Why will the sun expand?
So I know when the sun runs out of hydrogen, it becomes denser and contracts, but how does a red giant expand? What is happens in this process?
In: Physics
I absolutely read this as “why do red giant pandas expand” and got more excited than I care to admit because of the possible cute videos. I need some coffee.
A star in its normal phase (like our sun is at the moment) is actually a really delicate balancing act.
In the core, the enormous amount of energy being generated through nuclear fusion is trying desperately to expand outwards (like any “explosion”) — if it had its way it would blow the star apart.
But at the same time the sun is heavy. Really heavy. Its total mass is equivalent to over 300,000 planet earths. All that mass concentrated around a central point causes gravitational attraction, and gravity is trying to squash the star inwards — if it had its way it would squeeze the star into a tiny space.
So that’s the balancing act — nuclear-fusion energy pushing outwards, and gravity pushing in. If one of those forces changes significantly, the star will change. Depending on how massive the star was to begin with, different sets of changes will occur.
In the case of a medium-sized star like our sun, when it runs out of hydrogen in the core, hydrogen begins to fuse in the layers of the star outside the core. This causes a huge outpouring of energy and, because the mass of the star hasn’t changed, the star begins to expand. As it expands it cools, and eventually becomes the sort of star we know as a red ~~dwarf~~ giant (thanks /u/HorseDropping!).
Heat, fusing of hydrogen generates a lot of energy fusing of helium generates a lot more so once a star moves on to fusing helium it heats up and expands. https://youtu.be/64L0Dv55_Cw
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