Wednesday, March 12, 2008

A Death Star Pinwheel

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.

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.

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The Professor Astronomy Blog is based on work supported by the National Science Foundation under grant no. AST-0602288. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation (NSF).