BY KEITH COOPER
Posted: 20 May, 2009
A surprisingly simple method has been uncovered that allows astronomers to use supernovae to measure distances in the Universe to a far greater degree of accuracy than ever before, in just a single night.
Type Ia supernovae explode when a white dwarf star accretes enough matter onto its surface from a companion star that it kickstarts a runaway thermonuclear reaction.
Now analysis of 58 type Ia supernovae with full spectra collected by the Nearby Supernova Factory (SNfactory) has found an astonishing new technique for doing this. SNfactory member Stephen Bailey, of the Laboratory of Nuclear and High Energy Physics in Paris, discovered a ratio linking the power flux, or visible brightness, of the supernovae between wavelengths of 642 nanometres and 443 nanometres. By measuring this ratio it becomes possible to calculate the absolute magnitude of a supernova explosion, and by comparing that with its apparent magnitude you can then work out the distance. This can all be done with just a single night’s observations, it works for all type Ia supernovae independent of the properties of their host galaxy or any intervening dust that might redden their light, and is accurate to within six percent (the light curve method was accurate to within ten percent).
The bright colours on this graph indicate brightness ratios at specific wavelengths in nanometres. Image: SNfactory.
The physical mechanism that produces this ratio in the supernova’s light is still a complete mystery. “Astronomers have looked for spectral features that could be used to correct observed magnitudes before, but their searches tended to concentrate on a known physical feature such as a silicon or sulphur line,” says Bailey. “I decided not to make any physical assumptions about the SNfactory dataset but just see what the spectra could tell me by themselves.”
Other spectroscopic ratios yielded similar results, but the 642/443 ratio was the strongest. “While the luminosity of a type Ia supernova depends on its physical features, it also depends on intervening dust [which can dim the light from the supernova]. The ratio somehow aligns those two factors, and it is not the only ratio that does. It’s as if the supernova were telling us how to measure it,” comments Rollin Thomas, a member of the SNfactory from the Lawrence Berkeley National Laboratory.
The improved distance measurements that will come as a result of this will help better measure the expansion of the Universe and the effect of dark energy on that expansion, which was discovered in 1998 whilst using type Ia supernovae to measure distances to galaxies.
You can read the original paper describing the research, which is to be published in the journal Astronomy and Astrophysics, here.
Dark energy was discovered using type Ia supernovae, such as this one ten billion light years away and indicated by the arrow, to measure distances to faraway galaxies and then comparing them to the expansion of the Universe.
Image: NASA/A Riess (STScI).
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