Why is the sun expanding slowly

Adventure universe

And it is getting hotter!

Contribution from gravi ┬╗Sun 26 Mar 2017, 4:27 pm

Sometimes you have to be able to admit mistakes - which I am doing with this.
I've done a lot of research now, on the web and in many of my books. Unfortunately, I did not find a concrete statement anywhere-
but with a little thought it is simply crystal clear:
In the center, more and more of the hydrogen is fused to form helium nuclei. Currently at ~ 16 million K. About 35% of the H supply has been used up. The resulting helium now slows down the speed of the H fusion somewhat, which means that less energy is released. Gravitation is happy about that, because it can now compress the core further. The logical consequence of this is an increase in temperature and pressure. The core becomes hotter, expands and the sun's radius also increases due to the higher temperature. This also slowly increases the surface temperature. Then I read somewhere that as a result the average temperature on earth will have risen to over 30 degrees in 0.9 billion years - too much to allow organic life.

By the way, the radiation pressure in the core of sun-like stars can practically be neglected. It only has a noticeable effect on much more massive stars. In the sun it is mainly the density of the core that stands up to gravity, the electron gas has not yet degenerated here either.

greeting
gravi
Our knowledge is a drop. What we don't know is an ocean.
Sir Isaac Newton