Star May Have Escaped Galaxy

Astronomers have discovered the first ever star to be observed escaping the Milky Way galaxy, adding further evidence to the theory that massive black holes sit at the center of the galaxy.

Continued tracking of the star, which was first observed by a team from the Harvard-Smithsonian Center for Astrophysics (CfA), could provide important insights into the shape of the Milky Way in the future.

Telescopes based in Arizona spotted the star, which is three times the size of the sun, relatively hot compared to its neighbors, and bluish in color. The star has been traveling for 80 million years and is headed for the vast darkness of outer space.

“There is no defined edge to the Milky Way, but I’d say it’s about halfway out,” said Warren R. Brown, a fellow at CfA and the lead author on an upcoming paper announcing the find.

The star was discovered by chance, while the scientists were conducting a survey of faint blue stars along the outskirts of the Milky Way, which could provide clues as to whether the galaxy has merged with others in the past.

“You don’t expect to see a blue star that hot and young, and it was unusual,” Brown said.

The star is traveling at over 1.5 million miles per hour, a velocity two times that necessary to break free of the Milky Way’s gravitational pull. The star is relatively young, and it appears to have originated in the center of the Milky Way.

One explanation the researchers are advancing for the star’s remarkable speed is that the star was once part of a binary star system that passed too close to a black hole, which is a collapsed massive star that has a gravitational force so powerful even light cannot escape. Astronomer Jack Hills advocated such a scenario in 1998.

“Stars are falling into black holes, it is believed, all the time,” said co-author Scott J. Kenyon, a senior scientist at CfA. “If the orientation is just right, the black hole can grab a star and pull it in and give the other star a kick outward.”

Brown estimated that such an event happens once every 10,000 to 100,000 years. Consequently, there should be about 1,000 to 10,000 similar stars flying about the galaxy.

But with around 100 billion stars in the Milky Way, ”it’s a relatively rare occurrence,” Brown said.

This star provides scientists with further support for the theory that black holes are at the center of the Milky Way.

In the future, the star could also provide clues as to the uncertain shape of the Milky Way, said Michael J. Kurtz, a co-author and astronomer and computer scientist at the Smithsonian Astrophysical Observatory.

Because scientists can predict what the motion of the star should look like if the galaxy is flat. information about the galaxy’s actual shape—for instance, if it is in fact lopsided—might be gleaned from a comparison between the predicted and the actual trajectory of the star.

“It’s actually pretty neat,” Kurtz said about this potential implication. “I didn’t know about it myself at first.”