Seeing stars explode in real time

Image Credit: NASA/Swift Science Team/Stefan Immler

When a star ends its life by exploding, it tends to be a while before we see anything on Earth (and this is ignoring the millions of years it takes light to get to Earth). Because we don't know in advance which star in the Universe is going to explode when (as there is no "Upcoming Attractions" posting on the Universe's blog, and psychics continually fail to warn us of these things), we tend to see explosions after the fact. But even if we knew when a star was going to explode, it would be hours after the actual explosion before we saw any light on Earth.

When a massive star nears the end of its life, its core engine (a nuclear fusion reactor) is busily fusing silicon into iron and nickel and cobalt. These elements have absolutely no energy value, so they form a lump of inert ash at the center of the star. When the lump gets big enough, the forces between atoms can no longer counteract gravity, and the core collapses into a neutron star or a black hole. Suddenly, the star finds it has no support in its middle, and the star begins to collapse inward. All of the inward falling material collides, causing a shock wave to go rushing outwards toward the surface at the star at speeds of 10,000 miles or more per second. This shock wave is also probably driven by energy from a stream of subatomic particles called "neutrinos" that are formed by the collapse of the core of the star. When the shock wave reaches the surface of the star, it breaks free in a blinding flash of X-rays and ultraviolet light, as the first energy from the star being ripped apart is released into the empty vacuum of space.

But even though the shock wave is going at these very high speeds, it can take the shock wave a long time to reach the edge of the star. The stars that go supernova can be almost a billion miles in diameter. A shock wave starting at the star's center can take 14 hours to reach the surface of such a star! So, for at least half a day after a star explodes, we on Earth have no clue (in the form of light) that the star has exploded.

Even after the explosion, it often takes days for us to notice anything on Earth. The supernova explosion gets brighter for several days as the shrapnel from the star expands outward, exposing more and more of the bright debris to view. Then the debris starts to cool, and the supernova begins to fade away (though radioactive decay from elements created in the explosion help to keep the star from completely fading away in a matter of days). On Earth, the supernova appears as a point of light that didn't used to be there, and someone has to be looking in the right direction to see it. Because of this, most supernovae are discovered only around the time that they reach their brightest point, which can be days after the explosion.

Yesterday, NASA announced that their Swift X-ray telescope had discovered the break-out flash of a supernova. The telescope was looking at a galaxy when a bright X-ray "flash" was observed. Since part of Swift's mission is to look for flashes of X-rays (most of which come from gamma-ray bursts), the telescope immediately alerts interested astronomers around the world that a flash has gone off. After the alert, many professional telescopes went and looked at the spot of the flash, and were able to catch some of the earliest light ever to come from a supernova explosion. The picture above shows the X-ray picture of the supernova (top) and the optical-light picture of the explosion (bottom).

This research is interesting, because it allows astronomers to explore some of the earliest stages of a supernova. It was also very lucky, because the telescope happened to be looking in the right place at the right time, and the typical galaxy only has a star explode every few decades or so. But is the discovery important (the press release calls this supernova the "Rosetta Stone" for understanding exploding stars)?

Probably this is not going to be a crucial piece of data in understanding supernova explosions. The X-ray flash was expected, and now it has been observed, which does confirm one part of the theory of exploding stars. But it is hard to see that we will learn anything new from a single event. The theory of exploding stars seems to be pretty solid, and what we tend to learn from the earliest stages of a supernova is mostly what the star's outer-most layers looked like, and we already study the outer layers of stars (it's what we see when we look at them). Yes, there are things to be learned, but these are almost certainly just details, not grand over-arching themes. But a cool and lucky find, nonetheless.

This discovery would be great for someone interested in the psychology of astronomical research. Since many different research groups got notification of the X-ray flash, they all scrambled to produce papers and get credit for the discovery. There are grumblings under the surface about the group getting credit in this press release, but there are also good arguments for why they got credit -- I don't know enough to have a well-informed opinion. And the personalities in competing groups are always continually clashing, so there are some, um, colorful opinions floating around. Anyone who claims that scientists are a completely dispassionate people are wrong, and this particular discovery is a great piece of evidence that human psychology plays a large role in science.

The Hype of the Baby Supernova

Image Credit: NASA/CXC/NCSU/S.Reynolds et al.; NSF/NRAO/VLA/Cambridge/D.Green et al.

Have you ever had a time that too much hype can take the fun out of something? Like the Superbowl, or these presidential primaries that are talked about for weeks as "crucial turning points," and the results, while important, are far from crucial or deciding?

Yesterday NASA had a press conference that they had hyped for a week. All they would say was that it would be an important discovery of something astronomers had been looking for for more than 50 years. Speculation was rampant, running from black holes to planets to dark matter to aliens. Even from my inside perspective, I had little information. All I knew was that it involved both X-ray and radio telescopes. The X-ray telescope involvement meant it had to be something with tremendous energy -- planets and aliens don't produce nearly enough X-rays for us to detect on Earth. What in the world could this amazing, earth-shattering discovery be?

When it was announced yesterday, I felt a bit let down. The discovery was of the youngest known remnant of a supernova (exploding star) in our Milky Way Galaxy. This is both interesting and important, but I don't know that it is the culmination of a 50-year hunt, and I don't see why they kept all the secrecy about the press announcement, since the paper announcing the discovery was posted to our preprint server (a place where astronomers can put papers for each other to read before the journal with the official paper comes out) on April 15, one entire month ago. Granted, we did the same thing with our press release a couple of weeks ago, but we never tried to hype up some secret new discovery. If anyone had asked, we would have told them. But, NASA seems to get its jollies from hype, even though the science alone is usually sufficient to pique the public's interest.

So, what is the science here, and why is it interesting (even without NASA's hype)? It stars with exploding stars. Supernovae, or the explosion of stars, are rare events. Various people have estimated how often a supernova should happen in the Milky Way, and the best guesses end up once every 50 to 100 years. But keep in mind that this is a long-term average; stars don't know when other stars explode, so sometimes you could have several supernovae go off in a short time frame, and other times you could get really long lags between supernovae.

