A supernova is a star blowing itself apart. For a few weeks it can shine as brightly as an entire galaxy of stars, before fading over months. Stars have two ways to end like this.
Two ways to explode
The first starts with a white dwarf, the dense leftover core of a star like the Sun. If it pulls in material until it nears about 1.44 solar masses, runaway nuclear fusion tears it apart. This is a Type Ia supernova. A typical one peaks at about five billion times the Sun’s brightness, and because they are so alike, astronomers use them as measuring sticks across the universe.
The second is the collapse of a massive star, one born with at least about nine times the Sun’s mass. When its core runs out of fuel, nothing holds it up any more. It collapses in a fraction of a second into a neutron star or a black hole, and the rest of the star is blasted outward. About 10⁴⁶ joules pour out as neutrinos in some ten seconds. Only around a hundredth of that becomes the visible explosion, and that is still enough to outshine a galaxy.
The debris flies out at thousands of kilometres per second, up to several percent of the speed of light. It carries elements forged in the star and in the blast, from oxygen up to rubidium, out into space, where they become part of new stars and planets.
Seen with the naked eye
People have recorded supernovae for a thousand years. The one in 1006, in the constellation Lupus, reached about magnitude −7.5, far brighter than Venus, and was written down in China, Japan, Iraq, Egypt and Europe. Chinese astronomers recorded the one in 1054 whose remains we now see as the Crab Nebula. Tycho Brahe watched one in 1572, and Johannes Kepler another in 1604. That was the last supernova seen in our galaxy, even though estimates suggest a few should happen every century.
SN 1987A
The closest supernova of modern times went off in the Large Magellanic Cloud, a small galaxy orbiting the Milky Way, 168 000 light-years away. It was spotted on 24 February 1987 at Las Campanas Observatory in Chile, and reached about magnitude 3, easily visible to the naked eye from the southern hemisphere.
It was also heard in another way. Two to three hours before its light arrived, three detectors in Japan, the United States and Russia caught 25 neutrinos in a burst lasting under 13 seconds. They are still the only neutrinos ever caught from a supernova. In 2024 the James Webb Space Telescope found signs that a neutron star sits at the centre of the wreckage.
The star that exploded, Sanduleak −69 202, was a blue supergiant. In our own galaxy the most famous candidate for the next one is Betelgeuse, a red supergiant in Orion, but it is not expected to explode for perhaps 100 000 years.