Through a stroke of luck, astronomers have identified the star that exploded as a distant supernova last spring. The discovery beautifully confirms the prevailing theory that the most common type of supernova in the universe is produced by the death throes of cool, red supergiant stars. However, the newly discovered star is surprisingly lightweight.
The only way to check theories about supernovas is to find out what went kaboom. Astronomers try to do this by examining old photographs for stars at the exact position of the supernova. That's been impossible in our own Milky Way galaxy, which hasn't seen a supernova for centuries. And the progenitor stars of more distant supernovas are barely visible on older photographs. Compounding the frustration, the only two supernovas for which progenitors have been found so far were oddballs.
In contrast, supernova 2003gd was a very common type II explosion, believed to result from the core collapse of a massive star that runs out of nuclear fuel. It was discovered on 12 June in a spiral galaxy known as M74, which is some 30 million light-years away in the constellation Pisces. Now, a team led by Stephen Smartt of the University of Cambridge, U.K., has identified the precursor star of this supernova on older photographs.
The team found that--by sheer coincidence--M74 was imaged in crisp detail less than a year before the explosion, both by the Hubble Space Telescope and the Gemini North telescope on Mauna Kea, Hawaii. As they report in the 23 January issue of Science, the progenitor star of SN 2003gd turns out to be an orange-red supergiant star, about eight times more massive and 20,000 times more luminous than our sun. The authors write that the discovery "directly confirms stellar evolutionary models."
According to supernova expert J. Craig Wheeler of the University of Texas, Austin, "the most interesting thing about this work is that this classic supernova explosion is of very modest mass." As Smartt and his colleagues note in their paper, the behavior of a dozen other type II supernovas hint at much more massive progenitor stars. That makes the low mass of the current find surprising, says Wheeler, but it shouldn't create serious problems for theories of supernova formation.