For more than a decade astronomers have known that something strange is going on at the hub of our Milky Way galaxy. Now, using state-of-the-art electronics, a group of scientists from the University of California at Berkeley has gathered evidence that substantially strengthens the case that a massive black hole resides there.
``In the past, there have been a number of puzzles about this area. The pieces of the puzzle that are finally falling into place are quite impressive. With this new evidence, the case for a black hole now seems to be quite convincing,'' comments Nobel laureate Charles H. Townes, who headed the 10-year Berkeley effort.
If this explanation is correct, it is of considerable scientific importance. Confirmation of the existence of black holes could shed new light on the still mysterious process of galaxy formation.
In addition, because of their extreme gravitational fields, black holes would constitute one of the severest tests for Einstein's theory of relativity.
Over the years, unusual observations have been recorded at a number of points on the electromagnetic spectrum. Strong radio waves, the signature of dynamic astronomical events, are pouring out from a point lying on the galaxy's axis.
The entire core emits copious amounts of X-rays, characteristic of exploding gases, including short bursts of extreme intensity. The region also produces gamma rays, hard radiation that objects such as black holes are thought to produce.
But scientific sleuthing has been seriously hindered. From our solar system's location far out on a spiral arm, the galactic hub is heavily shrouded by stars and dust. In fact, it has generally been easier to study the nuclei of neighboring galaxies than our own.
Black holes are thought to be the remains of collapsed stars. When massive stars run out of nuclear fuel, they explode violently. This leaves behind a core that scientists think collapses to the size of a planet, becoming a super-dense object. Its gravity is so strong that nothing, not even light, can escape.
Like a celestial vacuum cleaner, a black hole should consume anything that ventures near, growing stronger and more massive in the process.
The Berkeley group, whose results are reported in the current issue of the journal Nature, focused on infrared light that passes freely through dust cloaking the galactic center.
They were able to make precise enough measurements to determine the rate at which clouds of hot gas sweep around the core, 30,000 light years distant. An analysis of these motions suggests that the gases are orbiting an object that has 4 million times the mass of the sun and is extremely compact, the group reports.
These results ``are very significant,'' says Wallace L. W. Sargent of the California Institute of Technology (Caltech), who has been searching for similar objects in other galaxies. In the past, the search for black holes has concentrated on detecting direct electromagnetic signals.
Another team of scientists at Caltech, Berkeley, and the Harvard-Smithsonian Center for Astrophysics at Cambridge, Mass., recently reported the results of their probe of the Milky Way's center.
The scientists linked six radiotelescopes around the country to form a high-tech lens 3,000 miles in diameter and carefully measured the size and shape of a compact, highly energetic object at the galaxy's center.
After studying the object, the team conclude that it represents further evidence that there may be a black hole in the center of the galaxy.
Still, the evidence gathered so far remains circumstantial. Black holes are shadowy objects that, if they exist, don't directly proclaim their presence.
``It is hard to think of anything that would be a very positive signature,'' says Roger Blandford, an astrophysicist at Caltech. So considerable powers of observation and reasoning will be required to unequivocally prove or disprove their existence.
There are other explanations for the unusual observations at the galactic center.
One is an extraordinarily dense cluster of stars. Another is a super star, an extraordinarily massive and rapidly rotating sun that has not yet burned out.
Then again, there is no guarantee that it might not be something that simply hasn't been conceived of yet.
But the black-hole hypothesis is clearly the current favorite.
``Step by step, the evidence is becoming more suggestive,'' says Dr. Blandford. And there is no end to the scientific probing in sight. An X-ray telescope flying on the next space shuttle will be focused on the galactic core in hopes of supplying yet another piece for scientists to fit into this elaborate puzzle.