When will the sun run out of fuel, and what will happen when it does?
The Sun will begin to run out of fuel in about 5 billion years, where it begins to run out of hydrogen to burn to create energy, and so it begins to burn helium, and thus leaves the main sequence and begins its evolution into a red giant.
The Sun expands and Eventually swallows the planets of Mercury, Venus, and possibly Earth As well.
As helium begins piling up at the core, increased amounts of it become electron degenerate. As a result, the temperature and pressure within the core of the Sun become 10 times their current values.
About 1.2 billion years after leaving the main sequence, the Sun’s core becomes hot and sense enough to cause it to ignite and burn.
When the temperature reaches high enough values (about 100 million degrees) helium will begin to fuse into carbon through the triple alpha process.
Unlike the Sun when it contained normal matter, electron degenerate helium does not expand and cool when exposed to heat, it simply gets hotter.
Doubling the temperature causes it to run a trillion times faster. Helium fusion heats the Sun’s core, which cannot expand and cool, and as a result the increased temperature causes the helium fusion to occur even faster, which produces more heat, which causes helium to fuse even faster, the helium core of the Sun explodes.
About 6% of the electron degenerate helium in the Sun’s core is rapidly converted to helium, which equates to the Sun releasing as much energy in a few minutes as it does in 200 million years, as a result of the burning of the mass of 10 Earths of helium being burned in seconds.
This event is known as the helium flash. For a brief instant during the flash, the Sun’s luminosity equates to that of all of the stars in the Milky Way.
The Sun’s core reaches temperatures high enough to allow for individual nuclei to escape and the Sun’s core reverts to a normal gas, which powerfully expands, releasing enormous amounts of energy to compensate for the enormous amounts of gravitational energy needed to revert the Sun’s core from its white dwarf state.
After its collapse, the Sun cools exponentially In a short amount of time and the Sun will become a sub giant.
It is a star with a double energy source, a sense carbon oxygen core which is not electron degenerate, and a shell surrounding the core where helium is burned into carbon, and a second shell where hydrogen is burned into helium.
To maintain its sub giant state, the Sun needs to burn fuel extremely fast.
After 100 million years in the helium main sequence, the Sun once again begins to evolve into a red giant, however there is no carbon equivalent of the helium flash, and thus there is nothing to stop the Sun’s expansion, and the Sun’s outer layers balloon past the orbit of Jupiter.
The core is now even more electron degenerate, and it is smaller and denser than before.
Eventually, the shells surrounding the core begin to separate from the core and, one by one, the shells are blown away about 100 thousand years apart, as the shells burn fuel at different speeds.
The shells become clouds of gas which orbit the core, and the solar system becomes a planetary nebula.
Eventually, the shells dissipate and all that is left is a hot, sense stellar corpse: a white dwarf, which gradually cools until it no longer gives off any light or heat and becomes an extremely dense black dwarf.
Eventually, this same fate will befall all stars.
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