What is the origin of the so-called Type 1a supernovae, which are widely used as celestial mileage markers?

LOS ANGELES — German astronomers using a U.S. telescope have provided scientists with at least a partial answer to a vexing question: What is the origin of the so-called Type 1a supernovae, which are widely used as celestial mileage markers?

Type 1a supernovae are of special significance to astronomers because all are believed to have essentially the same intrinsic brightness, and because they can be observed from great distances. Thus, by comparing the brightness of any one of them to what it is expected to be, researchers can estimate its distance from Earth and thereby judge the distance of objects near it.

A Type 1a supernova is created when a white dwarf, the remnant of an old star, accumulates sufficient mass to become unstable and explodes. But the cause of that accumulation has not been well understood. One school of thought suggests that the white dwarf sucks in mass from a companion star until it reaches a mass about 1.4 times that of the sun, at which point it explodes. The second theory is that the supernova results from the collision of two white dwarfs.

Astronomers Marat Gilfanov and Akos Bogdan of the Max Planck Institute for Astrophysics in Garching reported in Thursday's issue of the journal Nature that, at least in some galaxies, most of the supernovae are created by collisions.

To distinguish between the two possible scenarios, Gilfanov and Bogdan relied on the idea that, if a white dwarf were accreting matter from a nearby star, it should release tremendous amounts of X-ray energy as the gas from the star reached its surface.

The pair used the National Aeronautics and Space Administration's orbiting Chandra X-Ray Observatory to observe five nearby elliptical galaxies and the core region of the Andromeda galaxy. The two researchers found that the galaxies emitted only 2 percent to 3 percent as much X-ray energy as they would if the dwarfs were accreting matter from nearby stars. They thus concluded that collisions were the most likely source of the supernovae.