Center of the Galaxy
Radio and infrarred observations allow astronomers to observe the
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, much smaller than 1 ly in diameter.
- 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
- 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 Msun?
- It is debated.
- Perhaps it is a black hole.
- We will look for evidence of black holes at the center of other
- In principle, the region just next to a 106 Msun
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.
Davison E. Soper, Institute of Theoretical Science,
University of Oregon, Eugene OR 97403 USA