Key concepts

1. The main parts of the Galaxy are the encircling halo, the flat disk, and the central nucleus, which contains the nuclear bulge and the galactic core.

2. Stars of different types inhabit these regions: the disk contains young, metal-rich Population I stars; the nuclear bulge older Population I stars; and the halo, metal-poor Population II stars.

3. The Galaxy can be roughly mapped out by counting stars in different directions. The problem with this method is the dimming of starlight by interstellar dust.

4. Stars move relative to each other through space at speeds of a few tens of kilometers per second; this appears as a star's radial velocity and proper motion in the sky, which relates to the star's distance and transverse velocity.

5. The sun's motion relative to nearby stars shows up in their proper motions away from the apex and toward the antapex in the sky, and also in their Doppler shifts.

6. The motion in space of the stars and the sun allows us to measure the distances to stars far beyond that doable by the trigonometric parallax method, but the statistical and secular parallax methods provide only average distances to groups of stars.

7. Models of the Galaxy developed early in the century typically made it too small and heliocentric. The breakthrough in our understanding of the Galaxy's size and the sun's location. It came from Snapley's use of the distances to globular clusters.

8. Two types of variable stars, cepheids and RR Lyrae stars, can be used through a period-luminosity relationship to find their distances. This technique allowed astronomers to establish firmly the size of the Galaxy.

9. The disk of the Galaxy contains a layer of stars about 700 pc thick and a blotchy distribution of gas and dust about 250 pc thick that peaks in density in the region interior to the sun's orbit.

10. The core of the Galaxy emits intense radio and infrared radiation and x-rays; it contains both older stars and young massive stars, dust, and gas rotating at high speeds. To account for the motion of the gas requires a concentration of millions of solar masses of material in the inner parsec-perhaps a supermassive black hole.

11. The Galaxy's halo contains globular clusters, some gas, and perhaps other so far undetected objects.