A supernova marks the explosive death of a massive star. The cataclysm disburses into space all of the chemical elements that were spawned inside the progenitor star, making supernovae the primary contributors to enriching the cosmic ecosystem. Astronomers are keenly interested in supernovae for another reason as well -- they can be used to calibrate the cosmic distance scale. Because some are so bright, they can be seen at cosmological distances, while at the same time they are thought to be so well understood that they can be treated as "standard candles" whose intrinsic luminosity is known. Scientists can then estimate the distances to remote galaxies from the apparent brightness of their supernovae, which appear faint only because they are far away.
There are two basic kinds of supernovae reflecting the two basic ways a star can grow massive enough to explode in the final stages of its life. In the first case, the star is simply massive from birth. The second situation is more complex, and results when a star orbits with a companion star whose outer layers gradually shed material in a wind. If enough of this material accumulates onto the first star, it can become large enough to explode as a supernova. The second class is the one that astronomers regularly use as standard candles because, independent of its previous history, each star is thought to explode when it accumulates enough material to exceed a fixed, relatively well-defined limit. The first type of supernova shows evidence for hydrogen in its spectrum; the second shows no evidence for hydrogen.
CfA astronomers Stephane Blondin and Robert Kirshner were part of a team of seven astronomers led by CfA graduate student Malcolm Hicken that has identified a new kind of supernova -- one that was not the result of either of these other basic processes, but of two stars colliding. The stars formed a binary pair that orbited each other, but in this peculiar case they were very similar in mass and both had reached a similar advanced age. As they orbited, they gradually spiraled inward until they merged, touching off the titanic explosion. The new CfA observations provide the strongest evidence yet of what was, until now, a purely theoretical mechanism for creating a supernova.
The object, known as "SN 2006gz," had originally been considered to be a member of the second basic class of supernovae -- the standard candles -- due to its lack of hydrogen emission, among other characteristics. However, an analysis using new CfA data suggested that SN 2006gz was unusual and deserved a closer look. The team began observing the supernova from the Fred L. Whipple Observatory's 1.2m telescope starting three days after its discovery, and continuing for the next 22 nights. The object showed the strongest spectral signature of unburned carbon ever seen, as well as the presence of silicon. Thirdly, SN 2006gz was brighter than expected, suggesting that its progenitor exceeded the usual mass for exploding. The scientists conclude that all three peculiarities can be explained most easily as the result of the merging of two aged stars. In addition to providing the first example of this new way to make supernovae, the new result holds implications for the field of cosmology because it suggests that hydrogen deficient supernova may not be quite as "standard" as was once thought, and additional care needs to be taken to classify them before using them for distance estimates.
Three Top Young CfA Astronomers Honored
Three CfA astronomers have been recognized for their innovative work by three leading national magazines. David Charbonneau was named “Scientist of the Year” by Discover magazine for his contributions to our understanding of the worlds beyond the one we inhabit. Gàspàr Bakos was designated one of the year’s “Brilliant Ten” by Popular Science magazine, for creating the Hungarian-made Automatic Telescope network, or HATnet, to find planets orbiting distant stars on a shoestring budget using telephoto lenses and powerful computer software. Lisa Kaltenegger was honored as one of Smithsonian magazine’s “America’s Young Innovators in the Arts and Sciences under 36” for her work modeling the spectra of hypothetical planets orbiting distant stars.