Supernovae

What is a supernova?

A supernova is the explosion of a star. For a short time it releases so much light that a single star can outshine the hundreds of billions of stars in its home galaxy.

The word comes from "nova", Latin for new, because to early observers it looked like a new star had appeared. In reality an old star was ending its life.

Astronomers sort supernovae by their light and by the chemical fingerprints in that light, called a spectrum. Behind the many labels there are two basic ways a star can blow up.

Core-collapse: when a giant star runs out of fuel

Stars more than about eight times as massive as the Sun fuse heavier and heavier elements in their cores, from helium up to silicon. The chain stops at iron, because fusing iron takes energy instead of releasing it.

With no energy left to hold it up, the iron core collapses in less than a second. It shrinks to a ball only a few miles across and rebounds, and a flood of tiny particles called neutrinos helps drive a shock wave that tears the star apart.

What remains in the middle is either a neutron star or, for the heaviest stars, a black hole. Astronomers label these explosions Type II, Ib or Ic, depending on how much of its outer hydrogen and helium the star had kept.

Type Ia: a white dwarf pushed too far

A white dwarf is the dense, Earth-sized leftover core of a Sun-like star. On its own it simply cools for billions of years.

If it sits in a close pair with another star, though, it can gain extra mass, either by pulling gas from its partner or by merging with another white dwarf. When it gets close to about 1.4 times the Sun's mass, carbon fusion runs away and the whole white dwarf explodes, leaving nothing behind.

Because Type Ia supernovae reach a fairly predictable peak brightness, astronomers use them as "standard candles" to measure distances across the universe. In 1998 this method revealed that the expansion of the universe is speeding up.

Supernovae in history

In 1054, Chinese astronomers recorded a "guest star" in Taurus that was visible in daylight for weeks. Its wreckage is the Crab Nebula, about 6,500 light-years away, with a fast-spinning neutron star at its center.

In 1572, Tycho Brahe watched a new star in Cassiopeia and showed that it lay far beyond the Moon, which challenged the old idea that the heavens never change. It was a Type Ia supernova.

In 1604, Johannes Kepler studied another one in Ophiuchus. It remains the last supernova in our own galaxy that people saw with the naked eye.

In 1987, a star blew up in the Large Magellanic Cloud, a small galaxy near the Milky Way. SN 1987A was the first supernova whose neutrinos were caught by detectors on Earth, a few hours before its light was noticed, confirming how core collapse works.

How supernovae seed the universe

The early universe contained almost only hydrogen and helium. Nearly every heavier element came later, cooked inside stars and thrown out when they died.

Core-collapse supernovae release large amounts of oxygen, silicon and calcium, while Type Ia explosions are major sources of iron. The shock waves also help squeeze nearby gas clouds, which can trigger new stars to form.

Some of the heaviest elements, such as gold, appear to come mostly from other events like neutron star mergers. Either way, the calcium in your bones and the iron in your blood were made in stars that lived and died before the Sun was born.

Is Betelgeuse about to explode?

Betelgeuse, the reddish shoulder of Orion, is a red supergiant roughly 500 to 650 light-years away. It is massive and old, so it will almost certainly end as a core-collapse supernova.

In late 2019 and early 2020 it dimmed sharply, which caused a lot of excitement. Studies with the Hubble Space Telescope and ESO's Very Large Telescope showed the cause was a cloud of dust the star had puffed out, not a sign of an imminent explosion.

Current estimates put the explosion somewhere within the next roughly 100,000 years, which on human timescales means it is unlikely in our lifetimes. When it happens it will be bright enough to see in daylight for weeks, but it is far too distant to harm Earth.

How can I see one?

A naked-eye supernova in our galaxy is rare, so the practical way to see one is through a telescope, looking at other galaxies. Every year amateur and professional surveys find thousands of supernovae, and a few each year in bright nearby galaxies reach magnitude 12 or 13, within reach of a medium backyard telescope under dark skies.

You can also view supernova remnants. The Crab Nebula (M1) near the tip of Taurus's horn shows up as a faint, misty patch in a small telescope during winter evenings.

If a galactic supernova happens, neutrino detectors around the world will likely send an early alert before the light becomes obvious, giving observers a head start.