Center of the Galaxy

Here is a wide angle infrared view that shows the central bulge. (It if from the COBE satellite.)

We can see into the very center of the Galaxy (in the constellation Sagittarius) with infrared and radio telescopes. The results are discussed in Sec. 25-7 of our book, which includes nice pictures.

The main result is that there is a strong radio source called Sagittarius A* at the center of the Galaxy.

• Sagittarius A* is quite small, smaller than about 10 A.U..
• There is lots of molecular gas within a few light years of Sagittarius A*.
  Recall that this means that the gas is dense and cool.
• Within 5 light years of Sagittarius A* there is also lots of gas, but it is ionized
  Presumably it has been ionized by ultraviolet radiation from Sagittarius A*.
• Infrared observation shows 300 bright stars within 1 ly of Sagittarius A*.
• How much mass is concentrated at the galactic center?
  • Doppler shift measurements show how fast things are moving.
  • Things are moving fast because they are in orbit, pulled by a strong gravitational source.
  • The faster things are going, the more mass must be in the middle pulling.
  • Astronomers can use this to estimate the mass from observations of the speeds.
    - We will see precisely how to do this for the outside parts of galaxies.
  • There are observations of speeds for gas clouds and for the stars seen in infrared.
  • Result: mass in the central cubic light year is about 10⁶ times the mass of the Sun.
• What small source could produce substantial radiated energy and have a mass 10⁶ M_sun?
  • It is debated.
  • Perhaps it is a black hole.
  • We will look for evidence of black holes at the center of other galaxies.
  • In principle, the region just next to a 10⁶ M_sun black hole could radiate an enormous amount of energy from things falling in to the black hole.
  • If our Galaxy has a black hole in the middle, it is rather quiet.