25.4 The center of the galaxy  (Page 2/5)

Now let’s focus in on the central region using a more energetic form of electromagnetic radiation. [link] shows the X-ray emission from a smaller region 400 light-years wide and 900 light-years across centered in Sagittarius A ^* . Seen in this picture are hundreds of hot white dwarfs, neutron stars, and stellar black holes with accretion disks glowing with X-rays. The diffuse haze in the picture is emission from gas that lies among the stars and is at a temperature of 10 million K.

Galactic center in x-rays.

As we approach the center of the Galaxy, we find the supermassive black hole     Sagittarius A ^* . There are also thousands of stars within a parsec of Sagittarius A*. Most of these are old, reddish main-sequence stars. But there are also about a hundred hot OB stars that must have formed within the last few million years. There is as yet no good explanation for how stars could have formed recently so close to a supermassive black hole. Perhaps they formed in a dense cluster of stars that was originally at a larger distance from the black hole and subsequently migrated closer.

There is currently no star formation at the galactic center, but there is lots of dust and molecular gas that is revolving around the black hole, along with some ionized gas streamers that are heated by the hot stars. [link] is a radio map that shows these gas streamers.

Sagittarius a.

Finding the heart of the galaxy

Just what is Sagittarius A* , which lies right at the center our Galaxy? To establish that there really is a black hole    there, we must show that there is a very large amount of mass crammed into a very tiny volume. As we saw in Black Holes and Curved Spacetime , proving that a black hole exists is a challenge because the black hole    itself emits no radiation. What astronomers must do is prove that a black hole is the only possible explanation for our observations—that a small region contains far more mass than could be accounted for by a very dense cluster of stars or something else made of ordinary matter.

To put some numbers with this discussion, the radius of the event horizon of a galactic black hole with a mass of about 4 million M _Sun would be only about 17 times the size of the Sun—the equivalent of a single red giant star. The corresponding density within this region of space would be much higher than that of any star cluster or any other ordinary astronomical object. Therefore, we must measure both the diameter of Sagittarius A* and its mass. Both radio and infrared observations are required to give us the necessary evidence.