A new analysis of a survey of the stars in our galaxy has found a number that appear to have reached escape velocity, moving fast enough to exit the Milky Way. These aren't the first stars we've found that are moving this quickly, but all the previous ones are large, blue stars that were thrown out of the galaxy's core by its supermassive black hole. These are smaller stars, similar to our Sun, and they clearly do not originate at the galaxy's core. In fact, they're moving fast enough that they could have potentially originated in another galaxy.

The study relies on the Sloan Extension for Galactic Understanding and Exploration, a project that has mapped the location of 240,000 stars within our galaxy. With multiple exposures, it's possible to track stars as they move across the sky.

The authors of the new study limited their analysis to G- and K-class stars, those similar in mass to the Sun and often referred to as G and K dwarfs. Even so, that limitation provides a lot of candidates; there were about 43,000 G dwarfs alone in the survey's population. From there, the authors converted the apparent motion of the stars to motion relative to the galactic core. They could then identify those stars that are moving very quickly relative to the galaxy itself. They took only those stars that were clearly moving fast enough to escape our galaxy's gravitational clutches—600 kilometers a second and up.

That initial analysis picked out 42 possible high-velocity dwarfs, but there were potential problems with many of them, such as possible confusion with other nearby stars or too few exposures to get a good measure of the velocity. Even so, it was possible to identify 17 stars where the researchers could determine a velocity that they considered "reliable."

All of that seemed solid, which leaves a rather significant question: how did they possibly get moving so quickly? We know how this happens at the galactic core. A binary system of two stars can wander close to the supermassive black hole at the very center, which captures one of them. Nearly all the momentum of the system gets transferred to the other star, sending it rocketing out of the galaxy.

But many of the stars in the galactic core are larger and bluer than the dwarfs found in this study. More significantly, the direction these stars are moving indicates that they couldn't possibly have come from there. The stars are moving so fast that they could have come from the core of a nearby galaxy. M31/Andromeda is close enough that stars moving this quickly could cross the distance in just a billion years; these types of stars burn their fuel so slowly that they could last for tens of billions of years. Nevertheless, the stars don't come from the right direction, although other, more distant galaxies can't be ruled out.

Of course, that assumes the cores of nearby galaxies are very different from the Milky Way's and have a population of dwarf stars. The authors suggest that it might be time to consider alternate mechanisms for creating hypervelocity stars. One option could be complex interactions within dense groups of stars like dwarf galaxies and globular clusters. Still, these stars don't look like they came from globular clusters, either.

One prospect researchers are intrigued by is that the stars were once part of a binary system but got blasted loose when their partner went supernova. That, they suggest, should paint the fast-moving stars with debris, which should be possible to pick up with further observations. There should be a chance to return to this story when follow-up observations are made.

The arXiv. Abstract number: 1308.3495 (About the arXiv). To be published in The Astrophysical Journal.