Rude companion.
A white dwarf explodes inside the atmosphere of a red giant, in this artist's impression.

David A.Hardy/PPARC

Stellar Stomach Trouble

Call it a chronic case of stellar indigestion. In a not-so-far-away constellation, astronomers have witnessed a pair of stars so cramped together that the regular explosions of one actually take place inside the other. The findings are helping scientists to understand both stellar evolution and the origins of certain types of supernovas.

Over the past century or so, a faint star called RS Ophiuchi has flared up brilliantly five times, most recently on 12 February. RS Oph--as astronomers call it--is a binary system, comprising a white dwarf (the super-dense core of a former star) in close orbit with a much, much larger red giant. The two objects circle each another so closely that the dwarf's intense gravity continually strips hydrogen-rich gas from the outer layers of the red giant's atmosphere. Every 20 years or so, the dwarf accumulates enough gas to detonate a titanic thermonuclear explosion.

Knowing RS Oph was about to explode, a worldwide team of astronomers trained an array of space- and ground-based instruments in its direction. The combined readings showed that the red giant's atmosphere is so large that it envelops the entire binary system. That means the white dwarf's explosions take place inside the red giant. As the debris is hurled outward, it collides with the atmosphere at such a high speed--thousands of kilometers per second--it intensifies the blinding flash of the blast.

As expected, astronomers measured initial temperatures from the explosion of up to 100 million degrees Celsius--nearly 10 times that in the core of the sun. One of the most surprising results, says Michael Bode of Liverpool John Moores University in the United Kingdom, was a secondary increase in brightness, which might be caused by the blast temporarily cutting a hole through the red giant's atmosphere to expose the white dwarf's light. Bode, who heads the team that tracked the explosion with the Swift space telescope, reported the findings today at a Royal Astronomical Society meeting in Leicester, United Kingdom.

It's not clear what the future holds for RS Oph. It could be a type of binary star system that gives rise to so-called Type Ia supernovas, says Sumner Starrfield of Arizona State University in Tempe. Astronomers use the explosions as benchmarks to study the evolution of the universe. "So, anything we learn about RS Oph can confirm or deny the theories."

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