Lives of stars

Every star in the night sky has a life story. It was born at some point, it shines for a certain time, and one day it will fade. That sounds almost human, but it follows hard physics: a star is a giant ball of gas that fuses hydrogen into helium in its core, releasing unbelievable amounts of energy. Our Sun is such a star too, a rather average one in fact. How its life unfolds and how it ends is already decided at its birth.

Born in clouds of gas

Stars form in enormous, cold clouds of gas and dust. These clouds are many light years across and usually astonishingly thin, thinner than any vacuum we can make in a lab. But gravity is patient: wherever the cloud is a little denser, it attracts even more gas. The clump shrinks, grows denser and hotter. Once the core reaches about 10 million degrees, nuclear fusion ignites: hydrogen nuclei fuse into helium, energy streams outward, and a new star begins to shine.

The great balance

Why does a star not simply collapse under its own weight? Because two enormous forces work against each other. Gravity pulls all the gas inward. The pressure of the hot gas, powered by fusion in the core, pushes outward. The two forces balance each other exactly, and this equilibrium is what makes a star so stable. Our Sun has kept it up for around 4.6 billion years without a wobble.

Mass decides the fate

You might think: more fuel, longer life. For stars it is exactly the other way round. A massive star squeezes its core so hard that fusion runs at a furious pace. It shines blue and dazzlingly bright, but burns through its supply in a few million years. A lightweight red dwarf, on the other hand, sips its hydrogen and can shine for many hundreds of billions of years, longer than the universe has existed so far. Our Sun sits in between: it gets about 10 billion years in total, so roughly half still lies ahead. The finale depends on mass too: stars with several times the Sun's mass end in a supernova that tears the star apart and leaves behind a neutron star or a black hole.

What will become of the Sun

In about 5 billion years the Sun will run out of hydrogen in its core. It will then swell into a red giant with roughly a hundred times its present radius and gently shed its outer layers into space. It is far too light for a supernova, that would take several times its mass. What remains is its burnt out core: a white dwarf, roughly the size of Earth but immensely dense. It will glow for billions of years more, like the embers of a fire that has gone out.