Why do stars explode?
A star explodes when gravity finally wins and its core collapses or detonates in seconds.
A star is a standoff
Every star spends its life balanced between two forces. Gravity pulls all that mass inward, while heat from nuclear fusion in the core pushes outward. As long as fusion keeps burning, the star holds its shape.
Fusion works by stitching light elements into heavier ones, hydrogen into helium, helium into carbon, and so on. Each step releases energy, but only up to a point.
Iron is where the energy runs out
Fusing elements lighter than iron gives off energy. Fusing iron does the opposite: it costs energy. Once the core is iron, the furnace shuts down, and the outward push collapses in an instant.
The collapse and the rebound
The numbers are absurd. The core falls at roughly a quarter of the speed of light, and the collapse releases more energy in seconds than the Sun will produce in its entire ten-billion-year life. Most of it streams out as neutrinos.
Not every star dies the same way
There is a second path. A dense white dwarf can siphon matter from a partner star until it tips past a critical mass and detonates in a runaway thermonuclear blast, leaving nothing behind. The heavy collapse instead leaves a neutron star or, for the largest stars, a black hole.
Either way, the explosion seeds space with the elements forged inside. The calcium in your bones and the iron in your blood were scattered by a star that blew itself apart long before the Sun was born.