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This image, actually a painting, shows the entire sky. The Milky Way is the luminous band of light stretching across the picture. Note the clouds of dust which create dark patches within the Milky Way.
This is a computer simulation of a rotating disk galaxy. The disk is shown in blue, the central bulge in yellow-white, and the dark halo in red. As the galaxy spins, the camera pulls back to show the true extent of the dark halo.
This figure shows orbits in a galaxy like the Milky Way. Note that these orbits are not closed ellipses; Kepler's 1st law is not valid within a galaxy because the mass is spread out instead of concentrated at the center. The blue orbit is that of a typical disk star (Population I); it stays close to the disk plane. The red orbit is that of a typical halo star (Population II); the plane of its orbit slowly rotates.
A `globular cluster' is a swarm of about 106 stars held together by gravity. All the globular clusters in our galaxy are quite old (Population II); they are relics of the early stages of the Milky Way's formation.
`Open clusters' are younger (Population I) and smaller than the globular clusters shown above. In the Lagoon Nebula we see a gas cloud in which young blue stars are still forming. The Double Cluster a few million years older.
This is a computer simulation of the formation of dark halos by gravitational collapse. In this animation, color shows density. Note how several small halos merge to form a single larger one; such mergers probably built up the Milky Way's halo more than 10 billion years ago.
Homework 12: Shape &
Mass of the Milky Way, due 11/15.
NOTE: new policy on late homework! Too many people are turning in their homework late; this make it very difficult to grade and return homework on time. Starting with this assignment, I will deduct a full letter grade for any homework turned in after Monday's lecture.
Quiz 12: Understanding the Milky Way