Issue 3.10

The Astrophysics Spectator

May 24, 2006

This issue of The Astrophysics Spectator adds a page to the “General Relativity” topic path that discusses the black hole candidate at the center of our Galaxy. This issue also adds a new commentary on the forces within the scientific community that discourage scientists from considering new theories when an old theory have outlived its usefulness.

The gravitational signatures of a black hole in our Galaxy are invisible to us if the black hole has a mass typical of a star. If a black hole has a mass that is several million times the Sun's mass, however, the gravitational signatures become strong enough to possibly be seen with our current instruments. Such a black hole appears to live at the center of our Galaxy.

We can see the center of our Galaxy with radio, infrared, and x-ray telescopes. What we see with these instruments is a region dense with stars and gas. At the center of this region is an unusual radio source. This source, name Sagittarii A^*, is the center of the Galaxy, the point around which everything rotates.

We can measure the mass of Sgr A^* by measuring the velocities of the stars that orbit it. The stars within about 1 parsec of the galactic center move in Keplerian orbits around Sgr A^* By measuring the velocities of these orbits, astronomers have been able to determine that the mass of Sgr A^* is 2.6 million times the mass of the Sun. Under our current theories of physics, this object can be nothing other than a black hole.

The center of our galaxy is about 8.5 kiloparsecs from us. At this distance, we may be able to see some of the more unusual aspects of black holes. In particular, we may be able to see the last stable orbit of a black hole. Far from a black hole, objects can orbit indefinitely, just as a planet can orbit indefinitely around the Sun. Close to a black hole, however, stable orbits cease to exist, and objects fall onto the event horizon of the black hole. The boundary for this region is set by the radius of the last stable orbit, the radius where light can orbit in a circle around the black hole. Outside of this boundary, we would see the background stars as well as any objects orbiting the black hole, but inside of this boundary we would only see the blackness of the event horizon and the dim light from objects falling onto it. If we could measure the size of this dark region around a black hole, we could test whether black holes exist and whether they have the properties given to them by General Relativity.

Next Issue: The next issue of The Astrophysics Spectator is scheduled for publication on June 7.

Commentary

Intellectual Inertia. We like to hang onto our old ideas. This is as much true in business as in science. But while the free market provides a force to encourage and reward the successful development of new business ideas and punish the continued pursuit of outdated ideas, pressures within the scientific community tend to reinforce a scientist's natural tendency to cling to old theories that come into conflict with new data. As a consequence, changes within the economy are evolutionary, while changes within the sciences are revolutionary. (continue)

General Relativity

The Black Hole at Our Galactic Center. The largest black hole candidate in our Galaxy sits at the very center of the Galaxy. We see this object, called Sagittarii A*, by its radio and x-ray emission. From the velocities of the stars orbiting this object, we know that its mass is 2.6 million times that of our Sun. With this large mass, the basic features of a black hole predicted by General Relativity may be just visible with current telescopes. (continue)