Milky Way

3. a region of hr diagram in which stars are unstable to pulsation
6. stars that are poor in atoms heavier than helium
8. the spheriodal cloud of stars at the center of most spiral galaxies
9. stars rich in atoms heavier than helium
10. variable star with a period of 12 to 24 hours
11. the spherical region of a spiral galaxy containing a thin scattering of stars
12. a material confined to the plane of the galaxy
15. in astronomical usage, any atom heavier than helium
16. a graph of orbital velocity versus radius in the disk of a galaxy
17. the production of elements heavier than helium by the fusion of atomic nuclei in stars and during supernovae explosions
18. objects used to map the spiral arms

1. the process by which the birth of stars compresses the surrounding gas clouds and triggers the formation of more stars
2. the powerful radio source located at the center of the galaxy
4. the relation between period and pulsation
5. non luminous material that is detected only by its gravitational influence
6. the rate at which a star moves across the sky
7. a variable star with a period of 1 to 60 days
13. a unit of distance equal to 1000 pc or 3260 ly
14. orbital motion in accord with kepler's laws of planetary motion