Scientists have made an important step toward understanding the details of how Type Ia supernovae explode and the origin of their immense luminosity.

Type Ia supernovae are violent stellar explosions whose brightness is used to determine distances in the universe. Observing these objects to billions of light years away has led to the discovery that the universe is expanding at an accelerating rate, the foundation for the notion of dark energy.

Although all Type Ia supernovae appear to be very similar, astronomers do not know for certain how the explosions take place and whether they all share the same origin. The widely accepted theory is that Type Ia supernovae are thermonuclear explosions of a white dwarf star in a close binary system.

Now, a team of researchers has examined new and detailed observations of 41 of these objects and concluded that there are clear signatures of gas outflows from the supernova ancestors, which do not appear to be white dwarfs "If the star system originally contained two white dwarfs before the supernova, then there shouldn’t be any sodium,” remarked Nidia Morrell of the Carnegie Observatories.

"The fact that we detected the sodium shows that one of the stars must not have been a white dwarf,” he added.

The astronomers also ruled out other possible sources of the sodium absorption features including interstellar clouds or a galactic—scale wind blown by the host galaxy.

"The low velocities and narrowness of the features suggest that the absorption is from material very close to the supernova that was ejected by the parent system before the explosion,” explained co—author Josh Simon also from Carnegie.

"Typically, gas with these characteristics is attributed to the stellar wind blown by red giant companion stars, not white dwarfs,” he concluded.

The research is published in the August 12 issue of Science.

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