Astronomy Crossword Puzzle

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Across
  1. 3. A cloud of gas and dust in outer space
  2. 5. A celestial object consisting of a nucleus of ice and dust
  3. 7. A group of stars forming imaginary pictures
  4. 9. A small body of matter from outer space that enters the earth's atmosphere
  5. 11. A unit of astronomical distance equivalent to the distance that light travels in one year
  6. 12. Disk-shaped region of minor planets outside the orbit of Neptune
  7. 13. A system of millions or billions of stars, together with gas and dust, held together by gravitational attraction
  8. 15. Occurs after low or medium density star phase
  9. 18. The "brain" of the comet
  10. 19. Former Meteor
  11. 20. A massive and extremely remote celestial object, emitting exceptionally large amounts of energy
Down
  1. 1. A diffuse cloud of gas and dust surrounding the nucleus of a comet
  2. 2. Occurs after high-density star phase
  3. 4. Belt of asteroids
  4. 6. A spherical cloud of small rocky and icy bodies postulated to orbit the sun beyond the orbit of Pluto
  5. 8. Contains neutrons
  6. 10. A contracting mass of gas that represents an early stage in the formation of a star
  7. 14. Region of space from which nothing, not even light, can escape
  8. 16. An explosion in space
  9. 17. A small rocky body orbiting the sun