Stars and Galaxy stuff

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Across
  1. 2. hottest layer of atmosphere
  2. 5. cooler areas on surfacing away
  3. 6. steam of charged particles
  4. 7. Galaxy that has young stars in the arms and old in the middles
  5. 9. 6 trillion miles
  6. 10. loop of gas
  7. 12. fusion of H2 and He
  8. 13. Collapsed neutron star, no light escapes
  9. 15. cloud of gas and dust
  10. 17. Galaxy that has stars everywhere
  11. 18. Super massive black hole at the center of a galaxy
Down
  1. 1. warm rises, cool sinks
  2. 3. tells us galaxies are moving away
  3. 4. squeezing atoms together
  4. 8. The raisins spread apart as the bread rises
  5. 11. Galaxy that only has old stars
  6. 13. start of expansion
  7. 14. Collapsed core of a high mass star
  8. 16. apparent shift