work on.

On a personal level, I found Dr. Forestell's work quite interesting. My first idea for a dissertation topic back in the late 1990s was to do a very similar study, but using the Keck Telescope in Hawaii. My advisor and I even got time to start the project, but we were stymied by a breakdown of the instrument during our run. Rather than wait another year to start my project, I switched to studying white dwarfs instead.

Seeing Dr. Forestell's work, I'm glad that my project never got off the ground. The tools and supercomputing time that she required for the detailed analysis would not have been available to me a decade ago, and we would have struggled to make sense out of the data. Besides, I think I'm doing work that is just as interesting now, and I left today's grad students some projects to work on. It's a win-win situation.

Thursday, August 06, 2009

Kepler mission news: it works. It works really well.

The Kepler Mission, NASA's satellite dedicated to finding Earth-like planets, has just released its first science results via a news conference broadcast on NASA TV. Frankly, I was hoping to hear about planets that Kepler discovered during its early-mission check out, but the news conference did not focus on that. (I might know some things about those early results, but even if I do, I would not write anything more than what the press conference covered because I'd be getting some nice folks into deep trouble.)

Kepler works by measuring the brightness of stars. If a star has a planet, and if the planet's orbit takes it in front of the star as seen from Earth (only a very small fraction of planets will have that kind of orbit; it's all due to luck), then when the planet goes in front of the star (called a transit, it will block a tiny portion of the star's light. Planets the size of Jupiter block about 1% of the light of their parent stars, while Earth-sized planets only block about 0.01% of the light of the parent star. Since Kepler's goal is to find Earth-like planets, it has to demonstrate an accuracy of 0.01% (or 1% of 1%) for each of a hundred thousand stars. Today's news conference showed that Kepler is indeed getting data that accurate.

Here is a plot showing the brightness of a star seen by Kepler called HAT-P-7. HAT-P-7 is known to have a planet around it; the planet is called HAT-P-7b. HAT-P-7b is 1.8 times the mass of Jupiter and orbits its parent star every 2.2 days at a distance of 5.6 million kilometers (3.5 million miles), or less than 1/25th the distance between the Earth and the sun. The top line of the plot shows what Earth-bound telescopes see when they look at the star. A star with constant brightness should be just a straight line; instead you see a fuzz of points that is relatively straight. The fuzz is due to errors induced by Earth's atmosphere. But, in spite of the fuzz, you can see that the star is pretty constant, with a small but definite dip when the planet transits the star.

The second (bottom) curve is the same star as seen from Kepler. THAT is how much better Kepler is than telescopes on the Earth.

This plot shows the same Kepler data magnified 7 times (top) and 100 times (bottom). Magnifying it 7 times, you can just barely begin to see that the points are not a straight line, but vary a little bit. With the 100 times magnification, you can finally see Kepler's "fuzz." But you can also see a few other neat things.

First, you see a second dip in the light curve. The giant one is when the planet goes in front of the star. The little one is when the star eclipses the planet, also called the secondary eclipse. The fact that we can see this means that we are seeing light from the planet. It's just a tiny bit of light, but we can see it. And so when the planet goes behind the star, its light is blocked from view.

The other thing you can see in the 100-times magnified plot is that the light outside of eclipses is not constant, but it goes up and down as the planet orbits. This means that we are seeing phases of the planet, as it goes from "new" HAT-P-7b (the big transit) to crescent phases, to half phases, to a full phase (just before the secondary eclipse), and back to crescent and new phase again. Here's a movie illustrating the transit, secondary eclipse, and the phases.

Even more analysis of the amount by which the light is changing shows that the planet is not merely reflecting star light, like the planet Venus does as it goes through phases, but that HAT-P-7b is actually glowing. This isn't unexpected; at only a few million kilometers away from the surface of its parent star, the planet must be hellishly hot -- over 4400 degrees Fahrenheit (2300 Kelvin). At that temperature, the planet actually glows, just like the heating element on an electric stove.

