Universe

2. A nuclear reaction involving the spontaneous decay of unstable atoms
6. A nuclear reaction combining small atoms into larger atoms
8. A cloud of dust and gas in space likely to produce stars or planets
9. Theory of the origin and evolution of the universe
11. Electromagnetic waves from the Big Bang that have expanded to microwave frequency in the background of the universe
12. The further away a galaxy is, the faster it is moving away
13. One possible end to a giant star's life, caused by its core collapsing to a very dense state
14. The second stage in a star's life cycle - red giant or super-giant.
17. A nuclear reaction splitting large atoms into smaller atoms
19. Result of a radioactive atom converting one neutron into a proton and an electron to become a new stable atom
21. The amount of time in takes for half of a radioactive sample to decay and become stable
23. A force that allows elementary particles to transform
25. Extra high energy radiation released when a radioactive atom changes into a new stable atom

1. A young star in the early, stable stage of its life cycle
3. An observed change in frequency when the source of a wave or the observer is moving
4. The energy or brightness of a star
5. The result of a supergiant star imploding and blasting away its outer layers
7. Term for an atom with an unstable nucleus that is emitting large amounts of radiation (energy)
10. The end stages of a smaller star's life including losing outer layers then the hot inner core cooling
15. Attractive force between protons and neutrons in an atom's nucleus
16. One possible end to a giant star's life, caused by its core collapsing and vanishing
18. A large group of gas, dust, stars and satellites held together by gravity
20. Everything physical - matter and energy - that exists in space and time
22. A graph plotting stars by temperature and luminosity
24. Result of a radioactive atom losing 2 protons and 2 neutrons to become a new stable atom