Why does a stars diameter increase when burning Helium?

Question

Does Hydrogen fussion produce more energy than Helium fussion? If so why does the diameter of a star increase when burning Helium.

Answer 1

The star is not chemically burning. The reaction is nuclear fusion. The helium fusion takes place at a much higher temperature. It is this increase in temperature which is the cause of the increase in the diameter of the star.

Answer 2

I'll cover this in a bit of depth.

Stars expand during their entire main sequence life. However the expansion you are talking about is the red giant phase. This is indirectly caused by helium burning.

After hydrogen burn begins to stop the core shrinks and the temperature in the core raises until the density and temperature if high enough for helium fusion to start. However before this happens the heat from the contracting core raises the temperate in the shell of gas surrounding the core. This shell is hydrogen rich and the increased temperature starts hydrogen fusion. This is known as hydrogen shell burning (this can also occur when a star moves to from the pp cycle to the CNO cycle). This burning increases the pressure in the shell. As the star is in hydrostatic equilibrium the outer envelope of the star responds by expanding.

Because the luminosity remains constant the surface temperature must drop. This is seen as a reddening of the star. The star becomes a red giant.

Note that it has nothing do do with MORE energy being produced, just a change in location. Helium burning is LESS efficient that hydrogen burning by about a factor of 10. Hence stars burn helium ten times faster than hydrogen in order to keep in hydrostatic equilibrium.

If you wish to know more try "The Physics of Stars" by A C Phillips which is a standard work. If the maths looks daunting you can skip over it and still get a good idea of what is going on.

Answer 3

Helium fusion generates more energy than hydrogen. For hydrogen fusion to be sustained, 10-million degrees Kelvin is required. Helium requires 100-million degrees Kelvin. With the higher temperature involved with helium fusion, the diameter of helium-burning star will be forced outward.

Answer 4

yes, it's the red giant phase.