as the sun gets older why will it expand when the heat from fusion is keeping it from collapsing in on itself?

Question

why does it get hotter when it gets older

Answer 1

When the sun was first born it fused hydrogen in its core. Hydrogen requires a temperature of at least 10-million degrees Kelvin to begin fusing helium. Once the hydrogen supply is exhausted the sun will cool drastically and shrink under its own gravity. This compression heats up the core more and more until a temperature of some 100-million Kelvin is reached. At that point helium fusion begins and the higher temperature expands the sun well beyond its main sequence diameter. This cycle of fuel exhaustion-cooling-compression-renewed fusion of heavier elements-expansion continues until finally there's no elements left that allow further fusion and the sun shrinks down to a white dwarf star with a diameter about that of Earth.

Answer 2

The answer is complicated. The history of any given star depends on mass and initial composition. For most stars with about the same mass as our sun, there's enough hydrogen at the core to generate enough energy from hydrogen fusion reactions to maintain a hydrostatic equilibrum at the core. When this supply of hydrogen starts to run out, a gravitational collapse begins and helium fusion reactions take over, increasing both temperature and energy output. For a short time, the outer layers of the sun still containing hydrogen fuse at an accelerated rate, causing the sun to become a red giant, until this hydrogen, too, becomes exhausted. The core itself will continue to collapse, and fusion reactions will involve heavier atoms such as carbon and oxygen. Eventually our sun will become a white dwarf. This is a simplified answer. It's important to keep in mind that while many stars have similiar histories, no two are ever exactly alike, because it all critically depends on the varying populations of elements and their nuclear reactions within the star over its history.

Answer 3

As it uses up its hydrogen fuel, it starts burning helium at its core. This burns hotter than hydrogen, so the heat will cause the sun to expand because gravity isn't enough to keep it from expanding.

Answer 4

because it is getting hotter. also it is getting expanded due to gasses. also after they get really old they get tiny.