The life of a star.

Step 1 of 6

A ball of gas

A star is born when a cloud of gas and dust collapses under its own gravity. The collapse squeezes and heats the centre, until it is hot enough to light a reaction no bonfire could imitate. Age the star is a timeline here, from Birth to End. It is now at the long, steady stage, and the star has the mass of the Sun.

Step 2 of 6

Mass and power

Gravity squeezes the star inward, and the pressure of the hot gas in the core pushes outward. In a heavier star gravity is stronger, so the core has to be hotter and denser to hold it up, and fusion there runs far faster. On the main sequence, the stage when a star fuses hydrogen in its core, its power grows roughly like

$\frac{L}{L_\odot} \approx \left(\frac{M}{M_\odot}\right)^{3.5}$$\frac{L}{L_\odot} \approx$$\left(\frac{M}{M_\odot}\right)^{3.5}$⁠

Here $L_\odot$⁠ and $M_\odot$⁠ are the Sun’s power and mass. Starting mass is locked at $10$⁠ solar masses.

Step 3 of 6

Heavy and brief

A heavy star has more fuel, but it burns it faster. Predict which wins.

Step 4 of 6

How long

The time on the main sequence is the fuel divided by the rate of burning, $t \propto \dfrac{M}{L} \propto \dfrac{M}{M^{3.5}} = M^{-2.5}$$t \propto \dfrac{M}{L} \propto \dfrac{M}{M^{3.5}} =$$M^{-2.5}$⁠. The Sun will spend about ten billion years there, so

$t \approx 10 \cdot \left(\frac{M}{M_\odot}\right)^{-2.5}\ \text{billion years}$$t \approx$$10 \cdot \left(\frac{M}{M_\odot}\right)^{-2.5}\ \text{billion years}$⁠

The Scene shows this lifetime in billions of years. Starting mass is locked.

Step 5 of 6

The end of the Sun

Age the star has reached the end. When the hydrogen in the core runs out, fusion moves to a shell round it, and the star swells and cools into a red giant. What happens next depends on the mass.

Step 6 of 6

Heavy endings

A heavy star fuses helium into carbon, and then heavier and heavier elements, up to iron. Fusing iron gives out no energy, so the core loses its support and collapses all at once. The collapse blasts the rest of the star outward in a supernova, scattering into space the elements made inside it. The carbon in our bodies and the iron in our blood were made in stars like that.