In addition, when you look closely at the data, we can see that there is other stuff going on, too. Wiggles in the light are due to slight changes in the star itself. Our own sun does this, varying ever so slightly in brightness on time scales of minutes and hours. On the sun, these variations allow us to study the interior of the sun, so we should even be able to study the parent stars of every planet we find.

Lastly, this is just the first couple of weeks of data on this star. Imagine what we can do when we have three years of data!

So, when will we know if Kepler has found any Earth-like planets? Finding Earth-sized planets in Earth-like orbits takes time -- at least 3 years. Why? Because, in order to confirm a planet, mission scientists want to see the transit of the planet at least three times. The first transit tells you there may be a planet, the second transit confirms gives you an estimated length of the orbit, and the third transit confirms that the planet's orbit repeats with that period. This is crucial: suppose there are two Earth-sized planets around a single parent star at the distance of Jupiter. Each individual planet will only transit once every decade, but if there are two planets, we may get unlucky and see one transit and then the other a several months later. When a third transit doesn't happen months later, we'll know that we were faked out by something else. So, we need three transits, meaning three orbits, and the Earth takes one year to orbit the sun. So, that's three years. Stay tuned!

Wednesday, August 05, 2009

Congratulations Dr. Jeffery!

Newly minted Dr. Elizabeth Jeffery

Yesterday, University of Texas grad student Elizabeth Jeffery successfully defended her doctoral dissertation, becoming the newest PhD astronomer. Congratulations, Elizabeth!

Dr. Jeffery's thesis is on white dwarfs in open star clusters. She has been using white dwarfs to get the ages of star clusters, and comparing that number to the ages of the same star clusters that astronomers get using other methods. The idea here is that the physics behind how we get the ages of white dwarf stars is different than the physics behind the ages of normal stars. White dwarf stars are slowly cooling and fading, so we use the physics of heat loss to get their ages. Normal stars shine by using nuclear fusion, so we have to use nuclear physics to get their ages. And, in both cases, other things are important, like the structure of the stars and how energy gets from the core of the star to the star's surface. In both cases, the physics are different, and there is a fair amount of room for error. Thankfully, though, Elizabeth finds that the ages agree over the range that she studied: the star clusters with ages of a few hundred million years to several billion years. That's a big time span, and it also covers a lot of different physics tests in both normal stars and white dwarf stars. It was a lot of work, and there is more remaining to be done to tidy up the results for publication, but it is very impressive and quite important.

Elizabeth is now preparing to move to the Space Telescope Science Institute in Baltimore, MD, where she will be a postdoctoral researcher there. Congratulations again, Elizabeth, and best wishes on your future career!

Tuesday, August 04, 2009

Astronomers invade Rio; coffee in short supply

Yesterday saw the start of the 27th General Assembly of the International Astronomical Union. These meetings are among the largest meetings of astronomers in the world, and they bring together astronomers from almost 100 nations.

The meetings are held every three years; this year's meeting is in Rio de Janeiro, Brazil, and it runs through August 14. Alas, I am not there. Too many things were conspiring against my ability to attend this time. This year's IAU General Assembly also coincides with the International Year of Astronomy (IYA), so many activities at the assembly are devoted to promoting the IYA.

Evidently, all is going well, although I hear through the grapevine that there is an unfortunate lack of coffee at the meeting. If you had the misfortune of seeing the movie Airplane II, you may remember a scene where the passengers on a space flight remain calm while they are told that they are millions of miles off course, and that the entire flight crew has been ejected into deep space by a maniacal computer. But when the flight attendant announces that they are out of coffee, mayhem erupts. Astronomers are like that, only worse. In a few days, there better be coffee, or you may be hearing news about roving mobs of caffeine-starved astronomers in Rio.

As I hear news from the meeting, I'll let you know. You can also follow news from the meeting on Twitter by following astronomer Mike Brown (plutokiller) or reading his blog. For a more formal news source, the IAU is publishing a daily newspaper called the Estrela D'Alva ("The Morning Star") available online with stories from the meeting.