The last supernova in the Milky Way Galaxy seen from Earth was in 1604. This explosion, called "Kepler's Supernova", was observed by famous astronomer Johannes Kepler, but was before the invention of the telescope! Since then, the skies have been dark, although we are all ready and waiting for a new supernova.

So, are we in one of the long lags that can sometimes happen? Maybe, but maybe not. 400 years is a long time, even given the rarity of supernovae. More likely is the fact that the Milky Way is full of dust that is great at blocking optical light. If a supernova were to happen behind one of the many dust clouds, it would be invisible in optical light on Earth, even though the supernova would outshine the entire galaxy! And since most of the stars in the Milky Way are located toward the center of our galaxy, which is hidden behind an amazingly thick layer of this dust, we would expect that many, if not most, supernovae in our galaxy would be invisible to human eyes.

We have some evidence that this dust blocking has happened in the past. One of the brightest sources of X-rays in the sky is called Cassiopeia A, or Cas A for short. In X-rays and radio waves, Cas A looks like a supernova remnant -- a lot of material is shooting out from a common center at a very high speed, just like shrapnel from any explosion would. When we look at pictures of Cas A taken several year apart, we can see the expanding cloud of shrapnel growing in size. If we run time backward and see when all of the material was in the same spot (in other words, the start of the explosion), we estimate the explosion was 300 years ago. But no explosion was seen from Earth at that time, in spite of there being many astronomers with telescopes around the world.

So, astronomers have been looking for other young supernova remnants. These would be bubbles of hot gas expanding at very high speeds. Since high-energy X-rays and certain radio waves can pierce through the dust in our galaxy, most searches fave used X-ray or radio telescopes. And by measuring the rate at which the bubble is expanding, we can again estimate the age of the supernova.

Which brings us to yesterday's press conference. One supernova remnant with the less-than-exciting name of G 1.9+0.3 was studied with both X-ray and radio telescopes (the picture at the top of this page is the combined X-ray and radio pictures of this object; click on the picture to go to the press page for more pictures and information), and the estimated age of the explosion is only about 100-150 years. In other words, the explosion probably happened around the time of the U.S. Civil War, if not even more recent. There have been many astronomers with big telescopes around, but nobody saw the explosion.

This is not surprising. The supernova remnant is located only about 1000 light-years from the center of our galaxy (whereas we live 25,000 light-years away), and is located in a region with lots of dust to block visible light. The explosion could easily have gone unnoticed by astronomers at the time.

But the search for young supernovae is not over. There may be other very young explosions waiting to be discovered by the same method. Our galaxy continues for tens of thousands of light-years on the other side of the Galactic Center, and all that part of the galaxy has been invisible to our eyes until the invention of radio and X-ray telescopes. And we need to find these young remnants if we want to be able to estimate how often stars explode, which in turn is important for understanding what kind of stars do the exploding!

Last, could we miss a supernova today? I think the answer is probably not, for two reasons. There are a lot of astronomers, especially amateur astronomers, who are constantly scanning the skies, looking for the faintest pinpoint of light that wasn't there the night before. Even with all of the dust in our Galaxy, these amateurs might be able to see a faint new star. And, in a decade or so, astronomers will have the LSST, a large telescope scanning the entire sky every few nights to look for new and changing stars.

Second, we also have full-time neutrino observatories. Neutrinos are elusive subatomic particles that are produced in large numbers by supernovae, and they are not stopped in the least by dust in our Galaxy. In 1987, a supernova in the Large Magellanic Cloud (one of our closest neighbors in space) exploded, and we detected several neutrinos from that explosion. An explosion in our galaxy will produce many more neutrinos that we will detect, and we will know that, somewhere, a star in our galaxy has just exploded. And every professional telescope in the world would start scanning the skies for that explosion for the opportunity to bring modern astronomical instruments to bear on what will be the most exciting astronomical event in decades!

Vanishing Stars

Last week, some colleagues of mine made some news at the annual Royal Astronomical Society National Meeting in Belfast, Ireland. This team of astronomers, led by Stephen Smartt of Queen's University in Belfast, has been using the Hubble Space Telescope to take pictures of supernovae (exploding stars) in nearby galaxies. They then use pictures of the galaxies taken before the stars exploded, either from the ground or from the Hubble, to look for the star that exploded. From those pictures, Smartt and his team have been able to learn about what types of stars explode.

They've found many stars between about 10 times the mass of the sun and 30 times the mass of the sun have exploded, and they also have many supernovae where no star was seen beforehand. That doesn't mean the explosion came from no where, it just means the star was too faint to see from the Earth. These stars probably were "only" seven to ten times the mass of the sun.

Even more interesting, they haven't yet seen stars bigger than about 30 times the mass of the sun explode. We know these stars exist, so why don't we see them exploding?

One idea, which many astronomers at the meeting believe, is that Smartt and his team haven't looked at enough exploding stars yet. Monster stars are rare; probably only one in every 10 stars big enough to explode are bigger than 30 times the mass of the sun. And while Smartt and his team have looked at a couple dozen supernovae now, it is possible that nature is playing some trick on us. As any gambler knows, sometimes you have cold streaks, even if the odds tell you that you have to win sometimes. If this is the case, Smartt and his team will eventually find a big star exploding.

The other idea is that stars more than 30 times the mass of the sun don't make supernovae, but they collapse on themselves to make a black hole. This is the theory that Smartt was proposing, and it is a theory I've heard since I began studying astronomy. The idea is that these giant stars start to explode, but their gravity is so strong that the explosion can't break free.

We know that some stars make black holes at the end of their lives, because we can detect the presence of the black holes. But just because not even light can escape a black hole doesn't mean that we couldn't have seen an explosion. When the black hole forms in the center of a dying star, most of the star is far outside the black hole -- these black holes are less than 30 miles across, and the dying star is millions, if not tens or hundreds of millions, of miles across.

