Urrrp!

A giant black hole dipping into the cosmic cookie jar has been caught red-handed—the first time astronomers have seen a black hole eat a star from the first to nearly the final bites. The glutton was nailed by the ultraviolet space telescope known as the Galaxy Evolution Explorer, or GALEX—a NASA Small Explorer mission headquartered at Caltech. (See E&S, 2004, No. 2.) “This type of event is very rare, so we are lucky to study the entire process from beginning to end,” says Caltech postdoc Suvi Gezari, the lead author of the paper in the December 10 issue of Astrophysical Journal Letters.

For perhaps thousands of years, the black hole rested quietly deep inside an unnamed elliptical galaxy. But then a star ventured a little too close and was torn to shreds—a black hole’s gravity is so strong that even light cannot escape it. Part of the shredded star swirled around the black hole, then began to plunge into it, triggering the bright ultraviolet flare that GALEX saw. The spacecraft continues to watch as the black hole finishes the remaining crumbs of its midnight snack, observations that will ultimately provide a better understanding of how black holes evolve within their host galaxies. NASA’s Chandra X-ray Observatory and the Canada France Hawaii Telescope and the Keck Observatory, both in Hawaii, have also helped chronicle the event in multiple wavelengths over two years.

In the early 1990s, three other dormant black holes were suspected of having eaten stars when the joint German-American-British Röntgen X-ray satellite picked up X-ray flares from their host galaxies. Astronomers had to wait until a decade later for Chandra and the European Space Agency’s XMM-Newton X-ray observatory to confirm those findings, and show that the X rays had faded dramatically—a sign that stars were swallowed.

Active black holes are always feeding, creating glowing disks of material around themselves that are easy to see. But the black hole hiding in the heart of a typical galaxy may only snare an unsuspecting star once every 10,000 years. “Now that we know we can observe these events with ultraviolet light,” says Gezari, “we’ve got a new tool for finding more.” This black hole is thought to be tens of millions times as massive as our sun, and its host galaxy is located four billion light-years away in the constellation Boötes. —WC