November 29, 1996
Stars within 1.5 light years of the galactic core whip around at a furious rate, they discovered. Researchers surmised that the rapidly orbiting stars are caught in the strangle-hold of an unseen resident --an extremely massive, compact object too dense and too dark to be an ordinary grouping of stars.
R. Cowan, 1996
Our planets revolve around the Sun because of the gravitational force of the Sun. Compared to the planets, the Sun is a massive object. It is that massive Sun that keeps the planets in their orbits. When we see pictures of a spiral galaxy, the question immediately comes to mind: What is it at the center of a galaxy that keeps all of those stars revolving around it? The best guess would be that it is some object so massive that it would make our own Sun seem puny. If it is so big, why can't it be seen with our most powerful telescopes?
One theory to explain massive gravitational forces, where the source of those massive forces can't be seen, is the concept of a black hole. A black hole is an object so massive that there is no space between the components of atoms (electrons, protons etc.). It's gravity is so great that even photons(light particles) are absorbed by it. If no photons leave the object, there will be no light coming out. As a consequence, it will not be seen, as we see a star. No light will be reflected from it either.
Our solar system is inside of a spiral galaxy which we call the Milky Way. The Sun is the closest star to us and it can be studied in ways that are not possible with any other star. By the same token, the Milky Way is the galaxy that can also be studied in more detail than other galaxies.
Astrophysicists have postulated a black hole at the center of the Milky Way that has the mass of 2.5 million suns crammed into a space the size of our solar system. If you can't see a black hole, what is the evidence that it exists?
Reinhard Genzel and Andreas Eckart of Germany's Max Planck Institute, after four years of observing the movement of 40 stars near the center of the Milky Way, have some evidence for the existence of a black hole.
If you look at the constellation Sagittarius, which is in the Milky Way and can be seen in the summer sky, you will be looking toward the center of our galaxy. In 1974, astronomers discovered a very powerful source of radio emissions which they named Sagittarius A*. Astronomers believe this to be where the black hole -if it exists at all- is located.
Genzel and Eckart measured the motion of 40 stars lying within 1.5 light years of Sagittarius A*. (If you looked at a star that is 100 light years away from us, the light that you would see started out from that star 100 years ago.)
One way to measure how fast a star is moving is by examining the spectrum of its light. If it is moving away from you, the spectrum of the light will be shifted toward the red. If it is moving toward you, it is shifted toward the blue. However there are some uncertainties in doing this, particularly over vast distances. There is much less uncertainty if you measure how fast a star travels across the sky. If you know how far away a star is, and how much it moves in 4 years, you can calculate its speed. This is what those astronomers did. What they found was that stars close to the center of our galaxy are moving a lot faster than stars farther away from the core of our galaxy. This is fairly good evidence that something is pulling on those stars to make them speed up. The best explanation so far is -whoopee- a black hole.
So, some night when you look at the constellation Sagittarius, think that everything in our galaxy is being pulled toward it. More and more stuff keeps entering that black hole. Some day, perhaps many billions of years from now, that black hole will accumulate so much stuff that, in a spasm of indigestion, it will explode in one big bang. And then the whole cycle will start all over again.