L23. How Stars Live and Grow Old

Unit 5 · The Lives of Stars

R-report

L23. How Stars Live and Grow Old

The Sun has been shining steadily for 4.6 billion years and has about 5 billion left. Some stars last trillions of years. Others burn out in a few million. The difference comes down to one thing.

The long, stable middle

A star spends about 90% of its life on the main sequence, doing one thing: fusing hydrogen into helium in its core. What keeps it stable is a balance called hydrostatic equilibrium. Gravity pulls every layer inward; the pressure of hot gas, powered by fusion, pushes outward. As long as the two match, the star holds its size and brightness for billions of years. The balance is also self-correcting. If the core cools slightly, it contracts, which raises the temperature and speeds up fusion, which pushes back out. If fusion speeds up too much, the core expands and cools, slowing fusion down. A star is a remarkably steady machine. That stability matters for us. Life on Earth has had billions of years of nearly constant sunlight to develop, which would have been impossible around a star that flickered.

Mass decides the lifetime

Here is the counter-intuitive part. Massive stars have far more fuel — and die far sooner. A massive star's own weight squeezes its core to much higher temperature and pressure, and fusion runs extraordinarily faster at higher temperatures. So a big star burns through its huge fuel supply at a wildly disproportionate rate. - A star ten times the Sun's mass has ten times the fuel but shines thousands of times brighter, lasting only tens of millions of years. - The Sun will last about 10 billion years. - A red dwarf, a fraction of the Sun's mass, burns so slowly it could last trillions of years — longer than the current age of the universe. This has an interesting consequence: no red dwarf has ever died. Every one ever formed is still shining. It also means that when we see very massive blue stars, we know they must be young, because they cannot last long.

What happens when the hydrogen runs out

Eventually the core's hydrogen is exhausted. Fusion there stops, outward pressure drops, and gravity begins winning again. For a Sun-like star the sequence goes like this. - The core contracts and heats up, while hydrogen begins fusing in a shell around it. - That shell burning is fierce enough to inflate the outer layers enormously. The star swells into a red giant — larger, cooler at the surface, and far brighter overall. When the Sun does this it will likely engulf Mercury and Venus, and scorch Earth. - In the hot core, helium starts fusing into carbon and oxygen. - When that fuel is spent, the outer layers drift away as a glowing shell called a planetary nebula (a misleading name from early observers who thought the round shapes looked like planets). - The exposed core is left behind: a white dwarf, about the size of Earth but containing much of the star's original mass, so dense that a teaspoon would weigh several tonnes. A white dwarf makes no new energy. It simply cools and fades over billions of years.

Putting the investigation together

A star lives in balance: gravity inward, fusion pressure outward. When core hydrogen runs out the balance breaks, and a Sun-like star swells into a red giant, sheds its outer layers, and leaves a slowly cooling white dwarf behind.

The controlling variable throughout is mass. It sets the core temperature, the fusion rate, the lifetime, and the final outcome. Finding the one property that governs a whole family of behaviours is a recurring goal in science, and for stars, that property is mass.