Birth: How Stars Form

🚀 Birth: How Stars Form

Figure 2: The 4 stages of star formation — gravity contracts gas and dust until the core is hot enough for hydrogen fusion to begin

All stars begin in the same way — from a nebula, a vast cloud of gas (mainly hydrogen and helium) and dust, scattered across space.

From Nebula to Main Sequence

1. Nebula: A cloud of gas and dust, millions of kilometres across. Gravity very slowly begins to pull material together, but the cloud is so large this takes millions of years.
2. Protostar: As material collapses inward, gravitational potential energy converts to kinetic energy — the gas heats up. The collapsing, heating central region is called a protostar. It is not yet a true star because nuclear fusion has not started.
3. Main sequence star: When the core temperature reaches about 10 million degrees Celsius, hydrogen nuclei have enough kinetic energy to overcome their mutual electrostatic repulsion and fuse together. Four hydrogen nuclei fuse to form one helium nucleus, releasing enormous amounts of energy. This outward radiation pressure exactly balances inward gravity — the star is now stable and in equilibrium. This is the main sequence, and a star spends most of its life here.

Our Sun has been on the main sequence for about 4.6 billion years and will remain there for another 5 billion years or so.