Question

When the helium fuel runs out at the center of a horizontal branch star, what happens?

1) The star collapses under its own gravity
2) The star expands and becomes a red giant
3) The star undergoes nuclear fusion of heavier elements
4) The star forms a white dwarf

Answer 1

When the helium fuel runs out at the center of a horizontal branch star, the star undergoes nuclear fusion of heavier elements, such as carbon and oxygen, through the triple-alpha process.

Step-by-step explanation:

When the helium fuel runs out at the center of a horizontal branch star, the star undergoes nuclear fusion of heavier elements.

The star's core begins to collapse under its own weight, and the heat released by the shrinking of the carbon and oxygen core flows into a shell of helium just above the core. This helium, which had not been hot enough for fusion into carbon earlier, is heated just enough for fusion to begin and to generate a new flow of energy. This fusion process produces carbon and sometimes a bit of oxygen through the triple-alpha process.

In stars with masses about twice the mass of the Sun or less, fusion stops after the helium in the core has been exhausted. Fusion of hydrogen and helium in shells around the contracting core makes the star a bright red giant again, but only temporarily.