If you've ever had a bathtub filled with what looks like still water, and you open the drain to let the water out, you know that the water will start swirling around the drain. The water will not just disappear down the drain. The same would happen if you try to drain a star down the "drain" of a a black hole; the remains of the star will start to swirl. And as it swirls, it heats up due to friction, to temperatures of billions of degrees. At these temperatures, all kinds of nuclear reactions happen, and the dying star will release copious amounts of energy. You would think that at least some of that energy would escape the star to be seen as light on Earth.

Well, maybe, and maybe not. There are all kinds of complex physics to worry about, and our computers just cannot yet give a fully believeable model. So, for now, astronomers disagree on whether a star can just wink out of existence and make a black hole with no outward sign. And it will probably take Smartt and his team many dozens more supernovae before he can convince astronomers that the most massive stars are indeed disappearing without exploding, even if our computers can't tell us why.

Gammay Ray Bursts, part 2

As I started describing yesterday, gamma ray bursts are mysterious flashes of gamma ray radiation from deep space. There are two types of gamma ray bursts: the short bursts, which last less than a second or so and have very energetic gamma rays, and the long bursts (up to a minute long) that have less energetic gamma rays.  Up until the late 1990s, there were a plethora of explanations for these gamma ray bursts, but not a lot of observational data.

The big change started in the late 1990s with the launch of  href="http://bepposax.gsfc.nasa.gov/bepposax">BeppoSAX, an Italian X-ray satellite.  This satellite could detect gamma ray bursts and then turn and point X-ray imagers at the burst.  This allowed the position of the burst to be determined to within a few arcminutes (about 10% of a degree in size), whereas before we only knew the positions of gamma ray bursts to within a few degrees, or an area of sky 30 times the size of the full moon!

Coordinates accurate to a few arcminutes are good enough to try looking for the source of the gamma ray bursts in visible light using giant telescopes (which typically can only see an area of a few arcminutes in size).  And, when astronomers started to do this, they detected optical light from the gamma ray bursts, but only the long gamma ray bursts.

Further study found that these gamma ray bursts were happening in distant galaxies most of the way across the Universe, and that these galaxies were almost always making new stars at tremendous rates.  This was a crucial find, as it tells us that the sources of gamma ray bursts come from young stars but not from old stars.  And there is one other astronomical explosion with the same characteristic: supernova explosions.  One clinching piece of evidence came in 1998, when a nearby gamma ray burst was discovered, and after the optical light from the burst faded, a supernova appeared in the same spot.

So, it seems that gamma ray bursts are linked to supernovae.  The best current idea is that the entire system starts out as a star tens of times more massive than the sun.  These giant stars burn up all their fuel in just a few million years, and end up with a core of iron more massive than our own sun.  The special thing about iron is that there is no way to get more energy out of iron by nuclear fusion -- it is the ultimate ash.  But it is the energy from nuclear fusion that keeps gravity from collapsing the star.  Without that pressure from energy, the star collapses in on itself, forming a black hole at the middle.

The outer parts of the star start to fall in on the black hole, but, like most stars, this one is probably rotating slowly.  And, just like a figure skater who can spin up to tremendous speeds by drawing in her arms, the slowly-rotating star speeds up to a tremendous rotation of gas falling into a black hole.  The black hole can't swallow all of this rotational energy, and it begins to spew material outward in narrow beams moving at nearly the speed of light.  These beams of particles burrow out of the star and run into gas and dust in the space surrounding the star, where the violent collisions produce copious amounts of light in the form of gamma rays, X-rays, and even optical light.

Meanwhile, what is left of the star continues to collapse under gravity, but the stream of particles from the very center of the collapse causes the implosion to "bounce" outward, ripping the star apart in a cataclysmic supernova explosion.  The supernova material moves much slower than the speed of light, so it appears to us on earth only a few days after the gamma ray flash.

This model does a nice job explaining everything we know about the long gamma ray bursts.  It's not a perfect model, and there are still many holes in our understanding, so it would not be surprising if the true details are quite a bit different.  But that is how astronomy often works -- theories are developed to explain an observed phenomenon, those theories make new predictions that can then be tested with new observations, and then the theory is either disproven or shown to be in need of some revision, and the cycle continues.  

In the meantime, we still have the ever-mysterious short gamma ray bursts (which may be giant star quakes on neutron stars, or colliding neutron stars, or merging black holes, or something even more exotic) to study, and even the long gamma ray bursts continue to surprise us with complexities we never imagined!

ECHO...Echo...echo

Image Credit: NASA/CXC/Rutgers/J.Warren, J.Hughes

We all have experienced sound echoes. Sound waves from an event (preferably loud and short) bounce off of distant walls and travel back to our ears. Because of the finite time it takes sound to travel, we can hear individual echos. Sound echos are used quite often by living beings -- The Navy and fishermen use sonar to find fish (or submarines) under water; bats and dolphins use echolocation to get their food.

Light also travels at a finite speed, though much, much faster than sound. Humans have learned to make use of "light echoes" for all sorts of clever things -- we call this radar (for radio waves) or lidar (for laser light).

There are some events in space that make nice, short bursts of light, such as supernova explosions or eruptions from the surface of a star. The neat thing (at least to me) is that we are now able to detect these "light echoes" from astronomical sources, as light from the event bounces off of dust or gas and toward the Earth.

A press release from the Chandra X-ray Observatory shows one cool example of a light echo detected in the Large Magellanic Cloud (LMC), one of the Milky Way's companion galaxies. About 400 years ago, a supernova exploded. The supernova was probably quite easy to see from the Earth, but I'm not aware of records from it. This wouldn't be too surprising, as the LMC is only visible south of the equator, and there just aren't a lot of written astronomy records from civilizations down under during that time.

