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Astronomers watch stellar resurrection

"We need refined calculations to explain some still mysterious details in the behavior of SAO 244567," said astronomer Nicole Reindl.

By Brooks Hays
Astronomers watch stellar resurrection
The fast-paced evolution of SAO 244567 can be seen at the center of the Stingray nebula, a planetary nebula 2700 light-years from Earth. Photo by ESA/NASA/Hubble

LEICESTER, England, Sept. 13 (UPI) -- Over the last 30 years, astronomers have watched the resurrection of the star SAO 244567. Their observations -- detailed this week in the Monthly Notices of the Royal Astronomical Society -- mark the first time scientists have witnessed both the heating and cooling stages of rebirth.

"SAO 244567 is one of the rare examples of a star that allows us to witness stellar evolution in real time," lead researcher Nicole Reindl, an astronomer at the University of Leicester, said in a news release. "Over only twenty years the star has doubled its temperature and it was possible to watch the star ionizing its previously ejected envelope, which is now known as the Stingray Nebula."

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Between 1971 and 2002, astronomers measured a dramatic rise in the surface temperature of SAO 244567. More recently, data retrieved by the Hubble Space Telescope suggests the star is cooling down.

The star's fast-paced evolution is something one would expect to see from an aging star much larger than the sun. But data suggests the original mass of SAO 244567 is comparable to the sun's. Low-mass stars typically evolve slowly.

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In their latest study, Reindl and her colleagues suggest the star's flash of heat was triggered by the sudden ignition of helium just beyond the stellar core.

Researchers say such a flash would cause the star to rewind to an earlier phase of stellar evolution, which would explain the recent cooling.

"The release of nuclear energy by the flash forces the already very compact star to expand back to giant dimensions -- the born-again scenario," Reindl explained.

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Researchers have previously seen separate examples of both the cause and effect mechanisms of this type of stellar rebirth -- the heating and cooling -- but never before in the same star.

As such, this particular iteration of stellar evolution is forcing astronomers to rethink their models.

"We need refined calculations to explain some still mysterious details in the behavior of SAO 244567," Reindl added. "These could not only help us to better understand the star itself but could also provide a deeper insight in the evolution of central stars of planetary nebulae."

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