A star like the Sun ends as a white dwarf, a roughly Earth-sized stellar cinder that cools over billions of years. A white dwarf is about as big as Earth but about as massive as the Sun, so a teaspoon of its material would weigh several tons, more than a pickup truck. A white dwarf stays "white" hot with surface temperatures of about twenty thousand degrees Celsius for a billion or so years after its star collapses, and all its material is packed into a volume about one millionth the size of the original star. In a white dwarf, matter is so dense that quantum rules about electrons create a pressure that holds it up, solving a paradox that astronomers could not explain until the quantum theory of the 1920s.

A star's life is a balance of gravity pulling inward and pressure pushing outward, and the star is in hydrostatic equilibrium as long as the two are equal. In a massive star, fusion of silicon into iron is the last step of nonexplosive element production, because iron is the most tightly bound and most stable nucleus. By the time silicon fuses into iron, a massive star runs out of fuel in a matter of days; the core then collapses, rebounds, and sends a shock wave outward in a supernova. A supernova is a huge explosion at the end of a massive star's life.

Stars between about 8 and 20 times the Sun's mass end as neutron stars, which pack more mass than the Sun into a sphere about as wide as Manhattan Island is long. Neutron stars are the densest objects in the universe, and their surface gravity is 10^11 times that at Earth's surface; a neutron star is only 10 to 20 kilometers across. A pulsar is a rapidly rotating neutron star; its bright hot spots spin in and out of view like the beams of a lighthouse. In 1967 graduate student Jocelyn Bell discovered pulsars. Magnetars are neutron stars with magnetic fields about a quadrillion times greater than Earth's.

A Type Ia supernova is a sudden thermonuclear explosion of a white dwarf, which can happen when it passes 1.4 solar masses by taking matter from a companion star or merging with another white dwarf. Because Type Ia supernovas occur in a star of about 1.4 solar masses, they produce about the same amount of light, which makes them useful as a distance indicator. Astronomers estimate that on average one supernova explosion occurs in the Milky Way every 25 to 100 years, though none has been observable in our galaxy since the invention of the telescope. Chinese astronomers called the supernova of 1006 a "guest star"; a faint remnant is now found in the constellation Lupus.

Type II, Ib and Ic supernovas come from the collapse of a massive star's core; in less than a second a neutron star or black hole forms and the rest of the star is blown away at speeds above 50 million kilometers per hour. A massive star's core can survive a supernova as either a neutron star or a black hole. If a star has around 20 times the Sun's mass or more, its core collapses into a stellar-mass black hole, and as of November 2026, scientists think there may be as many as 100 million in our galaxy. Astronomers tracked the orbits of several stars near the center of the Milky Way to prove it houses a supermassive black hole, a discovery that won the 2020 Nobel Prize.

Words to know

• magnetars: magnetic neutron stars
• pulsar: spinning neutron star
• supernova: exploding star
• paradox: a puzzle