Anyway, astronomers studying this supernova's remains (an X-ray picture of them is at the top of this blog post; I think it looks like a celestial pufferfish) also looked in optical (visible) light at the area surrounding the supernova, and they found an echo of light from the supernova itself! This page has movies showing the movement of the light echo over a period of five years (be warned, the movies are large. If they are too big for your internet connection, you can look at the individual pictures). Pretty neat!

What can we learn from the light echo? First, astronomers have been able to analyze the light from the echo and determine the type of supernova that made the explosion. There are two types of supernova explosions -- those that come from a massive star ending its life, and those that come from white dwarf stars. From the light echo, astronomers were able to confirm that the explosion was from a white dwarf. This knowledge helps us understand the X-ray light the Chandra observatory sees.

We can also use the light echos to get a three-dimensional picture of the dust and gas in the region surrounding the supernova. You can see the shape of the echo change slightly from picture to picture in the movie, which tells us that the gas and dust in the LMC galaxy are clumpy.

Perhaps astronomy's most famous light echo is from an eruption on the star V838 Monocerotis, shown in this video from the Hubble Space Telescope. Over a period of 4 years, the light from the eruption lit up swirling dust surrounding the star. While the light echo in the LMC pictures is not quite as dramatic, it serves the same purposes!

(Thanks to Jason Harris for pointing this story out)

A Death Star Pinwheel

Image Credit: Peter Tuthill and collaborators

Astronomers (or at least the press who cover space news) seem to like the phrase "death star." I think of several reasons for this: it reminds everyone of Star Wars, and plays a little on our inherent paranoia: everything in space is out to get us.

A few months ago, I blogged about the Death Star Galaxy. And now, we have a news story about the "Real" Death Star. No, Darth Vader is not out to get us (that we know of).

So, what is out to get us this time? The scientific research is about a pair of stars known as "Wolf Rayet 104." Wolf-Rayet stars started life dozens of times more massive than the sun; they are so bright that the outer layers of the star are spewed back into space. In just a few million years, the stars may have lost 75% of their matter this way, and they've burned through their entire supply of fuel. These stars are very close (within a few hundred thousand years) of a supernova explosion.

There are many reasons to think that Wolf Rayet stars may be one of the sources of mysterious "gamma ray bursts," very energetic jets of gamma rays that we can see in galaxies most of the way across the visible Universe. These jets of gamma rays are created as material falls into a black hole formed by the collapse of the star, which releases tremendous amounts of energy.

Wolf-Rayet 104 is a pair of these stars, and so are supernovae waiting to happen. The stars are about 8000 light years away in the constellation Sagittarius. They are invisible to the unaided eye, but are located just north of the spout of the teapot shape of Sagittarius. The movie above shows a real infrared picture of the star system. Astronomer Peter Tuthill of the University of Sydney and collaborators used the Keck Telescope in Hawaii to take pictures in infrared light. What you are seeing is dust thrown off in the strong winds from these stars; the orbits of the stars around each other cause the dust to twist into a spiral pattern.

Tuthill and collaborators have been working on this star system for years, and are using the stars to measure the amount of matter the stars are spewing into space and how the winds from each star are colliding, helping to produce the dust that they see. But they also noticed from the nearly-circular shape of the spiral that we are looking at the south pole of the star system. If, when this star explodes, it produces a gamma ray burst, the gamma rays are likely to come out of the stars' north and south poles. In other words, nearly straight at us.

If a nearby gamma ray burst were to happen, it is possible that the Earth would suffer. Gamma rays are really energetic, and while our atmosphere protects us from the gamma rays normally streaming through space, it might be overwhelmed from the large amounts of gamma rays a nearby gamma ray burst would produce. And that could be bad for the Earth and our ecosystem. Some of the dire forecasts some astronomers have made are mass extinctions, collapse of the food change, horrible mutations from the flux of radiation, and other very bad things.

I'm not too worried about this right now. First, the chances of these stars exploding anytime soon are small -- maybe 1 in 100,000 every year. Second, we don't know for certain if either of these stars will make a gamma ray burst and, if they do, whether it will be pointed exactly at us. When it comes to gamma rays, it has to be a direct hit to cause the damage -- a near miss doesn't count.

But even more, when you look at the fossil record, there are very few mass extinctions, and most of these have other explanations. The dinosaurs died when an asteroid hit the Earth. The Permian extinction, which killed 96% of species in the oceans and 70% of those on land, seems to be associated with massive outpourings of lava (although this is still quite disputed). And some biologists claim we are currently in a mass-extinction event driven by humans. There is little evidence for a mass extinction event caused by a gamma ray burst in the past (though maybe we have mis-interpreted the evidence).

I think we should definitely keep studying Wolf-Rayet 104, and I think there is a lot to learn about dying stars from this pair of stars. But I will not lose any sleep over the supposed danger these stars might pose to Earth.

Supernovae in the news

Image Credit: The Onion

It's difficult to find good science reporting in the news. By the time science news has been translated into a press release, digested by journalists who have some (but typically not extensive) science training, re-written into an article, and trimmed by editors with little or no science background, the resulting news often bears little resemblance to the initial annoucement, and the science content is greatly watered down. So, when an article about science comes out that almost manages to get facts straight, it is a joy to behold.

Yesterday, I was made aware of this article in the satirical newspaper The Onion. (Note to readers: although this article is fine, the Onion often contains material inappropriate for people under 18 years of age, so some of the links from this article are likely not for young eyes.) The article jokes about how NBA superstar Shaquille O'Neal has become scared to play for the Phoenix Suns ever since he learned that the Sun is a star, and that aging supergiant stars tend to end their lives in tremendous explosions.

The article then goes on to describe several types of supernova explosions, and the descriptions are somewhat, though not completely, accurate. Whether the author of the article has taken an astronomy course or whether the article simply used some clever Wikipedia and Google skills, I don't know. But we in the Astronomy Department found it quite funny (probably funnier than we should have).

