Astronomy Chapter 15

The diameter is 100,000 light years and the thickness is 1,000 light years.

The disk, bulge, and halo [Spiral arms and global clusters]. The disk surrounds the bright central bulge. The entire disk is surrounded by a dimmer halo.

Stars in the disk follow roughly circular paths whereas the stars in the bulge and halo soar high above and below the disk on randomly oriented orbits.

The stars in the halo do not follow an orbit whereas the stars in the disk have a generally circular orbit.

The clouds and interstellar gas and dust.

In the spiral arms.

Star formation - nuclear fusion - returning gas - hot bubbles - atomic hydrogen clouds - molecular clouds. All stars return most of their original mass to interstellar space through the stellar wind and explosions. The wind sweeps the material creating a bubble of hot, ionized gas. The bubble breaks out of the disk and ejects gas. The gas cools which becomes atomic hydrogen gas. Gravity will draw blobs of gas together into tighter clumps which radiates energy. The molecular cloud will give birth to a cluster of stars.

Several stars have been observed to rotate an object in the center of our galaxy. The only possibility of having such a large mass in such a small volume is a black hole.