What happens when a main-sequence star exhausts its core hydrogen fuel supply? what happens when a main-sequence star exhausts its core hydrogen fuel supply?
a. the core shrinks, while the rest of the star expands.
b. the star becomes a neutron star.
c. the core immediately begins to fuse its helium into carbon.
d. the entire star shrinks in size.

Option A and C correct.

Explanation:

Both options are partially correct. The final cycle of a main-sequence star depends greatly on its mass when the hydrogen ran out.

For this case, let's take our sun for example. In billion of years, after the hydrogen is depleted, gravity will take over and start compressing the star, increasing the temperature to levels where the core can now fuse helium into carbon, this process will heat the star again, and the outer layers of hydrogen that remained during the main sequence, will be hot enough to reignite the fusion.

The sun will produce several times more energy than when it was in the main-sequence, and it will continue to expand, it will shallow mercury, venus, the earth itself and perhaps mars until it finally burns the remaining hydrogen. When there is no more fuel to power up the star, it will start to collapse itself, and shrink until it becomes a white dwarf, the last phase of the star.

The material of larger mass stars is burst out in massive explosions known as supernovas.

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