What a supernova is
A supernova is the explosion of a star, and for a short time it can shine about as brightly as a whole galaxy of billions of stars. Not every dying star explodes. Sun-like stars end more quietly, shedding their outer layers and leaving a white dwarf. Supernovae come from two main situations: the collapse of a massive star’s core, and the thermonuclear explosion of a white dwarf.
The word “nova” means new, because ancient observers saw what looked like a new star appear in the sky. In fact, a supernova is not a birth but a death, and the “new star” was an old one becoming suddenly bright enough to see.
Core-collapse supernovae
A star with roughly eight or more times the Sun’s mass can keep fusing heavier elements in its core, in shells like the layers of an onion. The process stops at iron, because fusing iron does not release energy. With no energy source to resist gravity, the core collapses in about a second, down to the size of a city.
The collapse halts when the core becomes a neutron star, and the infalling outer layers rebound and are driven outward. Most of the explosion’s energy is thought to be carried off by neutrinos, extremely light particles that barely interact with matter, while a much smaller fraction goes into the visible blast. How exactly the shock wave succeeds in blowing the star apart is still an active area of research, and computer simulations have taken considerable effort to reproduce it.
If the core is massive enough, it can collapse further into a black hole instead of a neutron star. In some cases, the explosion may be weak or may not be seen at all.
Astronomers sort these explosions into spectral classes, such as Type II (hydrogen lines visible) and Types Ib and Ic (little or no hydrogen, often because the star lost its outer layers earlier). The classification is about what the light shows, not about different physics at the core.
Type Ia supernovae
A white dwarf on its own is stable. But if a white dwarf has a companion star, it can pull in gas from it, or it can merge with another white dwarf. If the white dwarf approaches a limit of about 1.4 solar masses, called the Chandrasekhar limit, runaway fusion can be triggered throughout the star and tear it apart. Nothing is left behind in the standard picture.
Type Ia explosions are valuable because they tend to reach similar peak brightness, after a standard correction based on how quickly they fade. That makes them useful as markers of distance: the dimmer one looks, the farther away it probably is. Observations of distant Type Ia events were central to the discovery that cosmic expansion is accelerating. The precise nature of the companions that trigger them is still debated.
Famous examples
Records of “guest stars” in Chinese and other historical sources describe an object that appeared in the year 1054, and its remnant is the Crab Nebula, which contains the Crab Pulsar. Tycho Brahe and Johannes Kepler each documented a supernova, in 1572 and 1604 respectively. Kepler’s is the most recent one definitively seen in our own galaxy with the naked eye, though modern astronomers believe others have occurred unseen behind dust.
In 1987, a supernova called SN 1987A appeared in the Large Magellanic Cloud, a small neighboring galaxy. It was the closest observed in centuries. Detectors on Earth registered a burst of neutrinos hours before the light was noticed, which matched predictions and was a striking confirmation of core-collapse theory.
How often do they happen?
Estimates for our galaxy typically suggest a few core-collapse supernovae per century, though the numbers are uncertain. Astronomers therefore watch thousands of other galaxies, where explosions appear regularly.
Scattering the elements
Supernovae matter well beyond their brilliance. The early universe consisted almost entirely of hydrogen and helium. Elements such as oxygen, silicon, calcium and iron were created in stars and during their explosions, then dispersed into space. This enriched gas later formed new stars and planets, so much of the material in our solar system, and in rocky planets and living things, traces back to earlier generations of stars.
The origin of the heaviest elements, such as gold, is less simple. Some are made in rapid neutron-capture processes, and the likely sites are debated. Supernovae may contribute, and mergers of neutron stars are now considered an important source, although the exact balance is unsettled.
Shock waves from supernovae also compress nearby gas, which can help trigger the formation of new stars, and they accelerate cosmic rays. In this sense, stellar explosions are part of a cycle: gas forms stars, stars make elements, and explosions return those elements to the gas. This is part of why scientists describe a close link between atoms and stars.
Is Earth at risk?
A supernova would need to be very close, typically thought to be within tens of light years, to seriously affect Earth. No known candidates are that near. Some stars, such as Betelgeuse, are often discussed in the news, but they are far enough away that no danger is expected.