Weekly Science Update

Wednesday, January 24, 2007
Watching a Supernova Explode
Supernovae, the explosive deaths of massive stars, disburse into space all of the chemical elements that were spawned inside the progenitor stars. Furthermore, these violent explosions themselves generate most of the elements above iron in the periodic table, and scatter them as well. Chemical enrichment alone makes supernovae immensely important contributors to the cosmic ecosystem. The class of supernova called "type Ia" offer yet another powerful advantage. These objects are produced when a dense, evolved star has a large companion star that gradually spills material onto its surface. Once the mass of the compact star exceeds a fixed, relatively well-defined limit, gravity triggers its explosive demise. Type Ia supernovae are thus thought to be "standard candles," and are used by astronomers to estimate the distances to remote galaxies whose supernovae appear faint because they are far away: they calibrate our cosmic distance scale.

Astronomers, precisely in order to get the best possible calibration, want to understand all the possible ways these supernova can erupt. Observers have naturally tended to discover and monitor supernovae near or after they have peaked in brightness, but models suggest that subtle, important differences between kinds of Type Ia progenitor stars are lost after their brightness peaks. A team of five CfA astronomers, Arti Garg, Chris Stubbs, Peter Challis, Michael Wood-Vasey, and Stephane Blondin, together with sixteen colleagues, have now measured the growing brightness of eleven Type Ia supernovae *before* they reached their intensity maxima.

The team has been imaging the Large Magellanic Cloud, a neighboring galaxy of ours, for five years, each season taking about twenty images of the same regions of the galaxy over a three month interval. Using techniques designed especially for discovering small intensity variations, the astronomers found they had data on eleven Type Ia supernovae before their intensities had peaked. These brightness curves show that it takes such an explosion about 17.6 days to reach a maximum brightness in the optical; previous estimates had thought this time frame was 21.1 days. The significance of the discrepancy to the calibration of distances is still uncertain, but what is clear is that the precise new data can help sort out competing models of Type Ia supernova explosions, and thus improve the reliability of the critical calibrations they provide.


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