1. Light-year The distance light travels in one year. A unit of distance used to measure between stars.

2. Luminosity The amount of light energy emitted per second. "Brightness"

3. Apparent Magnitude Brightness of a star as seen from Earth

4. Absolute Magnitude How bright a star would look if it was close to Earth

5. Classification Placing things into categories based on shared characteristic

6. Absorption Spectrum Rainbow with dark lines. Produced when the light from a star passes through a cooler, less dense gas

7. Spectroscope An instrument that can be used to study the light that comes from stars

8. H-R Diagram Shows the temperature and luminosity of stars, also shows their classifications

9. Nebula Large cloud of gas and dust in space. Eventually forms a star.

10. Nuclear fusion When two atomic nuclei combine to form a larger nucleus with a higher mass. Produces the energy released by stars.

11. Red Giant What Sun-sized stars turn into when they run out of energy. They collapse and then rapidly expand and cool off.

12. White Dwarf The small, dense core of a giant star that remains. After sun-sized stars become red giants, they eventually become these.

13. Supernova The name of a supergiant star when it explodes before dying.

14. Neutron Star Remains after a supernova explosion. Very small and very dense.

15. Black Hole A region of space from which no matter or radiation can escape.

16. Galaxy Massive system of stars, dust, and gas held together by gravity.

17. Spiral Galaxy Can be barred or regular. Have a central bulge and spiral arms.

18. Milky Way Galaxy Our galaxy. Our solar system is located on one of the arms.

19. Elliptical Galaxy Oval-shaped galaxy

20. Irregular Galaxy Any galaxy that is not elliptical nor spiral

21. Big Bang Theory A theory that states that all of the matter in the universe was once contained in one tiny point. Since that time the universe has been expanding.