What Makes Supernovas Go Boom
This composite image of M31 (also known as the Andromeda galaxy) shows X-ray data from NASA's Chandra X-ray Observatory in gold, optical data from the Digitized Sky Survey in light blue and infrared data from the Spitzer Space Telescope in red.
Credit: X-ray: NASA/CXC/MPA/ M.Gilfanov & A.Bogdan; Infrared: NASA/JPL-Caltech/ SSC; Optical: DSS

The trigger that ignites a common type of stellar explosion has finally been uncovered with observations from NASA's Chandra X-ray Observatory, providing a major advance in the understanding of supernovas.

These supernovas, called Type 1a, result from the explosion of a white dwarf star. These types of supernovas are used as cosmic mile markers, and knowing what causes these stellar blow-ups is a critical key to studying the mysterious dark energy that astronomers think pervades the universe.

"These are such critical objects in understanding the universe. It was a major embarrassment that we did not know how they worked," said Marat Gilfanov of the Max Plank Institute for Astrophysics in Germany and a member of the team that made the new findings. "Now we are beginning to understand what lights the fuse of these explosions."

Type Ia supernovas are generally thought to go off when a white dwarf star ? the remnant core of a red giant star that has sloughed off its outer gas layers and cooled down ? exceeds its weight limit, becomes unstable and explodes.

But just what causes it to tip the scale and get blown to smithereens hadn't been pinned down. Two possibilities for pushing a white dwarf over the edge were considered the main contenders: accretion, in which a white dwarf siphons off material from a sun-like companion star until it exceeds its weight limit; and the merging of two white dwarfs into a bigger mass.

One way for telling which process was the culprit was looking at the X-ray light emission from supernovas, as each scenario would generate different amounts of X-rays. A Type Ia supernova caused by accreting material produces significant X-ray emission prior to the explosion, while a supernova from a merger of two white dwarfs would create significantly less.

To see which scenario was likely the cause of Type 1a supernovas, Gilfanov and his team used the Chandra Observatory to observe five nearby elliptical galaxies and the central region of the Andromeda Galaxy (or M31).

The scientists found that the observed X-ray emission was a factor of 30 to 50 times smaller than expected from the accretion scenario, effectively ruling out this mechanism and making white dwarf mergers the main suspect in these galaxies.

"Our results suggest that the supernovas in the galaxies we studied almost all come from two white dwarfs merging," said team member Akos Bogdan, also of Max Planck. "This is probably not what many astronomers would expect."

The unexpectedness stems partly from the fact that few double white dwarf systems seemed to exist and that such pairs are difficult to see even with the best telescopes.

"Now this path to supernovas will have to be investigated in more detail," Gilfanov said.

The difference between these two scenarios may have implications for how these supernovas can be used as "standard candles" to track vast cosmic distances. Type 1a supernovas have generally been thought to be excellent distance guides because they can be seen to large distances and follow a reliable brightness pattern.

But because white dwarfs can come in a range of masses, it means that the merger of two of them could result in explosions that vary somewhat in brightness.

One question that remains to be answered is whether this trigger that seems to be the cause of Type 1a supernovas in elliptical galaxies is also the fuse for these stellar explosions in spiral galaxies.