Space Exploration

3. a measure of distance used to describe the position of planets relative to the sun
6. Earth-centered model of the solar system
9. Sun-centered model
11. a meteoroid that enters Earth's atmosphere
13. a contracting mass of gas in the first stage of a star's formation
14. an enormous explosion that marks the death of a massive star
17. the distance that light travels in a year
18. grouping of millions or billions of stars, gas, and dust held together by gravity
19. super-dense remnant of a supernova; an object around which gravity is so intense that even light cannot escape
21. vast clouds of gas and dust in space, where stars form
22. either of the two times a year when the Sun crosses the equator and day and night are of equal length

1. small, rocky bodies orbiting the Sun and lying mainly in a narrow belt between Mars and Jupiter
2. a small body that orbits a larger one
4. a method of indirectly measuring distance by creating an imaginary triangle between an observer and an object whose distance away is to be estimated
5. groupings of stars that form patterns in the night sky
6. the force of attraction between masses
7. one of the latter stages in the life cycle of a Sun-like star during which the star collapses
8. celestial body composed of dust and ice that orbits the sun
10. orbital path of a planet around the Sun
12. refers to all the pieces of debris that have fallen off rocks, satellites, space stations and space shuttles and remain floating in space
15. the height of a celestial body above the horizon, ranging from 0 at sea level to 90 straight up
16. the apparent shift in position of a nearby object against a distant background when the object is viewed from two different positions
20. the angle between the most northerly point of the horizon and the point