Super-Asymptotic Giant Branch Stars

As I promised in my last post, here's an attempt to explain why in the world I would travel to London for a one-day visit. The challenge is to see if I can both explain the science and keep this post reasonably short.

First, we need to talk briefly about how stars work. Stars shine by nuclear fusion. They spend most of their lives turning hydrogen into helium. But eventually the hydrogen fuel runs low, and the star starts to burn helium into carbon. Almost all stars bigger than half the sun's mass go this far.

For stars up to about 7 times the mass of the sun, this is as far as the star gets. Once it has burned all of its helium, it ends its life, throwing off its outer layers as a planetary nebula and leaving behind a glowing lump of carbon ash called a white dwarf.

For stars more massive than about 10 times the sun's mass, the star is big enough to start fusing the carbon into oxygen, neon and magnesium, which then ignites and fuses into silicon and related elements, which then fuse to make iron and nickel and related atoms. And then the star explodes as a supernova star, spewing most of these elements out into the universe.

My story above, though, has a bit of a hole in it. I said that stars smaller than 7 times the mass of the sun make carbon white dwarfs, and stars bigger than 10 times the mass of the sun explode after fusing elements up to iron. But what happens to stars that are 8 or 9 times the mass of the sun? These stars are big enough to burn carbon into oxygen, neon and magnesium, but they are not big enough to fuse these elements into silicon and iron.

From the standpoint of theory, it is possible to make a white dwarf out of oxygen, neon and magnesium. It is also possible to have oxygen, neon and magnesium explode in a special kind of supernova explosion. But we don't know which of these scenarios happen.

Our meeting in London was therefore to discuss these stars. We talked about what we know about these stars between 7 and 10 times the mass of the sun, and what we don't know about these stars. Frankly, there are a lot of mysteries, and not a lot of answers. What happens to these stars depends a lot on how fast the stars are rotating, how quickly they are shedding their outer layers, and how much the inside of the star is mixed up by the slow boiling that happens inside many stars. But these mysteries are what makes the science interesting, and will keep me working for years to come!

I talked about the white dwarfs I have been studying, and a British group talked about their studies of the types of stars that explode as supernovae. If we combine our two areas of study, it seems that these stars must explode and not make white dwarfs. But there is a lot of careful study we need to do before blindly combining our work! I may make different assumptions than the other group which can affect the outcome of our data, or maybe one or both of us have made mistakes in our analysis. Only time will tell!

Crunching numbers

Image credit: Popular Science, Modern Mechanix

The trick to finding a needle in a haystack is not brute force (like Jim Moran above), but a clever sorting mechanism -- whether a metal detector, or some sifting machine, or another unique scheme.

In astronomy, some of the most interesting objects are some of the rarest. This is often because the interesting objects are those that are changing rapidly (at least in a cosmic sense). For example, stars like the sun are everywhere in the sky, because they live a middle-age life for ten billion years, nearly the entire age of the Universe. But stars ending their lives, making beautiful planetary nebulae, are very rare -- the phase only lasts ten thousand years, a blink in the cosmic eye. So, compared to stars like the sun, planetary nebulae are rare. Another rare object is a supernova, or exploding star. These are only visible for a year or so before fading from sight, and the stars that make them are rare. So, in our skies right now, there are no supernovae in the Milky Way galaxy. We have to look at distant galaxies to find supernovae.

But planetary nebulae and supernovae are pretty easy to find, relatively speaking. Planetary nebulae are big and glow in very specific colors of light, so you can design a search to take pictures of the sky in those colors, and you'll find lots of planetaries. Supernovae are very bright, and so can be seen far away. So, we just look at more and more distant galaxies until we see a supernova -- in essence, we are searching hundreds of billions of stars at once, looking for a "new", bright star in a galaxy. Both of these are clever ways of searching through the haystack of the sky for that elusive needle.

I am part of a group at the University of Texas Astronomy Department that is looking for a specific kind of white dwarf star in a specific patch of the sky. White dwarfs are faint, and bright ones are rare (because they have to be close by). And the patch of sky we are looking in is large, by astronomy standards -- about the size of both of your hands held at arms' length. A typical astronomical camera can only image part of the sky as big as a part of your pinky finger's fingernail. Out of the hundreds of thousands of stars in that patch of the sky, we expect to find about ten of these white dwarfs. So, how can we find them?

The brute force method, like searching each strand of hay for the needle, is long and complex. We would have to take pictures of the entire region of the sky (dozens of nights on telescopes available to us). Then we would analyze the images and look for stars that have just the right colors. Then we go back to the telescope and take spectra of all of those stars, splitting the light up into its component colors, and determining what each one is. We then would have to analyze each spectrum. And, lastly, we have to return to the telescope to double-check each of our best candidates. So, we're talking a few weeks (at least!) on the telescope, and hundreds of person-hours to find ten objects.

Thankfully, there is an easier way -- an automated sorting machine known as the Sloan Digital Sky Survey. The Sloan survey has taken images of one-quarter of the sky, and automated software analyzed each object. Interesting objects are targeted for follow-up work, such as the spectra, which automated routines then analyze. As of right now, the Sloan database has image analysis of 280 million stars and galaxies, and spectra of 1.2 million of those objects.

So, yesterday afternoon I sat at a coffee shop and used the Sloan database query tools to find a dozen interesting white dwarfs in our patch of sky. A grand total of less than one person-hour went into my search. Granted, Sloan has used hundreds of nights of telescope time and countless thousands of person-hours to create and maintain the database. But that enormous undertaking allows small teams such as the team I'm part of to do searches that used to be prohibitively time-intensive. Computers and clever software allow a single person to search through 280 million objects for the dozen he or she is interested in, and those searches only take minutes, not years.

So, I found the needles I was looking for in the haystack of the sky, all thanks to computers and a giant team dedicated to making this possible.

Groping about in the dark

Image Credit: photo.net

Today, I'll finish up my discussion on some of the science presented at last week's meeting of the American Astronomical Society by looking at the biggest scale we can -- the entire Universe.

