Friday, June 15, 2007

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.

No comments:

Post a Comment