Astronomers Hunt for Ticking Time Bombs

by Clara Moskowitz, SPACE.com Senior Writer

Date: 20 July 2010 Time: 08:09 AM ET

In this negative image of the Pinwheel Galaxy (M101), red squares mark the positions of "super-soft" X-ray sources. The Pinwheel should contain hundreds of accreting white dwarfs on which nuclear fusion is occurring, which should produce prodigious X-rays. Yet we only detect a few dozen super-soft X-ray sources. This means that we must devise new methods to search for the elusive progenitors of Type Ia supernovas.
CREDIT: R. Di Stefano (CfA)

Why certain stars explode in supernovas is a mystery to scientists, and a new study finds the situation is even murkier than thought because some of the light from these stars may be blocked.

To understand more about these cosmic explosions, astronomers are hunting for ticking time bombs ? stars that are on the track toward becoming supernovas. Yet the missing light could make the search even harder.

Death of a star

Supernovas mark the death of a star, when it stops shining light created through nuclear fusion and instead collapses into an extremely dense black hole.

A subset of supernovas called Type Ia supernovas are the gold standard in astronomy. They are thought to erupt when a dense, dim star called a white dwarf hits a certain upper limit in mass and explodes.

Thus, each white dwarf erupting in a Type Ia supernova will shine at roughly the same brightness. This allows astronomers to spot them across the universe and tell how far away they are by how bright they appear, compared to their known intrinsic luminosity. (They are called "standard candles," for this special distance measuring feature.)

But astronomers are still confused by just what cases white dwarfs to hit that mass limit and explode.

"The question of what causes a Type Ia supernova is one of the great unsolved mysteries in astronomy," said researcher Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics.

Missing X-rays

One scenario envisions a white dwarf gradually siphoning off mass from another white dwarf companion until it bulges over the limit. Another idea is that two white dwarfs merge to become one behemoth that topples the limit and explodes.

To get to the bottom of the mystery, scientists have been searching for white dwarfs in the stage before they get massive enough to explode. In particular, astronomers hunt for so-called "super-soft" X-rays, which are thought to be created when gas from one star hits another star's surface and undergoes nuclear fusion.

This should happen in either the siphoning or merging scenarioDi Stefano said, though the amount of light and particular signatures could tell scientists more about which occurs more often.

Yet for some reason, scientists have only been able to find a handful of these X-ray sources, where they expected to find hundreds.

In a new paper, Di Stefano proposes that some process is blocking this light from reaching our telescopes on Earth. Perhaps some other material surrounding the white dwarf absorbs the X-rays after they are emitted so we can't see them. Or maybe white dwarfs emit most of their light at wavelengths other than X-rays.

If this is true, "we must devise new methods to search for the elusive progenitors of Type Ia supernovae," Di Stefano said.

Di Stefano's paper will be published in an upcoming issue of the Astrophysical Journal.