What Is a Supernova?

A supernova happens when a star can no longer support itself against gravity and collapses catastrophically, or when a stellar remnant accumulates enough matter to trigger runaway nuclear fusion.

The energy released is difficult to convey. A single supernova can briefly outshine the combined light of the hundreds of billions of stars in its host galaxy, which is why they can be detected across enormous distances.

Core collapse supernovae occur in stars more than about eight times the mass of the Sun. Such a star fuses progressively heavier elements until it builds an iron core, and iron fusion consumes energy rather than releasing it. Support fails, the core collapses in under a second and the outer layers rebound outward.

Thermonuclear supernovae, known as type Ia, involve a white dwarf in a binary system pulling matter from a companion. Once it passes a critical mass limit, carbon fusion ignites throughout the star almost instantly and it is completely destroyed.

Because type Ia supernovae explode at a consistent mass threshold, they release a predictable amount of energy. That makes them standard candles: if you know the true brightness and measure the apparent brightness, you can calculate the distance.

Using them to measure distant galaxies produced one of the great surprises of modern astronomy in 1998, when observations showed the expansion of the universe is accelerating rather than slowing. That discovery led to the concept of dark energy and to a Nobel Prize.

Ordinary stellar fusion builds elements up to iron. About half of the heavier elements need a supernova or a neutron star merger; the rest form slowly inside ageing giant stars.

This is the origin of most of the material around you:

• •Iron in your blood, forged in massive stars and scattered by their explosions
• •Calcium in your bones, produced in stellar cores
• •Oxygen you breathe, made in massive stars
• •Gold and other heavy metals, from supernovae and colliding neutron stars

A core collapse supernova leaves a remnant. If the collapsed core is between roughly one and a half and three solar masses it becomes a neutron star, so dense that a teaspoon of it would weigh billions of tonnes. Above that, it becomes a black hole.

The expanding debris forms a supernova remnant, visible for thousands of years. The Crab Nebula is the remnant of an explosion recorded by Chinese astronomers in 1054, bright enough at the time to be visible in daylight.

Only a handful of supernovae in our own galaxy have been observed with the naked eye, and the records are historically valuable. Chinese astronomers documented a guest star in 1054 that remained visible for months, and its remnant is the Crab Nebula.

Two more were seen in 1572 and 1604, studied by Tycho Brahe and Johannes Kepler respectively. Those observations mattered scientifically, because they showed the heavens could change, contradicting the prevailing view that the realm beyond the Moon was fixed and perfect. No naked eye supernova has appeared in the Milky Way since, which is statistically overdue.

In February 1987 a star exploded in the Large Magellanic Cloud, a small galaxy orbiting the Milky Way, and became the nearest supernova seen in centuries, bright enough to see without a telescope from the southern hemisphere. It gave astronomers a chance to test their theory of core collapse in detail. That theory predicted something surprising: almost all of the energy released when a core collapses should escape not as light but as neutrinos, tiny particles that pass through ordinary matter almost without interacting.

Hours before the explosion was spotted in visible light, detectors in Japan, the United States and Russia recorded a brief burst of about two dozen neutrinos. That handful of particles, out of an enormous number streaming through Earth, confirmed that a collapsing core had formed, roughly as the models said. The light came later because it had to work its way out through the star's outer layers. Today a network of detectors watches for such a burst so that telescopes can be alerted before the next nearby supernova becomes visible.

The labels astronomers give supernovae come from their spectra, not their causes, which can be confusing. Type I explosions show no hydrogen in their light, while type II explosions do. Within type I, type Ia shows strong silicon and comes from exploding white dwarfs. But type Ib, which shows helium, and type Ic, which shows neither hydrogen nor helium, are actually core collapse supernovae. They come from massive stars that lost their outer layers before exploding, through strong winds or a companion star stripping the gas away. So Ia and Ib share a letter but not a mechanism, while Ib and II share a mechanism but not a letter.

The best-known candidate for the next nearby supernova is Betelgeuse, the red supergiant at Orion's shoulder. In 2019 and 2020 it dimmed dramatically, prompting excited speculation, but observations suggest the main cause was a cloud of dust it had thrown off. Betelgeuse is expected to explode eventually, but that could be at any point in roughly the next hundred thousand years. When it does, it may become bright enough to see in daylight. It poses no danger. Betelgeuse is several hundred light years away, while a supernova would need to be much closer, within a few tens of light years, to seriously harm Earth's atmosphere.

Some evidence about supernovae comes from the bottom of the ocean. Iron-60 is a radioactive form of iron made in massive stars and scattered when they explode. Its half-life is about 2.6 million years, so any that existed when Earth formed has long since decayed, and very little is produced on Earth today. Yet researchers have found small amounts of it in deep-sea crusts, rocky coatings that grow only a few millimetres in a million years. The iron-60 is concentrated in layers formed roughly two to three million years ago, which points to dust arriving from space at that time.

The same signal has since turned up in ocean sediments from several regions, in fresh Antarctic snow and in lunar soil brought back by the Apollo missions. The most likely explanation is one or more supernovae a few hundred light years away, close enough to sprinkle debris across the solar system but too far to cause serious harm. The Sun currently sits inside a vast region of hot, thin gas called the Local Bubble, which astronomers think was blown out by a series of supernovae over millions of years. The iron-60 in recent snow suggests the solar system is still passing through some of that debris.

A supernova is the explosion of a star, either through core collapse or runaway fusion in a white dwarf, and it is one of the processes that created and dispersed the elements heavier than iron in the universe.