Observatory · the life of a star
Stellar evolution
A star is a ball of gas that would collapse under its own weight if the fusion in its core did not push back. Everything about its life follows from one number it is born with, its mass: how bright it shines, how long it lasts, what it makes, and what is left at the end. Pick a mass and watch a whole life go by, on the diagram astronomers use for every star they see.
A life on the diagram
Fig. I. The Hertzsprung–Russell diagram with the star's track, and the star itself.
- Now
- Age
- In the core
- Size
- Surface
- Light
- Main sequence lasts
- It ends as
This is the Hertzsprung–Russell diagram, named after the Dane Ejnar Hertzsprung, who worked in Leiden, and the American Henry Norris Russell, who each plotted the stars this way around 1911–1913 and found that they do not fall just anywhere. Across it runs temperature, hot and blue on the left; up it runs brightness. Most stars, the Sun among them, sit on the faint grey band, the main sequence, for nine tenths of their lives, quietly turning hydrogen into helium. When the core runs out, it shrinks and heats, the outside swells and cools, and the star climbs to the upper right as a giant; the Sun will grow to roughly the size of the Earth's orbit. A star like the Sun then burns helium into carbon and oxygen, puffs its outer layers away as a glowing planetary nebula, and leaves its core, about the size of the Earth, as a white dwarf that cools for billions of years; Sirius B is one.
Above about eight Suns the core goes on, carbon to neon to oxygen to silicon to iron, each stage faster, the last in about a day. Iron is where it stops: fusing it costs energy instead of giving it. With nothing left to hold it up the iron core collapses in under a second, and the rebound blows the star apart, a supernova. What remains is a neutron star, a city-sized ball as dense as an atomic nucleus, or, from the heaviest stars, a black hole. The stages and places on the track here are sketched from simple rules, not computed star by star.
How long a star lives
Fig. II. How long a star lives, against its mass.
The heavy stars live fast: a star of thirty Suns burns for a few million years, while the first humans walked the Earth. The Sun gets about ten billion and has used up less than half. The small red dwarfs, a tenth of the Sun, are so frugal they will last thousands of billions of years, far longer than the universe has existed so far, so not one of them has ever finished its life. Everything left of the brass line is still in its first life.
Who worked it out
Where it already matters
You. The carbon in you, the oxygen you breathe and the calcium in your bones were made inside stars that lived and died before the Sun was born, and spread through space by their winds and explosions. The heavier elements came from slow neutron capture in old giants and from explosions and collisions; the chirp of two neutron stars in 2017 came with a glow of freshly made ones.
Betelgeuse, the red shoulder of Orion, is a red supergiant near the end of its life. In 2019–2020 it dimmed so much that people asked whether it was about to explode; it had puffed out a cloud of dust. It will go supernova at some point in the next hundred thousand years or so, bright enough to see in daylight.
1054. Chinese astronomers recorded a new star in Taurus, visible by day for weeks. Its remains are the Crab Nebula, with a neutron star at its heart, spinning thirty times a second.