6. THE GALACTIC CENTER:

Have yet another look at the Multiwavelength Milky Way. In the optical band you will see nothing special at the center of the image, because the light is obscured by dust clouds. But you will see a bright spot there at radio, infrared, and gamma rays. This bright spot is called Saggitarius A, because it was first noticed by radio astronomers as the brightest radio source in the constellation Saggitarius. Now we know that Saggitarius A is located at the very center of the Milky Way.

Below are X-ray and radio images of the center of the Milky Way taken at higher angular resolution (with the Chandra and VLA telescopes, respectively). This movie (source: Chandra) first pans the X-ray image of the galactic center and then fades from the X-ray to radio to infrared.

X-ray Image of the center of the Milky Way galaxy. Scale is 0.8o x 2o, or about 400 x 900 light years. Colors represent temperature, with blue referring to X-ray sources with T > 108 K while red refers to T < 107 K. Hundreds of blue point sources are neutron stars, black holes, and white dwarf stars. The diffuse red glowing areas are hot (> 106 K) gas between the stars. The bright source at the center of the image is due to X-rays emitted by hot gas flowing into a black hole at the galactic center. Click on image for a bigger image. See Chandra for further details.

Low Frequency radio image of the center of the Milky Way Galaxy (source), showing several supernova remnants and a bright radio source at the center of the Milky Way (called Sagittarius A). This image is tilted clockwise by about 60o compared to the X-ray image above. The scale is approximately the same.

We know that Sagittarius A is also producing anti-matter. Also notice the arc to the upper left of Sagittarius A: something wild is going on there! What? Be sure to check Antimatter in center of Milky Way. Based on these observations, astronomers suspected that there was some sort of monster at the center of the Milky Way -- perhaps a supermassive black hole. The gravity of such an object might account for the dense cluster of tens of thousands of stars there (which you can see as the bright spot in this infrared image: The galactic center across the infrared).

Recently, astronomers found compelling evidence for the black hole hypothesis. Using very precise infrared imaging techniques, they were able to see the images of stars moving around the center of the galaxy, as illustrated below.

The stars are moving as if they are orbiting a compact object weighing 2.6 million Solar Masses! We know of no other force than gravity that can account for the motions of these stars, and a black hole is the only object we know that can be so massive and yet so compact. But even this amazing object is rather puny when compared to the supermassive black holes that we can see in other galaxies, as you will see later in this course.

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Last modified March 9, 2002
Copyright by Richard McCray