National Geographic News
An illustration depicting two ''blue straggler'' stars.
Two stars merge to form a blue straggler in an artist's conception.

Illustration courtesy ESA/NASA and G. Bacon/STScI

Candidate ''blue straggler'' stars are circled in green.

Possible "vampire" stars in the Milky Way's bulge are circled in green. Image courtesy ESA/NASA, W. Clarkson/IU/UCLA, and K. Sahu/STScI

Charles Choi

for National Geographic News

Published June 9, 2011

The stellar version of vampires—stars that drain life away from other stars—have been discovered for the first time in the heart of our Milky Way galaxy.

(See Milky Way pictures in National Geographic magazine.)

Called blue stragglers, these cannibal stars have been spotted in other parts of the Milky Way. They seem to lag in age next to the other stars with which they formed—appearing hotter, and thus younger and bluer.

Astronomers suspect blue stragglers look so youthful because they've stolen hydrogen fuel from other stars, perhaps after colliding into their victims.

(See related picture: "'Cannibal' Galaxy Gets Fatter.")

These cannibal stars are routinely found in dense star clusters, where stars have many chances to feed off each other. Now, however, scientists have found blue stragglers in the Milky Way's galactic bulge, a dense region of stars and gas surrounding the galaxy's center.

"For a long time, it was suspected there were blue stragglers in the bulge, but no one knew how many there might be," said Will Clarkson, an astronomer at Indiana University Bloomington and the University of California, Los Angeles.

"At long last, we've shown they're there."

Milky Way Vampires Formed Differently?

Using NASA's Hubble Space Telescope, astronomers looked at 180,000 stars in and near the bulge. The team discovered 42 unusually blue stars that appeared much younger than the other stars.

(Also see "Galaxy 'Crumbs' Found in Milky Way—Proof of Recent Feeding.")

From these 42 stars, researchers estimate that 18 to 37 of them are likely real blue stragglers that are about 10 billion to 11 billion years old. The remainder may be genuinely young stars in the bulge, or stars not actually in the bulge.

It's also possible the blue stragglers did not form by slamming into other stars and absorbing extra hydrogen fuel, as occurs in other parts of the universe. 

Instead, the blue stragglers in the galactic bulge may have formed by ripping hydrogen off their companion stars. This possibly occurred either when one star fed off its partner in a two-star system, or perhaps after gravitational interactions in a triple-star system had caused two of its members to merge into one.

"There's still a lot we don't know about the details of how blue stragglers form," Clarkson said.

"Finding them in the bulge provides another set of constraints that can help refine models of their formation."

Vampire-star findings presented May 25 at an American Astronomical Society meeting in Boston.



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