While astronomers have known of runaway stars flung out of their home galaxies, they've now discovered that entire galaxies can be "homeless," tossed out of their galactic cluster homes to forever wander through intergalactic space.

Eleven such runaway galaxies have been observed, the victims of gravitational disturbances that have left them alone, drifting and marooned, in the space between galactic clusters.

"These galaxies are facing a lonely future, exiled from the galaxy clusters they used to live in," says Igor Chilingarian, an astronomer working with the Harvard-Smithsonian Center for Astrophysics and Moscow State University.

Stars — and now, it turns out, entire galaxies — can be tossed out of their homes if they are accelerated by gravitational upset to a speed greater than their home location's "escape velocity," the researchers explain.

For example, for a star to escape our Milky Way galaxy, it would need to achieve a speed of more than 1.2 million miles per hour.

For an entire galaxy to escape the gravitation pull of its entire home cluster, it could need a speed as high as 6 million mph, depending on the mass of the entire cluster, the researchers note.

Chilingarian and his study co-author Ivan Zolotukhin initially were working to identify new examples of a class of galaxies dubbed compact ellipticals, small groupings of stars just a few hundred light years across, bigger than a star cluster but smaller than a typical galaxy.

Of the almost 200 previously unknown compact ellipticals they found, 11 surprised them by being completely remote and far from any larger galaxy or galactic cluster.

"The first compact ellipticals were all found in clusters because that's where people were looking. We broadened our search, and found the unexpected," says Zolotukhin.

The phenomenon suspected of creating the homeless galaxies is similar to processes known to happen in the center of our own Milky Way galaxy, when two stars in a binary system get to close to a supermassive black hole, which will swallow one of the pair and fling the other out of the galaxy, he says.

"This is the same phenomenon, but working on a different scale, a slingshot effect, when during a three-body encounter the lightest body flies away from the system," he explains.

However, Chilingarian says, being flung out of a galactic cluster can help one of these compact galaxies survive, because otherwise they would likely spiral into the center of the cluster and be torn apart by their massive hosts — although it might take a billion years.