The Universe

2. GIANT A very large, luminous star with a low surface temperature.
3. The theory that the universe originated from an extremely hot, dense state.
5. A natural satellite that orbits a planet.
7. A large group of stars, gas, and dust held together by gravity.
9. A spacecraft that orbits Earth, used for communication and observation.
13. An explosion of a star that releases a tremendous amount of energy.
16. A massive cloud of gas and dust in space where stars form.
17. A small, dense star that has exhausted most or all of its nuclear fuel and will eventually cool to become a black dwarf.
18. The force that holds the universe together.
21. Galaxies that resemble giant rotating pinwheels with a pancake-like disk of stars.
23. The state of having no gravity.
25. The most luminous type of active galaxies.

1. The phenomenon where a star collapses under its own gravity.
4. Galaxies that emit light across the electromagnetic spectrum and appear the brightest since the light is directed towards Earth.
6. Galaxies that have unusual shapes, like toothpicks, rings, or even little groupings of stars.
8. A very large and bright star; often among the most luminous of its type.
10. HORIZON The point of no return around a black hole.
11. A small, dense star that emits regular pulses of radio waves.
12. The most common type of star in the universe, relatively cool and dim.
13. The most common active galaxies that exhibit the lowest energies.
14. The path an object takes around another in space.
15. A bright object that leaves a trail as it enters Earth's atmosphere.
19. About 6 trillion miles.
20. A small rocky body orbiting the sun.
22. Galaxies that have the central bulge and disk common to spiral galaxies but no arms.
24. HOLE A region of spacetime exhibiting such strong gravitational effects that nothing—not even light—can escape from inside it.