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A better way to size up distant galaxies

supernova, astronomy, standard candleNew research may shed light on the stellar explosions used as cosmic mileposts.

Type Ia supernovae, known as "standard candles" in astronomy, have consistent properties that make them good markers of distance when peering at galaxies across the universe. In fact, studies of this kind of exploding star a decade ago produced one of the most significant scientific discoveries in recent history: that the universe is expanding at an accelerating rate, thanks to the influence of so-called dark energy.

In a paper set to be published in Astronomy & Astrophysics, members of a consortium known as the Nearby Supernova Factory present a new way to ascertain type Ia supernovae's relative distance quickly and accurately, thereby increasing their usefulness as markers. Whereas past approaches painstakingly tracked the light output from the explosion over time, the new method can be used to measure a supernova's luminosity, and hence distance, much more quickly, and with a level of accuracy the study's authors say surpasses the traditional approach.

The researchers found that the ratio of the brightness of two colors in the supernova's light spectrum strongly correlate with the explosion's magnitude. Specifically, the ratio of 642-nanometer-wavelength (orange-red) light to that of light with a wavelength of 443 nanometers (indigo) gives an accurate estimate of the supernova's brightness, in concert with other observed characteristics.

Type Ia supernovae are useful as markers because of their fairly standard brightness, which facilitates gauging their distance across the universe. Their uniformity derives from unusual birth circumstances: they are believed to arise from white dwarfs that have swollen to 1.4 times the mass of the sun by drawing material from a nearby companion star. At that point, says study co-author Greg Aldering, a cosmologist at Lawrence Berkeley National Laboratory, "a white dwarf star cannot support itself against gravity," exploding in a thermonuclear blast that is visible even in distant galaxies. "The conversion of a fixed mass—1.4 solar masses—into energy," Aldering says, "sets a reasonably narrow range to the resulting brightness."

Image of remnant from the type Ia Tycho supernova: X-ray: NASA/CXC/SAO, Infrared: NASA/JPL-Caltech; Optical: MPIA, Calar Alto, O.Krause et al.

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