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!

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)

Keeping warm

Image credit: NASA/CXC/IoA/A.Fabian et al.

Yesterday was a day full of meetings, most of them useful but boring. But one was our weekly colloquium, where an astronomer comes in (usually from outside the University of Texas) and talks about his or her research work, and it was quite interesting. It involves a problem that not too many people outside of astronomy have heard of, but is a big mystery -- the mystery of "cooling flows."

Cooling flows exist in large clusters of galaxies, such as the Coma Cluster. In between the galaxies in galaxy clusters, a very hot gas, tens of millions of degrees, permeates the otherwise empty space. This gas is invisible to human eyes, but it radiates a lot of light in the X-rays.

Like anything hot, the gas in galaxy clusters is cooling off. In some clusters, it is cooling so quickly that we should be able to see it changing from hot gas to cool gas and into stars, which should cause the galaxy at the center of these clusters to form stars at a rate a hundred times that of the Milky Way. But the galaxies in these clusters don't show new stars -- they are all very old.

The only explanation for this is that something is heating the gas up again before it gets too cool. But what?

Most people think it has to be a giant black hole, billions of times the mass of the sun, at the center of the galaxy cluster. These black holes are known to exist, and they often shoot giant plumes of gas and particles a million light-years or more into deep space. These black holes should produce enough energy to heat the cooling X-ray gas.

A few years ago, the Chandra X-ray Observatory found giant bubbles blown in the X-ray gas by the central black hole. The picture at the top of this post is one such galaxy cluster, and you can see several bubbles. The thought is that these bubbles, which contain a lot of energy, dissolve (or "pop") and release all of that energy into gas, perhaps as heat energy, or perhaps as sound waves.

The talk yesterday by astronomer David De Young was technical, but it dealt with the physics behind these bubbles. And, strangely, the physics seems to show that these big bubbles don't want to pop -- they want to stick together and slowly float off out of the galaxy cluster, kind of like a blob in a giant lava lamp. And if the bubbles in galaxy clusters do that, they don't transfer their energy to the X-ray gas. And so the mystery remains as to why the gas stays hot.

Maybe the physics used to explain these bubbles isn't complete -- we don't yet have the computing power to determine what happens in these bubbles over very long times, so the physicists have to make simplifying assumptions. Perhaps the black holes also emit energy in forms other than the giant bubbles, and that energy is what does the heating. We really don't know. And so, the mystery surrounding some of the most energetic things in the Universe -- giant black holes at the centers of giant clusters of galaxies, remains.