By Karen Adams, Updated Mar 24, 2022
The death of a star is not a final end but a transformation that seeds new cosmic structures. Because the universe is still young, astronomers rely on models and observations to piece together the lifecycle of stars.
Stars with masses up to about 0.5 M☉ (half the Sun’s mass) avoid core collapse. After exhausting hydrogen and helium, they shed their outer layers and leave behind a dense, electron‑degenerate core—a white dwarf.
A white dwarf is the remnant of a low‑mass star. Its core, composed mainly of carbon and oxygen, is supported by electron degeneracy pressure. While it cannot fuse fuel, it gradually cools over billions of years, radiating its residual heat into space.
In the red‑giant phase, a star’s core contracts while its outer envelope expands. Helium fusion in the core produces carbon and oxygen, and the star’s outer layers are eventually expelled, forming a glowing planetary nebula and leaving behind a new white dwarf.
The Chandrasekhar limit—1.4 M☉—defines the maximum mass a white dwarf can support. Stars below this threshold end their lives as white dwarfs. More massive stars exceed this limit, collapse into neutron stars, and, if they reach about 5 M☉ or more, can explode as core‑collapse supernovae, leaving behind neutron stars or black holes.
