Astronomers See Early Stages of Star Explosions

Mugshots of some of the two dozen supernova explosions captured by NASA's Swift satellite. CREDIT: NASA

Satellite observations have confirmed how a certain type of star explosion occurs and revealed a previously unknown stage of radiation release during the stellar burst.

Using NASA's Swift satellite, astronomers observed two dozen supernovas [image] shortly after they happened. Two, especially, were of special interest.

One, called SN2005ke [image], was a Type 1a supernova. These explosions are thought to occur when small, dead stars called white dwarfs siphon gas from the atmosphere of either another white dwarf or a red giant star. The white dwarfs accumulate matter until they reach a critical mass of 1.4 solar masses and explode.

Astronomers use Type 1a supernovas as "standard candles" to measure cosmic distances because all Type 1a's are thought to blaze with equal brightness at their peaks, although this has been challenged by recent observations.

X-ray and ultraviolet observations by Swift showed shock waves from SN2005ke ramming into very dense gas that could have only come from a red giant star, confirming at least one of the Type 1a formation scenarios.

"The only explanation that we have for X-ray and ultraviolet emission is that this Type 1a supernova has to have a companion star. This companion has to be a massive star that is losing a lot of mass from its stellar wind particles," study leader Stefan Immler of NASA Goddard Space Flight Center in Maryland told reporters at a press conference.

Also of interest was SN2006bp [image], a Type II supernova created when a massive star ran out of fuel and collapsed under its own gravity. Swift detected X-ray radiation immediately after the explosion that had never been seen before and which faded within days. The researchers were also surprised to find evidence of hot gas lingering near the exploded star's remains. This suggests that the star's radiation did not create a cavity around the star before the explosion, as is commonly thought.

"That was our surprise finding," Immler explained. "We had expected that the stellar wind blows a cavity around the star and that there is nothing there that can be shock-heated to high enough temperatures to give off X-rays."

One possible explanation for the anomalous gas, Immler said, is that the exploded star might have been a red supergiant whose stellar wind was slow and dense.

"That's just the nature of the stellar wind, if it's a slow enough, dense enough stellar wind, it will not blow a hole in the environment," he said.

The research was presented last week at the annual meeting of the High Energy Astrophysics Division of the American Astronomical Society in San Francisco.

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