In ancient times the Sun was revered in cultures around the world as a God, worshipped for its light and life giving powers. Thousands of years of research and vast improvements in technology have led to the realisation that our Sun is in fact a very average star, just one amongst billions in the Milky Way, which is in turn just one of trillions of galaxies in the Universe. And like all these trillions of stars, the Sun, which has given rise to so much life, will one day, inevitably, die.
It will not be a slow process, nor do you need to worry as it won’t happen for a few billion years. Happen it will though, and there is, as yet, nothing that humanity can do to prevent it.
Like all stars, the Sun generates its power by fusing billions upon billions of Hydrogen nuclei into Helium nuclei every second in its core, which requires immense pressure (~3.84 trillion psi) and temperature (15 million degrees Celsius) in the solar core. The energy released by this fusion is what bathes our tiny world in light and has fuelled the planet for untold millennia. More importantly for the Sun, it prevents the Sun from collapsing under its own weight due to gravity. The Sun weighs in at 2 x 10^30 (2 followed by 30 zeros) kg, or around 330,000 Earths, and generates a strong enough gravitational field to keep all the planets in orbit around it, so the struggle to overcome its own gravity is a pretty tough one. It’s a struggle that the Sun has been able to win so far, thanks to nuclear fusion, but gravity always wins in the end.
The Sun has only a finite amount of Hydrogen in the core and once it’s all been fused into Helium, in about 5 billion years, the Sun will enter its final stages. Gravitational collapse will begin, heating the stellar material and compressing it further until the regions around the core are hot enough for Hydrogen fusion to begin. The energy released by this fusion heats the outer layers of the core significantly, causing them to expand greatly. The Sun will become a Red Giant, around 100 times larger than it is currently, swallowing up and vaporising Mercury, Venus and likely Earth. The material ejected in this expansion will form a beautiful iridescent nebula that may birth stars of its own in the distant future.
As for the core, gravity will continue its merciless compaction. Temperature and pressure will rise and rise until the Helium fused in the main stage itself fuses, becoming Carbon, which may also fuse with Helium to become Oxygen. Much as before, the energy released by the fusion will stave off further collapse for a time, though for a far shorter time now, as Helium fusion is less efficient than Hydrogen fusion and there are simply fewer Helium nuclei to fuse. Once the Helium is exhausted, gravity will once more assert itself.
The core, now comprised of Carbon and Oxygen, will collapse further, becoming a White Dwarf. Now collapse is finally halted, but not by nuclear fusion now. Instead the collapse has forced electrons so close together that Pauli’s exclusion principle from quantum mechanics comes into play. Simply put, this principle states that two electrons cannot occupy the same space. This leads to what is known as electron degeneracy pressure, as the electrons exert a force to comply with the exclusion principle. This will hold the final structure of the Sun up against gravity, at a fraction of its current size and brightness. In time, even this dead remnant will cool and fade, to a hypothetical black dwarf. The Sun will be no more.
This is no special path, no unique destiny; our star is exceptional only in that it is very average. Indeed, this is why we can know so much about the fate of the Sun, as we can study similar stars across various stages of their lives. Such a detailed knowledge of future events on the grand scale of the cosmos is a testament to the millennia of development of scientific advancement and development. Even Gods may die, but knowledge and ideas cannot.