Hubble spies old stars that shed their skins to look younger

Using NASA’s Hubble Space Telescope, a University of Washington-led team has peered deep into the neighboring Andromeda galaxy to find what look like young blue stars in a neighborhood that should be populated by old stars.

Blue is considered a telltale signal for hot, young stars but, in a “surprising and intriguing” twist, it turns out that some old stars can also be blue, said Philip Rosenfield, a UW doctoral student in astronomy who discussed the findings Jan. 11 at the annual meeting of the American Astronomical Society in Austin, Texas.


As the team inspected more closely, it turned out that the stars are similar to our sun but have jettisoned much of their outer material to expose deep blue cores. The scientists know the stars have to be old because they are in an area of Andromeda that stopped making stars eons ago.

“These stars lost just about everything except their cores,” said Rosenfield. “What we’re trying to figure out is how they lost their envelopes, how they lost their mass.”

The discovery, coupled with previous observations of these blue stars, shows there are thousands of such stars in the center of Andromeda. That region is called a “bulge” because of the mutual gravitational attraction of millions of stars distributed through the center of the galaxy, Rosenfield said. The very center is a supermassive black hole.

The astronomers used Hubble’s Wide Field Camera 3 to find about 8,000 such stars in a census done in ultraviolet light, which traces the glow of the hottest stars. The study is part of the multiyear Panchromatic Hubble Andromeda Treasury survey, led by Julianne Dalcanton, a UW astronomy professor.

“We were not looking for these stars. They stood out because they were bright in ultraviolet light,” Dalcanton said.

Rosenfield is the lead author of a paper describing the finding that has been submitted to The Astrophysical Journal for publication. It explains that the astronomers analyzed the stars for more than a year before determining they were past their prime and getting on toward “retirement.”

“The stars are dimmer and have a range of surface temperatures different from the typical hot, young stars we see in the disk of Andromeda,” said Rosenfield.

As these stars evolved and grew into red giants, they ejected most of their mass to expose their blue-hot cores. When normal stars with masses similar to the sun swell up to become red giants, they don’t lose as much material. The compact, blue stars follow a longer path to their last years, ending up as white dwarfs, the cooling embers of stars like the sun.

Astronomers aren’t quite sure why the ultra-blue stars lose their mass, and Dalcanton’s team has as its next goal trying to figure out precisely what is happening.

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Additional information is available from the Space Telescope Science Institute.