Star Formation

Stars begin life inside huge, cold clouds of gas and dust called nebulae. These clouds contain lots of hydrogen. Over very long times, parts of a nebula clump together under gravity and a new star is born.

From cloud to star: key stages

• Nebula: A vast, thin cloud of hydrogen gas mixed with dust in space.
• Gravity takes over: Slightly denser regions attract more material, like a snowball growing as it rolls.
• Protostar: As the clump collapses, it squeezes in. The particles fall inward, speeding up and heating the centre. The temperature and pressure rise.
• Accretion disc: The collapsing cloud often spins, flattening into a disc around the hot centre. The middle becomes the star; the disc can later form planets.
• Ignition of fusion: When the core gets hot and dense enough, hydrogen nuclei fuse into helium, releasing energy.
• Stable star: The star settles when two forces balance.

The balance of forces

Inside a stable star, inward gravity is balanced by outward pressure from the very hot core (heat and radiation pushing out). This balance keeps the star steady for millions to billions of years.

In simple terms: $$\text{gravity in} \;\approx\; \text{pressure out}$$

Where the energy comes from

Nuclear fusion powers stars. In the core, hydrogen joins to make helium and releases energy as light and heat:

$$4\,^{1}_{1}\mathrm{H} \;\rightarrow\; ^{4}_{2}\mathrm{He} + \text{energy}$$

A small amount of mass turns into energy (as in $E = mc^2$), which keeps the star shining.

What decides the kind of star?

Mass is the key. Bigger starting clumps make hotter, brighter, shorter-lived stars. Smaller clumps make cooler, dimmer, longer-lived stars. All stars begin in nebulae and pass through the protostar stage.

Common misconceptions

• Stars do not “burn” like fires; they do not need oxygen. They fuse hydrogen.
• Not every star comes from an explosion. Most form quietly by gravity in nebulae.
• Protostars can be hidden by dust, so they may not be visible in ordinary light.