Origin of key cosmic explosions unraveled

CHICAGO (AFP) – Astronomers who have long used supernovas as cosmic mile markers to help measure the expansion of the universe now have an answer to the nagging question of what sparks the massive stellar explosions.

"These are such critical objects in understanding the universe," lead author Marat Gilfanov of the Max Planck Institute for Astrophysics in Germany said Wednesday in describing his team's study.

"It was a major embarrassment that we did not know how they worked. Now we are beginning to understand what lights the fuse of these explosions."

Most scientists say Type 1a supernovae are formed when a white dwarf star -- the collapsed remnant of an old star -- becomes unstable after it exceeds its weight limit.

Instability could come from the merging of two white dwarfs or accretion -- a process in which the gravity of the star draws in enough material from a sun-like companion.

Using NASA's Chandra X-ray Observatory, Gilfanov and his team studied the supernovas in five nearby elliptical galaxies and the central region of the Andromeda galaxy.

"Our results suggest the supernovae in the galaxies we studied almost all come from two white dwarfs merging," said co-author Akos Bogdan, also of Max Planck.

"If the supernova were produced by accretion, the galaxies would be roughly 50 times brighter in x-rays than actually observed."

Further study will be needed to determine if merging is also the primary cause of supernovae in spiral galaxies.

Pinpointing a possible supernova prior to the explosion is also extremely difficult, the researchers noted.

Pairs of white dwarfs are incredibly difficult to find. And once the white dwarfs spiral into distances when they are about to merge, it takes just a few tenths of a second for them to explode.

Knowing how supernovas are formed is key to better understand how they can be used to measure cosmic distances and tracing dark matter, said Mario Livio, an astrophysicist with the Space Telescope Science Institute in Maryland.

"We use Type 1a supernova to determine the properties of dark energy in order to be able to do that accurately we need to fully understand the evolution of the power of the luminosity of these supernovae," Livio said in a conference call with reporters.

"There is (also) this process that we call feedback -- namely all supernova play an important role in the evolution of their host galaxies."

The study is published in the February 18 edition of the journal Nature.