Stars begin their-lives when hydrogen fusion ignites in their dense, hot cores. When that process starts, it’s game on. The gravitational pull of all the mass of the star tries to squeeze it down into a small point, but the energy released by fusion pushes outward, creating a delicate-balance which will persist for millions or even trillions of years.
Small stars live an incredibly long-time. Due to their small stature, they do not need tons of energy to balance the inward gravitational pull, in order that they only sip at their hydrogen reserves. In a bonus boost, the atmospheres of those stars constantly circulate, pulling fresh hydrogen down from the outer layers into the core, where it may fuel the continuing fire.
All told, a typical red dwarf star will happily burn hydrogen in its core for trillions of years. Not too-shabby.
As these small stars age, they steadily become brighter until they only kind of vaguely sputter out, becoming an inert, boring lump of helium & hydrogen just hanging round the universe minding nobody’s business but their own.
It’s a sorrowful fate, but at-least it is a quiet one.
The grand finale
When the huge stars in our universe die, it’s far more violent. Due to the increased bulk of those stars, fusion reactions got to happen much faster so as to sustain the balance with gravity.
Despite being such a lot heavier than their red dwarf star cousins, these stars have much shorter life spans: Within only a couple of million years, they die.
But when massive stars die, they go-out in all their glory. Their huge size, means there’s enough gravitational pressure to not-only fuse hydrogen, but as well as helium. And carbon, oxygen, magnesium, silicon, an honest number of the element on the periodic table are produced inside these giant stars near the end of their lives.
But once these stars form an iron core, the music stops & the party’s over.
All that material surrounding the iron squeezes in on the core, but iron fusion doesn’t release energy to counter-act it. Instead, the core contracts to such incredible densities that electrons get shoved inside protons, turning the whole core into an enormous ball of neutrons.
That neutron ball is able to temporarily, at least, resist the crushing collapse, triggering a supernova blast. A supernova will release more energy in a week than our sun will release over the course of its whole 10 billion years lifetime. The blast wave & material ejected during the explosion carves bubbles in the interstellar medium, disrupts nebulas and even sends material spewing-out of galaxies themselves.
It’s one among the most spectacular sights in the whole universe. When supernovas happen in our neck of the galactic woods, the explosions are bright enough to seem during the day and may even be brighter than the complete moon in the night.
Pretty intense & what a way to go.
One last show
It’s the medium-size stars that suffer the worst-fate. Too big to only go-off quietly into the night and too small to trigger a supernova blast, they instead become gruesome monsters before finally turning themselves inside out.
For these medium stars (which includes stars like our sun), the matter is that when a ball of oxygen & carbon forms in the core, there’s not enough mass surrounding it to fuse it into anything heavier. So, it just sits there, getting hotter by the day. The remain of the star reacts thereto inferno in the core, swelling & turning red, producing a red giant. When our sun turns into a red giant, its edge will reach near the orbit of the Earth.
That red giant star phase is unstable and stars like our sun will convulse, collapsing & reinflating over & over, with each event launching winds carrying the bulk of the sun’s mass-out into the solar system.
In its final death throes, a medium-size star spews-out its guts to make an effervescent planetary nebula, thin wisps of gas & mud surrounding the now-exposed core of carbon & oxygen at the center. That core gets a new-name when exposed to the vacuum of space: a white dwarf star.
The white dwarf star illuminates the surrounding planetary nebula, energizing-it for about 10000 years before the stellar corpse cools an excessive amount of to enable such light shows.
While beautiful & bewildering to behold in a telescope, planetary nebulas are the products of a violent, tortured death of a star. Alluring, yes, but also haunting to contemplate.