A lot of people, both astronomers and non-astronomers, are very interested in dark matter and dark energy. What are they? Where do they come from? Do they even exist?

These questions are easier to answer for dark matter than for dark energy. Certainly dark matter is fairly convincing. When we look at galaxies and clusters of galaxies, we can see that the pull of gravity is stronger than we expect based on the "normal" matter there. And emerging theories of physics predict dark matter particles that have properties that mimic what we surmise dark matter should act like. But these particles haven't been proven to exist yet, so they remain a very compelling hypothesis, not proven particles.

Dark energy is a little more subtle to explain the evidence for it. Most important is to realize that there are at least two independent lines of evidence for dark energy. The first method involves using what is known as a "Standard Candle," or something that we think we know exactly how bright it is. For dark energy, we use a certain type of supernova that always appears to be about the same brightness. We look and see how faint the supernova appears, we know how bright it actually is, and we use geometry to get a distance. Then we measure how fast the supernova is moving away from us due to the expansion of the Universe. And what we find is that the expansion of the universe is speeding up, when gravity should be causing the expansion of the universe to slow down. Since some form of energy has to be speeding the Universe up, we call that "dark energy."

The second piece of evidence for dark energy comes from a satellite called WMAP. WMAP explored the echoes of the Big Bang visible on the sky, and determined that the total amount of energy in the Universe. We add up all the energy we know about (light, visible matter, and dark matter -- remember, Einstein told us that matter is energy!), and we only get about 30% of the total energy measured by WMAP. The remaining 70% is called "dark energy."

Let me admit here that the distinct possibility exists that both dark matter and dark energy are related, and maybe they just indicate that we are missing some fundamental understanding of physics in the Universe. Dark matter and dark energy make up about 95% of the Universe, and we have very little clue what it is! So maybe our theories about gravity are incomplete. However, Einstein's General Relativity, which explains how gravity works, has worked so well in every experiment designed to test it, that most astronomers are not about to throw it out yet.

So, let's assume that our theories of gravity are correct. What could dark energy be? First, maybe it is a mistake. If the brightness of the supernovae we are looking at changes over the age of the Universe, then the supernovae are not "standard candles," and the measurements we make for them would naturally give us the wrong answer. This is where it is important that we have a second indicator or dark energy, the WMAP satellite. Those data are much harder (but not impossible) to misinterpret. Since both the satellite and supernovae give us the same answer, it seems likely that this is not a mistake.

So, many smart theorist astronomers have developed some ideas as to what dark energy could be. These theories explain all of our observations so far, but they make differing predictions about dark energy that better observations can test.

But there is one alternative theorists don't like, and that is Einstein's Cosmological Constant. The Cosmological Constant is a value that Einstein put into his General Relativity. Early on, Einstein saw that General Relativity predicted the Universe had to either be growing or shrinking. At the time, we had no evidence that it was doing either. The math of general relativity allowed Einstein to put the constant in, so he did. Later, when Einstein learned from astronomers that the Universe was indeed expanding, he set the constant equal to zero. But the cosmological constant, if it is not equal to zero, allows the universe to expand faster and faster, just like dark energy.

Astronomers don't like the cosmological constant, because it has no explanation -- it's just a number allowed by math. Let's suppose I ask you, "How long will it take a car to drive to El Paso if the car is moving at 60 miles per hour?" The answer depends on how far away from El Paso the car is. Now suppose I tell you that the car is 60 miles away. Simple! The answer is 1 hour!

But, then you can ask, why is the car 60 miles from El Paso? Did it just magically appear there? Is there an auto factory 60 miles from El Paso? Does the owner live 60 miles from El Paso? Or is the owner a vacationer from St. Louis, who just happens to be driving west at the time? For the purposes of math, these are silly questions. But if you truly want to understand all that there is to know about the car, these are important questions!

It is the same with the Universe and the Cosmological Constant. The Cosmological Constant is like a starting point. The value of the starting point affects the answers we get from doing math problems involving general relativity. But the math doesn't care where the value came from, it just wants to know what the value is.

For the astronomer and the physicist, that is not good enough. We want to know why the cosmological constant has the value that it does. Is it a fluke of nature? Is there some underlying process that we don't understand that sets the value? Does the value change over time?

So, back to the American Astronomical Society meeting. A few groups presented research trying to make detailed observations of "dark energy" and to determine if one of the existing theories was better than another, or if dark energy continues to act just like a cosmological constant. And the answer is, dark energy acts like the cosmological constant. Now, we can still make better measurements -- if dark energy is mostly like a cosmological constant, but slightly different, we can't yet measure that. But it makes a world of difference in understanding dark energy if the expanding universe acts just mostly like the cosmological constant, or exactly like a cosmological constant. And, if the answer is the latter, we have a lot of tough philosophical questions ahead.

In the meantime, though, astronomers are happy to say that we need to do better measurements first. And this means that you can all keep wondering, "What is Dark Energy? Is Dark Energy Real?" And we'll just smile, shrug our shoulders, and say, "Let me get back to you on that."

CSI: Universe --- Who set off that explosion?

Image credit: Jon Morse (University of Colorado) and NASA

In 2004, astronomers reported a possible supernova (the explosion of a dying star) in the galaxy UGC 4904, a barred spiral galaxy about 75 million light-years away. However, the apparent explosion was awfully faint for a supernova, and it faded away too quickly for a supernova. And so, the "transient" (as such events are called) was forgotten.

On the night of October 9, 2006, amateur astronomers in Japan detected another transient in the same galaxy, a transient that was confirmed as a supernova several nights later. Not only that, but the supernova seemed to be coming from the same part of the galaxy as the first transient. What was going on? And just recently, European astronomers were able to do a careful alignment of the images of both events, which confirms that they are coming from the same spot. What's going on?

