Every star, the Sun included, was born from a cloud of gas and dust. The most likely birthplaces are giant molecular clouds: cold (about 10 to 20 K), dark, and dense by the standards of space, with enough gas for thousands of stars. The Orion Nebula is a nearby nursery, about 1300 light-years away, where new stars are forming today.
Why it works
Two things fight in a cloud: gravity pulls it in, and the gas pressure of its warm, jostling molecules pushes out. Cold gas pushes weakly, so in a cold, dense clump gravity wins and the clump collapses. As it falls inward it loses gravitational energy, and roughly half of that energy heats the gas while the rest is radiated away (the virial theorem). So a collapsing protostar heats up even as it shines. It keeps shrinking and heating until its core reaches about 10 million K. Then hydrogen begins to fuse, as in how the Sun shines, the core makes as much energy as the star radiates, and the outward pressure balances gravity. A star is born on the main sequence.
Key idea
Stars form when cold, dense clumps of gas and dust collapse under their own gravity. Contraction heats the protostar until its core reaches about 10 million K and hydrogen fusion begins.
- Clouds fragment, so stars are born in groups, from a few to many thousands at once.
- The spinning collapse leaves a disk around each new star, where planets can form; jets often shoot out along the spin axis.
- Objects lighter than about $0.08$ solar masses (about 80 Jupiter masses) never get hot enough to fuse ordinary hydrogen: they become brown dwarfs.
Worked example
Worked example: Seeing star birth
A dusty cloud core is at $20$ K. Where does its thermal glow peak? Use $\lambda_{\text{peak}} = \frac{2900\ \mu\text{m·K}}{T}$.
- $\lambda_{\text{peak}} = \frac{2900}{20} = 145\ \mu$m.
- That is far infrared, invisible to the eye. Dust also blocks visible light from inside the cloud, so astronomers watch stars form with infrared and radio telescopes such as the James Webb Space Telescope and ALMA.
Answer: About $145\ \mu$m, in the far infrared.
Common mistake
Some people picture stars forming from the leftovers of planets, or from older stars. It is the other way around: stars form first, from collapsing gas clouds, and planets form afterward in the disks around them. Check: a star contains hundreds to millions of times more mass than all its planets together, so planets could never supply the material.