Cassiopeia A supernova remnant based on data compiled from Spitzer, Hubble, and Chandra observatories in 2008. Credit: NASA
About 220 million years ago, deep within the galaxy NGC 266, a blue hypergiant star exploded. When the light from that supernova reached Earth in 2005, astronomers began piecing together something of a stellar mystery: a star that died before its time.
The key piece of evidence in this mystery came from astronomers Avishay Gal-Yam, of Israel’s Weizmann Institute, and Douglas Leonard, of San Diego State University. Publishing in Nature on March 22, their findings include the second-ever set of images that conclusively link a supernova with its progenitor, the star it used to be.
Gal-Yam and Leonard established this link by comparing historical images of the galaxy, taken by Hubble in 1997, with those taken in the same location 10 years later. Noting the absence of a single star is harder than it sounds. It was only because the progenitor was in one of the brightest, hottest, and largest classes of stars — luminous blue variables — that scientists could distinguish it from its neighbors and pick it out from the objects in the intervening millions of light-years between it and Hubble.
But the star’s luminosity is one of the reasons its death was so unusual. LBVs are older stars, but they still have large stores of hydrogen to burn. That this one exploded a hundred thousand years ”too early” calls into question the standard model surrounding supernovae.
The conventional thinking on what causes stars to explode at the end of their lives focuses on the collapse of their cores. Aging stars that are more than 50 times the size of the Sun gradually accumulate heavy elements, such as nickel and iron, as they run out of the hydrogen and helium that power their cores’ continuous nuclear fusion. When that fuel is gone, the cores can no longer support the weight of the star’s outer layers. The cores then collapse into small, gravitationally intense objects — generally, a neutron star or a black hole — that should suck the outer layers of the star in.
But it’s still unclear what mechanism can reconcile those crushing implosions with the dazzling explosions we see. “If you put that theory to the test, if you plug it into a computer and run it with all of the laws of physics, the star doesn’t actually explode,” says Leonard. “There’s a problem with the theory and everyone knows it.”
Avi Loeb, an astronomer at Harvard University, agrees, saying that more investigations like Gal-Yam and Leonard’s are the way to begin refining this theory. “We need to find many more of these progenitors to determine the conditions where you get implosions versus explosions,” he says.
Leonard believes that older supernovae will start generating a renewed interest as astronomers begin more closely scrutinizing before-and-after images from Hubble’s historical data. But whether a new theory of supernovae will emerge from this mystery is an open question.
“The universe keeps us humble,” says Leonard. “You can work out all kinds of theory on paper, but it keeps showing you something that makes you have to go back to the drawing board.”
Originally published March 27, 2009
