A supernova is the explosion of a star, and it ranks among the most energetic events humans have ever observed. For a short time, a single exploding star can shine as brightly as an entire galaxy before fading over weeks or months. How Do Black Holes Actually Form. Why Do Stars Twinkle?.
One way a supernova happens is through the collapse of a massive star. A star is held in balance between gravity pulling inward and the outward pressure created by nuclear burning in its core. When a massive star runs out of fuel, it cools, the pressure drops, and gravity wins. The core collapses in seconds, triggering enormous shock waves that blast the star's outer layers into space. What is left behind is either a very dense core, called a neutron star, or, for the most massive stars, a black hole, surrounded by an expanding cloud of hot gas known as a supernova remnant.
A second type of supernova involves a white dwarf, the dense leftover of a smaller star that has already exhausted its fuel. If a white dwarf shares a system with a companion star, it can pull in too much matter from its partner or collide with another white dwarf. That extra material can push it past a stability limit and cause it to detonate. Astronomers call these Type Ia supernovas, and because they reach a predictable brightness, scientists use them like cosmic measuring tapes to gauge distances across the universe and to track how fast the universe is expanding.
Supernovas are rare in any one galaxy. Astronomers estimate only a few occur each century in a galaxy like the Milky Way, but because the universe contains so many galaxies, a few hundred supernovas are observed in other galaxies every year.
These explosions also build and spread the ingredients of worlds. Stars forge heavier elements from simpler ones in their cores, and massive stars produce elements such as gold, silver, and uranium. When a star explodes, it scatters those elements into space, where they can later become part of new stars, planets, and living things.
The short answer
A supernova is the explosion of a star. It happens when a massive star collapses under its own gravity or when a white dwarf gains too much matter from a companion, and the blast can briefly outshine a galaxy while spreading newly made elements into space.
Sources
- NASA Space Place, "What Is a Supernova?" β https://spaceplace.nasa.gov/supernova/en/
- NASA, "Supernovae." β https://www.nasa.gov/universe/stars/supernovae/
- NASA Science, "Type Ia Supernova." β https://science.nasa.gov/resource/type-ia-supernova/
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