Very massive stars, those nearly 100 times the mass of the sun, live short and violent lives. These stars live their lives on the brink between gravity holding the star together and the radiation from the nuclear reactions at the star's center ripping the star apart. Sometimes these very massive stars become unstable, and can rapidly lose large amounts of material, many times our sun's mass. As that material flies off in a massive eruption, the star can get significantly brighter -- just like the first transient in UGC 4904. Typically, these stars seem to settle back down after the eruption, just like a little burp can make you feel better after you've eaten too much. And we think (or thought) that these stars would go on to live for another 200,000 years or more.

But the supernova throws that into question. Did the same star that erupted a few years ago then go supernova, meaning it had used up all of its nuclear fuel much faster than astronomers thought?

Maybe, maybe not. Massive stars tend to be born in clusters of stars, with many other very massive stars around. And many of these massive stars have companion stars in tight orbits. Because we don't have pictures of this galaxy taken with the Hubble Telescope, we can't see individual stars in this galaxy, so we can't know if the eruption of the star in 2004 and the explosion of a star in 2006 came from the same star. Based on the coincidence and the close timing, it would make sense that they are related. But maybe this just was two separate stars, and the timing was a coincidence.

If eruptions of material from massive stars often results in a supernova shortly thereafter, we should see this occurrence more often. It's only been in the last decade that astronomers have been diligently searching nearby galaxies for supernovae, so more time is needed before the book can be closed on this case.

But maybe we don't have to look too far away. In the southern hemisphere, the star Eta Carina is a massive star, 120 times the mass of the sun. In 1843, the star temporarily became the brightest star in our sky after the sun, despite being 8000 light-years away. Then the star rapidly became fainter than the human eye can see, and it has slowly gotten a little brighter since. This is thought to be the same type of eruption that was seen in UGC 4904 in 2004. The picture above shows a Hubble Space Telescope picture of Eta Carina -- the star is buried in the middle of two giant, expanding bubbles of material. Those bubbles were probably created in the eruption 160 years ago. So, will Eta Carina go supernova soon? 160 years seems a lot longer than 2 years, but in astronomical terms, they are both almost instantaneous. But maybe we will have to wait 200,000 years to see Eta Carina explode. If Eta Carina were to explode in the next several decades, then we would have to re-think these massive eruptions.

The brightest supernova ever?

Photo credit: Lick/UC Berkeley/J.Bloom & C.Hansen

This morning, there are dozens of news stories on the internet about what is being called the brightest supernova ever seen. To be honest, I was a bit surprised about the hubbub, as the details of this supernova were published in December. Although the basic idea behind this explosion is relatively unchanged, new evidence has come in that supports the hypothesis, and, as always, NASA is hungry for press releases.

So, what's the big deal? The explosion we are seeing is the death of a very massive star. Typically, when a massive star dies, it has run out of fuel in its core, having burned all of its hydrogen gas through successive stages into iron. The iron tries to burn in a nuclear reaction, but that absorbs energy instead of releasing it, and the center of the star collapses into a neutron star or a black hole. The rebound from this explosion rips the star apart.

But that doesn't seem to be the case for Supernova 2006gy (the name astronomers use for this supernova). The supernova has stayed very bright far longer than expected for this type of supernova. The only explanation we can think of is a strange physics phenomenon called "pair instability." Pretty much, that means that the center of the star gets so amazingly hot that the light radiation in the core of the star (in the form of gamma rays) becomes so energetic that the gamma rays are converted into matter and antimatter. This is a direct result of Einstein's equation E=mc2, which means that energy can turn into matter and vice versa. It takes some special circumstances (very high temperatures and densities), but in these extreme stars, those conditions are met. And, as you may know, when the antimatter hits normal matter, it annihilates and releases energy, which then is turned back into matter and antimatter again because of the extreme conditions. This circumstance is unstable, and the entire stare is ripped apart in a huge explosion. We don't know if a black hole might be left behind or not, but most of the star (which is 120 times the mass of the sun or more!) is blasted into space.

This supernova mechanism has been proposed before, but astronomers didn't expect to see it. In most galaxies these days, the giant stars are polluted by lots of metals made in previous generations of stars. Before the star finishes its life cycle, these metals act almost like a sail, and light from the star pushes the metals away from the star. When the metals leave, they take a lot of hydrogen and helium with them. In this way, a star that started life with a mass 100 times that of the sun is whittled down to fewer than 10 times the mass of the sun in just a few million years. The resulting star still explodes, but in the "normal" way.

So, if supernova 2006gy is a pair instability supernova, astronomers will need to ask how such a thing could happen in the modern Universe. Can some monster stars somehow manage to hold on to their envelopes? Or did this star form in a galaxy or a part of a galaxy where there were far fewer metals than normal? Or have we mis-interpreted the data so far? These questions will keep supernova astronomers occupied for years to come!

How things go boom in the night

Image credit: F. Roepke & W. Hillebrandt

In the past few days, I've talked about the controversy of what types of stars explode as Type Ia supernovae. But the mystery surrounding these explosions is not limited to what explodes, but also how.

When a white dwarf explodes, it is undergoing an uncontrolled nuclear reaction, turning carbon and oxygen into silicon, iron, nickel, cobalt, and many other elements. In a matter of a second or so, a sun's worth of carbon and oxygen is burned. But there is a question as to how it burns. Burning comes in two flavors. In a deflagration, or what you would call a normal flame, a flame moves more slowly than the sound speed. A good example is a wildfire, which can race across the ground at 60+ miles per hour (more than 100 km/hr), but even seemingly explosive burning, like gunpowder or an automobile engine, is still just a deflagration.

In a detonation, the flame moves faster than the speed of sound. Most high explosives do this sort of explosion. Detonations are more destructive than deflagrations, which is why plastic explosives are used in modern warfare instead of kegs of black powder.

