How Stars Die

Nothing in this Universe lasts forever—not even the stars.

Throughout most of history, people believed that stars were eternal. They were seen as immortal deities in the sky or as timeless gems decorating the heavens. One of the greatest breakthroughs in twentieth-century astronomy, however, was the discovery that stars go through life cycles just like humans. They are born, they age, and eventually, they perish.

Today, I want to talk about that final chapter: the death of stars. Specifically, I will explore the different paths stars take as they die, which largely depend on their mass.

A Carbon Corpse

Nuclear fusion reactions are what keep stars alive. The intense gravitational forces at the core of a star generate extreme densities, temperatures, and pressures that force atomic nuclei to collide and fuse into heavier elements.

For most of a star’s life, it’s hydrogen—the simplest element—that fuses into helium, powering the star’s light and counteracting the force of its own gravity. However, the hydrogen supply at the core, where fusion occurs, is not limitless. Once the hydrogen is depleted, the star is living on borrowed time, and its initial mass becomes a critical factor.

Stars that begin their lives with less than eight times the Sun’s mass never achieve the conditions necessary for anything beyond helium fusion. As hydrogen runs out, the energy flowing outward diminishes, and the surrounding material weighs down on the core, causing it to shrink. Eventually, core temperatures soar to the point where helium can fuse into carbon—around 100 million degrees Celsius.

This phase temporarily prolongs the star’s life, but the helium fuel is also limited. When it’s gone, the star lacks the mass needed to ignite carbon fusion, leaving behind a dense, dead remnant composed of tightly packed carbon nuclei. This remnant, known as a white dwarf, is a stellar corpse that slowly fades to black.

A Big Star’s Fate is Written in Iron

Stars that start out with more than eight times the Sun’s mass meet a different fate. Once their cores convert helium into carbon, they possess enough mass to reach the extreme temperatures (500 million degrees Celsius) needed to ignite carbon fusion. The star then burns through a sequence of heavier elements, each leaving behind an “ash” from previous reactions: carbon turns into neon, then oxygen, and eventually silicon. But once iron accumulates in the core, the star’s journey ends; iron fusion consumes more energy than it produces.

With nothing left to support it, the iron-laden star collapses under its own gravity, triggering a massive explosion known as a supernova. The remnants of this explosion can take one of two forms: on the lighter side of massive stars, the result is a neutron star—a compact remnant supported by quantum forces. For the heaviest stars, gravity overwhelms even these forces, leading to the formation of a black hole.

The Stellar Figure is Eight

The classification of stars can be divided into two groups: low- and intermediate-mass stars with less than eight solar masses, and high-mass stars with more than eight solar masses. Low- and intermediate-mass stars end their lives as white dwarfs, while high-mass stars transform into neutron stars or black holes.

That critical number—eight solar masses—defines the boundary between different stellar deaths. Although there are many intricate aspects to a star’s end, this figure is the key to determining a star’s final fate.