Answer to Question 1
The protostar assembles from the molecular clouds, heats up from gravitational contraction, and begins hydrogen fusion in the core. The star settles onto the main sequence, where it will fuse hydrogen in its core for 10 billion years. When the core hydrogen is used up, the core contracts until it is degenerate, hydrogen fusion continues in a shell outside the core, and the outer layers expand and cool the star becomes a red giant. Helium fusion begins in the core, but since the core is degenerate a helium flash takes place and rapidly spreads throughout the core. Helium fusion stabilizes, and the star moves left on the H-R diagram. Core helium is used up and helium begins fusing in a shell outside the core, with hydrogen still fusing in a shell above it. The outer layers expand, and the star again becomes a red giant.
The star undergoes thermal pulses and loses its outer layers through a stellar wind. The core shrinks and heats up but is not able to fuse any more elements. The star becomes a planetary nebula as heat from the core blows away and heats up the gas left over from the red giant phase. Only the naked degenerate core is left, a white dwarf.
Answer to Question 2
B
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