The MILKY WAY GALAXY (for the printable version, click HERE)

What is a Galaxy? Some history: Galileo and stars in the Milky Way. Spiral nebulae or island universes? The issue was resolved only in the 1920's. The view from here: The forest and the trees; need to use different wavelengths (infrared and radio waves to see through interstellar matter).

Our position: About 28,000 light years from the center, moving at 220 km/s.

The Milky Way: Size and Structure Main parts: Disk, halo, bulge; Compare with Andromeda galaxy (M31). The halo: Shape (from globular clusters); no gas/dust; redder, stars with few heavy elements [population II]. The disk: Shape (from 21 cm radio waves); spiral arms and interstellar matter; stars with heavy elements [population I]. Size: Diameter about 30 kpc; thickness varies, about 300 pc here; containing about 100 billion stars. The central bulge: Football shaped, 6 kpc ¥ 4 kpc. The core: Towards Sgr A, near M8; difficulty to see, look with IR and radio; A black hole with more than a million solar masses, possibly surrounded by many thousands of smaller ones.

The Milky Way: Formation, Motion, Mass Galaxy: Merger of previous fragments; flattening from rotation. Spiral arms: Just by rotation? Spiral density waves? Self-propagating star formation? Motion of stars in our neighborhood: Up to tens of pc apparently random. More distant stars: Rotation around center, and bobbing up and down the disk; 220 km/s; P = 225 Myr; motion along huge random ellipses in halo. Differential rotation: Rotation curve in disk; Cannot be explained in terms of the matter we see. Mass: Use Kepler's laws and rotation curve; inside Sun's orbit, 1011 solar masses!

Resource page.