The Milky Way, it appears, has an ejection seat.
Evidence is mounting that certain stars get kicked free from the gravitational pull at the center of the galaxy - rocketing outward into intergalactic space at breakneck speeds.
The latest sighting of two such "hypervelocity stars" - bringing the grand total to five over the past year - suggests that these Milky Way escapees are not bizarre oddities. Rather, they give an indication of conditions and processes that exist at the center of the galaxy, a place that can be difficult for astronomers to see.
The discovery of the two newest hypervelocity stars was no accident. Astronomers at the Harvard-Smithsonian Center for Astrophysics, the first to go looking for stellar speedsters, say the duo are zooming away from the center of the Milky Way at up to 1.43 million miles per hour. At that pace, an object would cover the distance between the Earth and the moon in about 10 minutes - fast enough to allow a star to escape the galaxy's gravitational grip.
The same team reported finding the first of these objects last February, and since then theories attempting to explain their travel patterns have multiplied like rabbits.
"The first one was just sort of an odd object - you know: 'Look at this! Wow!' " says Warren Brown, an astronomer at the Center for Astrophysics in Cambridge, Mass., who stumbled across last year's hypervelocity star. "But now we have a sample of five, so it starts to get interesting. They come from the galactic center and are ejected by the supermassive black hole there. So they tell a story of what's happening at the galactic center."
To find these stars, astronomers hunt for them in the Milky Way's halo - a spherical region around the galaxy. (Picture the Milky Way as a dinner plate, and the halo as a silver domed cover - one above and one below the plate.)
The halo contains the galaxy's oldest stars, the bulk of its mass, and hot gas. Most of the mass resides in so-called dark matter - a form of matter astronomers invoke because the gravity from the matter they can detect isn't strong enough to hold the whirling galaxy together on its own.
By tracking the travels of hypervelocity stars, astronomers hope to determine the distribution of the galaxy's dark matter, as well as trace the shape of the galaxy's gravitational field, adds Margaret Geller, a member of the team hunting the "hypers."
The galaxy's halo is home to other fast-moving stars, typically kicked out of the plane of the galaxy by interactions with other objects. But these "runaway" stars fail to reach the velocities needed to flee the galaxy entirely.
Many theories of hyperspeed
The tale behind hypervelocity stars begins in 1988, when Jack Hills, a theorist at the Los Alamos National Laboratory in New Mexico, calculated that under the right conditions a supermassive black hole at the center of the Milky Way could eject stars in its immediate vicinity at speeds up to nearly 9 million m.p.h.
Over the years, other scientists toyed with the concept, trying to devise scenarios to identify the configurations of stars and black holes that could give a star the kick it would need to exit the galaxy.
One approach, for example, holds that a hypervelocity star comes from a tightly bound pair of stars drawn toward the galaxy's central black hole. As this binary star gets too close, the black hole's gravity rips the two apart, consuming one of them. The sudden release frees the second star from the black hole's clutches - and sends it packing.
Last February, Dr. Brown and his colleagues reported finding the first hypervelocity star while searching for star "streams" in the halo that would testify to past mergers between the Milky Way and other galaxies.
Hypervelocity stars give themselves away by their age and composition, which are more akin to stars in the center of the galaxy than to those in the halo, as well as by their speed and direction.
Then, in November and December, two separate groups of European astronomers published their detections of another hypervelocity star each. One appears to have come from the Large Magellanic Cloud, a satellite galaxy to the Milky Way. If that's its true source, the star could testify to the presence of a supermassive black hole in the LMC.
The two latest hypervelocity stars, however, clearly came from the Milky Way, Brown says. The team used data from the Sloan Digital Sky Survey as the starting point for its hypervelocity star search.
Once candidates are found, the team uses the Smithsonian's 6.5-meter telescope on Mt. Hopkins in Arizona to make detailed measurements of their compositions and velocities.
Hypervelocity stars are thought to be rare - some scientists estimate that only about 1,000 exist in the vast space the halo occupies. But that view could change if searches using more powerful telescopes expand to include more types of stars than the extremely hot blue stars astronomers have been looking for so far.
Blue stars, while very faint, are the easiest to spot at the distances involved. And because they are relatively young, they provide the biggest age contrast with the halo's stellar geezers.