Death of a Star: What Happens When Things Go Dark

Article March 22, 2025

This week came news of a study suggesting the possibility of explosive star deaths playing a role in some of Earth’s mass extinctions. It got me thinking about star deaths (although let’s face it, the odds are pretty decent that at any given time I might be thinking about star deaths because it’s a fun topic to think about).
When you ask kids in a planetarium show what happens to stars at the end of their lives, they’ll almost always tell you they explode. But of course there’s more than one way for a star to die, and it all comes down to the mass of the star. So join me in pondering the deaths of stars and lets find out (admittedly on a very basic level because that’s what I have the space for) what can happen when a star reaches the end of its life.

Diagram showing the different layers of fusion the most massive stars are undergoing by the ends of their lives. Credit: Astro-Edu

How to Shine

To start with, what do I mean when I say a star reaches the end of its life? I mean when the star is no longer able to fuse elements in its core. Stars shine mostly by using the intense temperatures, gravity, and pressure in their insides to smash together atoms and force them to undergo nuclear fusion.

All stars can fuse hydrogen atoms into helium (this is, fundamentally, what makes them stars). The more massive the star, the heavier the elements it can force to fuse. For instance, the smallest stars don’t have the internal conditions to force helium atoms to fuse, so once they can no longer fuse hydrogen they will reach the end of their lives.

Stars like the Sun can reach a stage towards the end when they are able to fuse helium into carbon, but that’s the end of the line for them. Towards their ends the biggest stars will look like a crazy onion, with different layers managing different kinds of fusion, all the way into the core where silicon fuses into iron.

All of this fusion helps a star maintain an equilibrium that carries it through its life. Think of a star as a battle between two extreme forces. They are massive objects and have a lot of gravity. Gravity wants to pull all parts of the star inward towards the core. But the star is also generating a whole lot of heat and energy inside via all that fusion, which creates an outward pressure force that tries to make the star expand. That drive to expand is counteracted by the inward pull of gravity.

For a star that is still fusing elements steadily, these two forces will be more or less in balance and you will have a healthy star in the middle of its life. Once a star has reached the point where it is incapable of fusing anything more, one of those forces is going to win.

The bright star in this image is the Sirius. The arrow points out its companion, Sirius B, the closest white dwarf to Earth at roughly 8.6 light years away. Credit: NASA/ESA/H. Bond and M. Barstow

Tiny Stars

Although bigger stars are able to go through many levels of fusion, it’s the tiny stars that live the longest. Technically we don’t know what happens when the smallest stars die because none ever have. The very first red dwarf stars that were born early in the universe’s history are still around. They are expected to last for trillions of years, and the universe is only 13.8 billion years old. But we expect that when these little ones go, they’ll go quietly.

We think that once they’re out of hydrogen to fuse they’ll start to collapse under their own gravity until the electrons in their atoms refuse to get any closer to each other. At this point the electrons will start pushing against each other, creating a force called electron degeneracy pressure. The mass of these stars is low enough that this pressure can counter their gravity and they stabilize in this state. We now call them white dwarfs. Technically they’re still shining, but at this point they are considered stellar remnants—dead stars.

Medium Stars

The planetary nebula known as the Southern Ring Nebula as imaged by the James Webb Space Telescope. Credit: NASA/ESA/CSA

For stars up to 8 solar masses, the core will also collapse down into a white dwarf, so gravity will win that bit. But pressure wins the rest of it. As these stars begin fusing helium it increases the outward pressure shoving at their outer layers. They begin to expand. As their outer layers move farther from the heat of the core, they’ll also cool off and turn redder. At this point we call it a red giant.
When this happens to our own Sun, in about 5 billion years, it will expand out as far as Earth’s orbit. Mercury and Venus will get swallowed. Earth miiight get swallowed, but the expansion of the star may also cause the planets to move outward a bit, and it’s possible Earth will survive this stage.

A star will maintain this state for a relatively short amount of time, no more than 100 million years. Eventually the contraction of the core will generate so much heat (and therefore outward pressure) that the outer layers will be ejected completely and begin to drift outward into space.

The abandoned core, now a white dwarf, is still shining and lighting up these lost outer layers of gas. This creates what is known as a planetary nebula, among the prettiest things you can find out in the cosmos (in my humble opinion).

Theoretically the white dwarf will eventually cool itself off and go dark. At that point it would be known as a black dwarf. However, as this is also expected to take many trillions of years, nobody has bothered to look for black dwarfs. They wouldn’t exist yet.

Big Stars

Then, of course, there are the big stars, the ones with more than 8 Suns worth of mass. They will also go into a red giant phase at the end of their lives, as their multiple layers of fusion generate more and more outward pressure. But once their core has turned entirely to iron, no more fusion will happen there. The outward pressure drops and the tremendous gravity of these enormous stars takes over.
Everything collapses inwards toward the core. A whole lot of physics happens, but the basics are that the sudden implosion of so much material into such a small space causes a rebound outwards, generating a massive shockwave. That shockwave intercepts the collapsing outer layers and violently reverses their course, throwing them outwards at ridiculous speeds.

The Crab Nebula, a supernova remnant from a supernova that erupted in the year 1054, as seen by the Hubble Space Telescope. Credit: NASA/ESA/J. Hester and A. Loll

Meanwhile all of this is accompanied by a huge release of energy, hundreds of times what the Sun will produce over the course of its entire lifetime. The brightness of this energy is what we can see as a supernova, some of which can be seen way on the edge of the visible universe. When the light of the explosion fades, the rapidly expanding bubble of the ejected outer layers is visible as a supernova remnant (also a very pretty sight, if slightly secondary to planetary nebulae in my personal opinion).

The cores of these stars continue their collapse. They’re massive enough that the push of electron on electron cannot counteract their gravity and they collapse right through the white dwarf phase until the neutrons of the atomic nuclei are pushing on each other. This is called neutron degeneracy pressure, and if the core is less than a little over 2 solar masses, the collapse stops here. It’s now a stellar remnant called a neutron star, a weird little thing.

If the core mass is greater than that, then not even the neutrons can stop the collapse. It continues its collapse until it is so dense that it forms that most mysterious of objects, a black hole.

When the Lights Go Out

This is all, obviously, a highly simplified version of what goes on. It also doesn’t account for the fact that recent studies have found evidence of giant stars collapsing directly into black holes without going supernova first, which is still mind-boggling to me. The universe is an incredibly complicated place and it likes to keep proving that fact to us.

But one thing is certain, and that’s that stars eventually die. It can take ridiculous trillions of years for the tiniest ones or only a few million for the most massive ones, barely an eyeblink in the history of the universe. But they all go sometime, and they usually have something to say as they do.

A diagram showing the possible lifecycles of stars based on their masses. Credit: Encyclopaedia Britannica, Inc.

But it’s thanks to the death of stars that we exist! If you’ve ever heard Carl Sagan’s speech about us being made of star stuff (or Delenn’s on Babylon 5 if you’re a particular type of sci-fi nerd like me), that’s a reference to the fact that many elements that make up our planet and our bodies were first made inside of stars through stellar fusion, or were generated in the immense energies of a supernova explosion. The deaths of these stars scattered those elements into the cosmos where they became part of the next generations of stars and planets, including our own.

So we are made of star stuff, and without the deaths of the stars that came before us we wouldn’t be here to admire the stars that burn today. Take a moment the next time you see a clear night sky to appreciate those long-lost lights for our stardust-y existence. And, you know, also just enjoy looking at the stars. They’re not going to last forever after all!