Stars, Galaxies, and the Universe

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Across
  1. 2. difference in position
  2. 3. very, very bright star
  3. 5. small, but very dense
  4. 7. system of many stars
  5. 9. neutral subatomic particle
  6. 10. brightness of a star
  7. 11. radiation from early universe
  8. 17. nothing can escape it
  9. 19. stars fusing hydrogen to helium
  10. 21. distance light travel
  11. 23. change in frequency of waves
Down
  1. 1. magnitude of celestial object
  2. 3. suddenly increases in brightness
  3. 4. likely a rapidly rotating neutron star
  4. 6. cloud, gas and dust
  5. 8. increase in brightness
  6. 12. universe originated from explosion
  7. 13. bigger than similar stars
  8. 14. reaction of atomic nuclei
  9. 15. proton without electric charge
  10. 16. small, very dense star
  11. 18. displacement of spectral lines
  12. 20. light, according to wavelength
  13. 22. scatter graph of stars