HARVARD GAZETTE ARCHIVES

Black Hole Seen at Center of Milky Way

By William J. Cromie
Gazette Staff

The bright, round spot (center) marks the source of radio signals coming from the center of the Milky Way. Astronomers, including Jun-Hui Zhao (below), who tuned in to the broadcast, believe the signals originate from hot gases being ripped from stars and swallowed by a supermassive black hole. The view is in the direction of the constellation Sagittarius.

For the first time, astronomers have gotten a good view of one of the most awesome sights in the universe: a supermassive black hole at the center of our galaxy.

Packed in a dark maw a scant 9 million miles across, one-tenth the distance from the Earth to the sun, is a mass equal to 2.5 million sunlike stars. The gravitational strength of such a colossus is so great, not even light can escape from it.

But light aplenty exists at the edge of the black hole. The immense gravity is chewing up stars within its reach, creating an intense hell of gases swirling into the black hole like water down a drain.

"It's a history-making event to see in such detail what lies at core of the Milky Way galaxy," says Jun-Hui Zhao, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. "Theoretical models of the core envision a flat disk spinning rapidly and pulling in the gaseous remains of stars at its edges. But we see an odd, cigar-shaped region."

The astronomic "cigar" stretches 90 million miles wide. It is 270 million miles long, or about three times the distance from Earth to the sun. The black hole sits inside.

Zhao thinks the shape comes from jets of material fired out of the hole probably at right angles to the plane of the whirling disk. "Pressure may build up to the point where materials accreting on the edge of the black hole are ejected outwards," Zhao says.

Tuning in to the Center

Gasses being ripped from stars in temperatures as high as a billion degrees result in a storm of electrons. These electric particles moving at tremendous speeds generate short-wave radio signals that can be heard all the way to Earth, 150,000 trillion miles from the galactic center.

A radio telescope with receivers capable of picking up such broadcasts, however, only recently became available. It consists of a system of 10 antennas stretching from Hawaii to the U.S. Virgin Islands. This so-called Very Long Baseline Array (VLBA) serves as a single radio telescope with a lens more than 5,000 miles across. Signals from each antenna are brought and correlated together at the National Radio Astronomy Observatory in Socorro, N.M.

Zhao and Paul Ho from the Harvard-Smithsonian center, together with colleagues K.Y. Lo and Z. Shen from the Institute of Astronomy and Astrophysics in Taiwan, aimed the VLBA at a radio source thought to mark the center of the Milky Way. The source, called Sgr A*, is in the direction of the constellation Sagittarius.

At first, electrons whizzing around between the galactic center and Earth produced only a noisy blur. The researchers tuned their enormous antenna to five different wavelengths before receiving a clear signal at 7 millimeters.

Mapping the area from which the signal comes yields an image of a surprising small, peculiarly shaped structure enclosing what everyone believes is a supermassive black hole. "It's the first good look at the structure since it was discovered in 1974," Zhao says.

Why it's so small and peculiarly shaped remains a mystery. Lo, Zhao, Ho, and Shen are now tuning the antennas to a shorter wavelength (3 millimeters). "We expect that this will provide more details and answer some of the questions we have about its structure," Zhao notes.

The biggest mystery is the origin of such black holes, which apparently exist in the center of other galaxies. Are they leftovers from the "Big Bang" that created the universe some 15 billion years ago, or did they evolve after the galaxies formed?

"Right now, that's a chicken-or-egg question," Zhao remarks. "We can't say which came first."