Space.

2. This is formed when a contracting cloud of gas and dust becomes so dense and hot that nuclear fusion begins.
6. There are three basic kinds of star clusters: open clusters,______, and globular clusters.
7. This shows that the universe is expanding.
8. This has a gravity so strong that anything falling into it is trapped.
10. This cannot be directly seen.
11. This is an area of gas that is cooler than the surrounding gases.
12. Astronomers theorize that at this single moment the universe was formed.

1. This is an explosion so violent that the dying star becomes more brilliant than an entire galaxy.
3. Enormously bright centers of distant, young galaxies.
4. A large cloud of gas and dust spread out over a large volume of space.
5. This galaxy had a bar through the center with the arms extending outward from the bar on either side.
9. The sun stays stable because the inward pull of _______ balances the outward push of thermal pressure from nuclear fusion.