How to find something you can't see

Astronomers are getting better at tracking the strange objects called black holes. They can't see these objects, which are so dense that not even light escapes them.

Astronomers do see the radiation that comes from stars, dust, and gas caught in a black hole's grip. Satellites that scan the electromagnetic spectrum, more powerful data processing, and better ground instruments sharpen that view. In recent weeks, research teams have reported finding new black holes, watching a black hole gobble up a star, and detecting a mysterious new type of black hole explosion.

Just Wednesday, on the journal Nature's website, Thomas J. Maccarone at Britain's University of Southampton and colleagues reported finding a black hole in a globular cluster. Astronomers suspected that these clusters of several thousand stars would be good black hole habitats. Now Dr. Maccarone's team has found one in a globular star grouping in galaxy M49 in the nearby Virgo Galaxy Cluster. Team members can't yet estimate the object's mass. But they say that X-ray data gathered by Europe's Multi-Mirror Newton satellite leave no doubt it is a black hole.

This past November, an international team reported seeing black hole radiation suddenly appear 25,000 light-years away. Europe's INTEGRAL satellite spotted a gamma-ray burst as the black hole began slurping down gas from a nearby star. NASA's Swift satellite picked up the surveillance. Its ability to respond quickly provided the additional data needed to certify that this is, indeed, a previously unknown black hole. Surprised at the object's sudden appearance, team member Volker Beckmann at the University of Maryland in Baltimore said, "We suspect these powerful [gamma ray] emissions are caused by big chunks of the star's matter falling into the black hole."

Suvi Gezari at the California Institute of Technology in Pasadena and colleagues have no doubt they have the first start-to-finish observation of a black hole eating a star. In the Dec. 10 issue of Astronomical Journal Letters, they tell how ultraviolet data from NASA's Galaxy Evolution Explorer satellite enabled them to watch the meal. The first chunk swallowed emitted a UV burst. Every bite thereafter sent out a UV signal. "Now that we know we can observe these events with ultraviolet light, we've got a new tool for finding more," Dr. Gezari said in announcing the discovery.

Black holes come in two sizes. Lightweights range from a few times to thousands of times the mass of our sun. Heavyweights dwell in the centers of galaxies and have masses of many millions of suns. Heavy or light, they give themselves away by the radiation from the matter they accrete around them. Ian McHardy at the University of Southampton explained in the Dec. 7 issue of Nature that, light or heavy, black holes shine by the same processes that make matter in their grip emit a broad spectrum of radiation.

Scientists think black holes form when a star with sufficient mass explodes and collapses at the end of its life or when two stars collide. They should think again. In the Dec. 21 issue of Nature, several teams reported seeing a black hole explosion that doesn't quite fit those scenarios. Caltech's Avishay Gal-Yam said, "Its very mystery shows how much we still have to learn about the universe."