Stellar Evolution
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ACROSS
2. In a main-sequence star, the gravitational forces are balanced by ____.
4. An atom becomes a(n) ____ when it loses or gains an electron.
6. An exploding star. Our Sun will not become one of these.
8. A star that retains more than 3 solar masses at the end of its life become this kind of hole.
10. At the "event ____" of a black hole, the escape velocity is equal to the speed of light.
11. Where in the Sun the most energy is produced.
12. In the Sun, 657 million tons of hydrogen are converted to 652½ million tons of helium every second. The other 4½ million tons of material are converted into ____.
16. When the core of a massive star (between 1.4 and 3 solar masses)is crushed to the point where electrons combine with protons, it becomes a ____ star.
19. A weakly interacting particle produced during nuclear reactions.
20. A "black ____" is an object so dense that it has disappeared, in the sense we can no longer receive radiation from it.
21. Type II supernova explosions generally occur in ____ (size) stars.
23. The surface temperature of a red giant star is ____ than it was when the star was a yellow dwarf.
24. His equation (e=mc^2) allows us to calculate the energy produced by the Sun.
26. This kind of dwarf star retains less that 1.4 solar masses at the end of its life. Our Sun will become one of these.
27. Stars are probably being born in these regions rich in dust and gas.
 DOWN
1. The age of the Sun is about five ____ years.
3. A(n) ____ is an alternate form of an element having a different number of neutrons.
5. The formation of new elements that occurs during supernova explosions.
7. When a red giant star becomes unstable, it may eject shells of gas into space forming this kind of nebula.
9. The electrical charges on the ____ and electron are equal but opposite.
10. The most abundant element in the Universe.
13. The particles found in the ____ of an atom are the proton and the neutron.
14. Compared with low-mass stars, high-mass stars have main-sequence lifetimes that are ____.
15. A "white ____" is the planet-size, hot remnant of a lightweight star after its red-giant phase is completed.
17. A ____ becomes a true star when its core finally becomes hot enough for fusion.
18. Rapidly rotating neutron stars.
22. The fusion process, which produces energy in most stars, forms ____ from the nuclei of hydrogen.
25. Explosions produced by interaction between a red giant and a white dwarf star in binary systems. These do not destroy the stars.