In white dwarfs, the speed of sound is very high. In Earth's atmosphere, it is around 770 mi/hr at Earth's surface. Sound moves faster through dense material. For example, the average speed of sound through the Earth (as measured by earthquakes) is about five miles per second, or 18,000 miles per hour! Even so, this means it can take nearly half an hour for a sound wave to go through the Earth. White dwarfs, on the other hand, are much denser yet. Although the size of the Earth, sound waves can travel all the way through a white dwarf in less than two seconds at a whopping speed of over 14 million miles per hour! So, a white dwarf can completely burn in a matter of a couple of seconds and still not be a detonation.

How can we tell how a white dwarf burns? By what it leaves behind. In a deflagration, the white dwarf burns pretty thoroughly, leaving behind almost no carbon. The same is true of deflagrations on the Earth -- in your car's engine, you burn all of the gas you put into the piston. In a gun, you burn almost all of the gunpowder. Otherwise, you are just wasting fuel! In a detonation, pieces of unburned material are sent flying. So, for a white dwarf, we would expect to see lots of carbon and oxygen left if it detonates.

When we look at the Type Ia supernovae, we usually see a little bit of carbon and oxygen -- not much, but some. Neither the detonation or deflagration models really work! The truth seems to lie somewhere in between -- most of the white dwarf burns "gently," but somehow, somewhere a detonation explosion occurs, adding some extra energy and keeping all of the carbon from burning.

The problem is, it really is difficult for theorists to explain why one type of burning would turn into another type. So, at meetings such as the one last week, scientists argue very heatedly about their latest models. This isn't work that I do, so I am not qualified to make a guess as to which models are more likely to be correct.

The picture above is a model of a white dwarf deflagrating ("gently burning"). The yellow-orange is the unburned white dwarf, while the grey/white indicates where the flame has already burned the white dwarf. You can see that, as the burning continues, the heat causes the white dwarf to get larger. Soon after the phases pictured above, the flame reaches the surface, and the ashes are spewed into space at a speed of 10,000 miles per second!

Things that go boom in the night, part 3

Image credit: NASA/Dana Berry, Sky Works Digital

This update is a few days later than promised; I happened to get involved in some discussions during breaks in our conference, and by the evening, I was too tired to go online. So, although the conference is over, I'll continue to try to bring things up to date.

So, remember that we were talking about Type Ia supernovae, some of the most intriguing explosions in the Universe. There are two suspects for objects that form these particular explosions. The first, discussed last time, involves a white dwarf pulling mass from a companion star until it reaches the all-critical "Chandrasekhar mass," about 40% larger than the sun's mass, at which the white dwarf would undergo a thermonuclear explosion. Today we talk about the second mechanism, the double degenerate model, so called because it involves two white dwarfs, and white dwarfs are also known as "degenerate stars," because when you pack the mass of the sun into something the size of the Earth, the matter enters a state called degeneracy.

The way the double degenerate model works is fairly simple. If you have two white dwarfs orbiting each other, over time they will get closer and closer. This is because under Einstein's Theory of General Relativity, the objects radiate "gravitational waves," which carry energy away from the system. When the white dwarfs get close enough, they will merge together. If their total mass is over the Chandrasekhar mass, the combined white dwarf will explode!

Of course, the devil is in the details. As the white dwarfs get close, if one is more massive than the other, the bigger one will rip the smaller one to shreds. We'd get the white dwarf version of Saturn, in which a white dwarf (with the mass of the sun squeezed into a ball the size of the Earth) would be surrounded by rings with a mass of half the sun or more, still in that weird state of degeneracy. This is really hard to study from theory, and we certainly have never observed a star in this state. Maybe the remnants of the second star will fall onto the bigger star quickly, which would cause an explosion. But if they don't fall on quickly enough, the system may cause big winds that will blow the smaller star's remnants away.

Perhaps a bigger problem is that we only know of a few white dwarfs in double degenerate systems, and only one of these is above the magic "Chandrasekhar limit." For these to cause Type Ia supernovae, there should be dozens in our Milky Way that we should be able to see. Maybe we are there and haven't found them yet. But maybe these aren't the parents of Type Ia supernovae.

So, in short, we really don't know what causes Type Ia supernovae. This is really disturbing, since we use these explosions to study the distant Universe. But it is a mystery I think we can solve, given some more time.

Over the next few days, I'll summarize a few other controversies from the conference that are quite interesting.

Things that go boom in the night part 2

Image credit: NASA/STSci

Yesterday I mentioned that I am at a conference discussing exploding stars known as "Type Ia Supernovae." I also mentioned that there are competing hypotheses about what causes these explosions. Today we'll briefly mention one of these: the so-called "single-degenerate" channel.

A degenerate star is a star made out of a type of matter that is packed so densely that it cannot be packed tighter. It is like taking the sun (which is nearly a million miles across) and squeezing it into a ball the size of the Earth. We also call such a star a "white dwarf," because it is small and glowing white-hot.

The vast majority of white dwarfs slowly fade and cool away. Yet if a white dwarf gets heavy enough, the material it is made of is squeezed until it stars a nuclear reaction that, in a matter of a few seconds, burns the entire star and causes a supernova explosion. The magic mass limit is known as the "Chandrasekhar mass" after famous astrophysicist Subrahmanyan Chandrasekhar, who calculated this magic level.

If a white dwarf is not at the Chandrasekhar mass, it will have to gain matter until it reaches the Chandrasekhar mass in order to cause a supernova. One way to gain matter would be if the white dwarf happened to be close to another star, the white dwarf's gravity could pull gas from the other star onto the white dwarf.

We see white dwarfs pulling matter from nearby companions. These are called "cataclysmic variables." Dozens of these systems are known. But what we don't know is if these cataclysmic variables will end up sending enough matter to the white dwarf to cause the white dwarf to explode. The reasons for this uncertainty involve very complex physics that we can only partially model on computers. So, we don't know if any of these systems can ever make a Type Ia supernova. Many astronomers at this conference believe this is the best way to make the supernovae, but others disagree. Tomorrow I'll discuss the other competing theory for making Type Ia supernovae.