At what core temperature does hydrogen begin to fuse to helium?

Answer to q. 1

D

Answer to q. 2

As the spiral-density wave moves through the gas and dust clouds, it prompts the rapid formation of the massive blue stars we find in the associations, born from hot, pink emission nebulae. But these massive stars evolve very quickly, blowing up as supernovae when hardly out of the nursery; their supernova shock waves reshock the medium, leading to formation of many more less massive Population I stars like our own Sun farther behind the density wave.

Answer to q. 3

No, normal matter adds up to only about 1-3 of that to reach the critical density. Dark matter is perhaps 10 times more abundant, but it still adds up to only 20-30 of the mass needed for closure.

Answer to q. 4

C

Answer to q. 5

These brilliant explosions are consistent standard candles, visible out to billions of light-years. Instead of finding the most distant galaxies slowing down as expected in a closed universe, the supernovae were even more distant than expected, as dark energy kept the universe forever expanding.

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