How stars die: White dwarfs, neutron stars and supernovae

Indian Express

20-05-2025

The stars in the sky seem to shine forever, but they too are subject to lifespans determined by their mass and internal physics. Some last for billions of years and others are transitory in comparison, with far shorter lifetimes.
Regardless of duration, all stars survive by maintaining a balance between two competing forces: the inward pull of gravity and the outward pressure produced by nuclear energy. In the dense core of a star, nuclear fusion converts hydrogen into helium. This process occurs because the extremely high temperatures and pressures force atomic nuclei — normally repelled by their positive charges — to collide and merge, releasing enormous energy. That energy heats the surrounding gas, generating thermal and radiation pressure that resists gravitational collapse.
This elegant explanation of how stars shine wasn't always known. It was only in 1938, while riding a train to Ithaca, New York, that German-American physicist Hans Bethe scribbled down the equations describing how nuclear fusion powers stars — a breakthrough that won him the Nobel Prize and solved one of astrophysics' most pressing mysteries.
But even fusion has its limits. When a star's fuel is exhausted, gravity takes over — and then the nature of its death depends entirely on its mass.
Stars with masses similar to or less than our Sun meet their end through a relatively gentle transformation. Once the core exhausts its hydrogen, fusion slows, and gravity causes the core to contract. As it contracts, it heats up. When it crosses a critical temperature threshold — about 100 million Kelvin — helium fusion begins, converting helium into carbon and oxygen.
Meanwhile, the surrounding shell of hydrogen just outside the core also heats up and reignites in a thin layer. This shell-burning dumps energy into the star's outer layers, causing them to expand dramatically. The star becomes a red giant—swollen, cooler on the surface, but far more luminous than before.
Eventually, the star cannot sustain further fusion. The outer layers are gently expelled into space, forming a glowing planetary nebula, while the core is left behind as a white dwarf: a hot, dense object roughly the size of Earth, composed mostly of carbon and oxygen. About 95% of stars in an average galaxy like our Milky Way end up as white dwarfs.
The Sun, too, will follow this path. In about five billion years, it will swell into a red giant, likely engulfing Mercury and Venus, perhaps even Earth. It will shed its outer layers and leave behind a white dwarf at its center— a slow, fading remnant that will radiate heat into space for billions of years. No explosion will mark the Sun's end — only a quiet dimming.
But even quiet deaths have their boundaries. In 1930, a young Indian physicist named Subrahmanyan Chandrasekhar, while sailing to England, calculated that a white dwarf above a certain mass — about 1.4 times that of the Sun — could not support itself against gravity. Beyond this Chandrasekhar limit, collapse would be inevitable.
His idea, initially ridiculed, later laid the foundation for our modern understanding of black holes and won him the Nobel prize decades later.
For stars more than eight times the mass of the Sun, the end is anything but quiet. These stars can reach the extreme core temperatures required to fuse progressively heavier elements — carbon, oxygen, silicon — all the way up to iron.
At this point, the physics changes dramatically. The fusion of iron does not release energy; instead, it consumes it. Without any source of pressure to oppose gravity, the core collapses in on itself within seconds. It becomes incredibly dense, forming a neutron star, or if massive enough, a black hole.
Meanwhile, the outer layers of the star, still falling inward, slam into the stiffening core and rebound outward. This violent interaction, combined with a flood of escaping neutrinos and thermal energy, drives a powerful supernova explosion. The result is one of the most luminous and energetic events in the universe — a dying star momentarily shining brighter than an entire galaxy.
Fusion in stars builds elements only up to iron. But the heavier elements—gold, uranium, iodine—are forged in the final moments of a star's life. The heat, pressure, and rapid neutron bombardment during a supernova enable a cascade of nuclear reactions that form these rare, heavy elements.
The explosion disperses them across the galaxy, enriching the gas clouds that will form the next generation of stars and planets. Much of Earth's matter —including the iron in our blood and the calcium in our bones — originated in the fiery death of long-gone stars.
Not all stellar explosions require high-mass stars. In binary systems, a white dwarf can siphon material from a nearby companion. If it accumulates enough mass, it reaches a tipping point, triggering a runaway thermonuclear explosion — a type Ia supernova — that obliterates the star. These explosions serve as cosmic mile-markers for measuring the expansion of the universe, and they too help enrich the interstellar medium with heavy elements.
Whether a star dies with a whisper or a bang, its death is not just an ending — it is an act of cosmic renewal. The materials scattered by dying stars seed new solar systems, new worlds, and, ultimately, the conditions for life itself.
We are built from atoms forged in the hearts of stars — refined in their lifetimes and released in their deaths. In every stellar ending lies the promise of a beginning.