Thursday, May 22, 2008

Seeing stars explode in real time

The SWIFT X-ray Telescope detects a flash of X-rays from an exploding star
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.

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