A supernova is the violent explosion that marks the end of certain stars’ lives. It happens in one of two main ways: a massive star runs out of fuel and its core collapses under its own gravity, or a dead star called a white dwarf pulls in enough extra material from a companion star to become unstable and detonate entirely.
For a brief period, a supernova can outshine every other star in its entire host galaxy combined, releasing more energy in a few weeks than the sun will produce over its entire ten billion year lifetime. The explosion blasts material outward at tens of thousands of kilometers per second.
Supernovae are also how the universe makes many of its heavier elements. Every atom of iron, calcium, or gold in your body was likely forged either inside a dying star or in the explosion itself, then scattered into space to eventually become part of new stars, planets, and eventually, us.
Historical supernovae have left their mark on astronomy for centuries. In 1054, Chinese astronomers recorded a ‘guest star’ bright enough to see in daylight for weeks, the remnant of which is now known as the Crab Nebula. Danish astronomer Tycho Brahe and, decades later, Johannes Kepler each documented naked-eye supernovae in 1572 and 1604, events so unexpected at the time that they helped undermine the old belief that the heavens beyond the moon were fixed and unchanging. The most recent nearby example, SN 1987A, exploded in a neighboring galaxy in 1987 and remains the closest supernova observed since the invention of the telescope, giving astronomers an unprecedented close-up look, across every wavelength of light along with a burst of neutrinos, at exactly how a massive star’s death actually unfolds.
Supernovae are thought to occur in a galaxy the size of the Milky Way roughly once or twice a century on average, though the last one confirmed to have happened within our own galaxy was observed by Johannes Kepler in 1604, before the invention of the telescope. Astronomers suspect several supernovae have likely occurred since then hidden behind thick clouds of interstellar dust, undetectable from Earth even though their light technically reached us.