The Sun is, without a doubt, the most important the single most important object that humanity, and any living thing, will ever come across. Each and every day that humanity and its genetic ancestors have been alive, they have looked toward the Sun. It’s the source of countless myths, stories, legends, and religions.
It is the reason the earliest forms of life on Earth were able to pump oxygen, the by-product of photosynthesis, into the atmosphere. It is, to this day, the source of almost all of the energy we use and consume.
And we have only recently reached the point where we can investigate the Sun directly, in fact, in late 2021, the Parker Solar Probe touched the Sun for the first time.
The is a delicate balance of exploding nuclear fusion and constricting gravity, a balance that was struck many millions of years ago when the Sun first formed.
Stars can go one of three ways when they form in solar nebulae. They can, like ours, gather enough material that it almost perfectly balances the internal pressures off nuclear fusion within their cores. This is known as a Main Sequence star, and they are stable for billions of years as they chew through their not quite endless supply of fuel.
What if the protostar doesn’t gather enough material in the solar nebula to itself? If it never reaches the critical point of ignition (which is 0.08 the mass of our Sun), then the star is considered a Brown Dwarf. In short, there wasn’t enough hydrogen and pressure at its core to generate the heat needed for nuclear fusion on the scale that is required for a stable main sequence star.
‘A light that burns twice as bright burns half as long’.
This quote, adapted from Taoism and popularised in the movie Bladerunner, applies perfectly to the final type of star, the giants. If a solar nebula has enough available fuel the core will burn hot enough that it runs out of hydrogen and helium and begins to burn heavier elements like lithium and carbon. Once a star hits this point it has effectively run out of fuel, but its mass is incredibly large and without the outward pressure of nuclear fusion, it will collapse under its own gravitational pull.
The result is a supernova which are some of the largest explosions in the Universe.
Our star will run out of hydrogen one day and when that happens the helium filled core will continue to be compressed by gravity, but the lighter, outer layers of the Sun will expand until it consumes all of the inner planets, including Earth. At this point the Sun will have moved into one of the final stages of its lifecycle, a Red Giant. As fusion continues and the Sun burns carbon and lithium, reactions that don’t create anywhere as near as much energy as hydrogen burning, the Sun will collapse and take its final form, a White Dwarf, surrounded by an ephemeral aura of gas known as a planetary nebula.
But if you’re worried about this happening anytime soon, don’t be. The Sun is barely halfway through its life, it has another 5 billion years before it will expand and consume Earth.
It will be here far longer than the rest of us.
References:
https://astronomy.swin.edu.au/cosmos/s/Stellar+Evolution
https://www.atnf.csiro.au/outreach//education/senior/astrophysics/stellarevolution_deathlow.html
https://www.aanda.org/articles/aa/full_html/2017/03/aa28260-16/aa28260-16.html
https://www.frontiersin.org/articles/10.3389/fspas.2020.617946/full
Physics and Fiction 
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