Star escaping clutches of galaxy

At a recent yearly congregation of the American Astronomical Society this week, it was announced that a team of scientists has discovered a new type of star. Unlike most stars, there celestial bodies are traveling at speeds fast enough to escape the gravitational field of their home galaxy. The details about these “hypervelocity stars” were also published in the Jan. 1 issue of the Astrophysical Journal.

The fast moving stars were first discovered when a graduate student, Lauren Palladino, of the Vanderbilt University was observing stars in the Milky Way. Palladino was mapping certain sections of the galaxy using a database of stars and galaxies called the Sloan Digital Sky Survey. This database contains information about both stars and galaxies scattered across a large area representing about a fourth of the night sky seen from the Earth. After reviewing calculations regarding the orbits of certain stars, Palladino noticed a number of stars moving at abnormally fast velocities.

“These new hypervelocity stars are very different from the ones that have been discovered previously,” says Palladino. “The original hypervelocity stars are large blue stars and appear to have originated from the galactic center. Our new stars are relatively small – about the size of the sun – and the surprising part is that none of them appear to come from the galactic core.”

Palladino was working under Vanderbilt assistant professor of astronomy Kelly Holley-Bockelman when the discovery was made and the pair has noted that these stars seemed to be travelling at velocities that normally indicate interaction with the theorized supermassive black hole at the center of the galaxy. Due to the trajectories, however, it seems that the newly discovered “hypervelocity stars” are moving due to forces yet to be explained.

“It’s very hard to kick a star out of the galaxy,” says Holley-Bockelmann. “The most commonly accepted mechanism for doing so involves interacting with the supermassive black hole at the galactic core. That means when you trace the star back to its birthplace, it comes from the center of our galaxy. None of these hypervelocity stars come from the center, which implies that there is an unexpected new class of hypervelocity star, one with a different ejection mechanism.”

To escape the powerful gravitational field of the galaxy, a star needs to travel at more than 1 million miles per hour. These speeds can be reached when a star, usually a binary pair, “slingshots” around a supermassive black hole using the massive gravitational forces involved to reach the velocities required for galactic ejection. With the new “hypervelocity stars,” however, more observations will need to be taken to get to the root of the problem.