How a Star Is Born — Stellar Evolution | IGCSE Physics

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

Life cycle of stars and the Hertzsprung-Russell diagram appear as descriptive multi-mark questions.

All stars, regardless of how massive they end up, start the same way.

Nebula

A nebula is a giant cloud of dust and gas (mostly hydrogen) drifting through interstellar space
Some patches of a nebula are denser than others. The dense patches have stronger gravitational pull on the surrounding gas

Protostar

Inside a dense patch, gravity pulls the gas and dust together. The cloud begins to collapse inwards
As the gas gets squeezed:
- The density rises
- Particles collide more frequently, transferring kinetic energy
- The temperature rises sharply
The hot ball of contracting gas is called a , a star not yet fully alive, because fusion has not yet begun

Main-sequence star

Once the core of the protostar reaches about 10 million K, the hydrogen nuclei in the core start to fuse into helium, releasing huge amounts of energy
A star that is fusing hydrogen in its core is called a
The Sun has been a main-sequence star for about 4.6 billion years and will continue for another 5 billion or so before its hydrogen runs out
During the main-sequence stage, the star is stable, because the outward pressure from fusion balances the inward pull of gravity. The star stays the same size for billions of years (covered in topic 21)

Every star starts the same way: → protostar → main-sequence star

Exam tip

Describing how a star forms from a nebula

What comes up: a 3-mark "describe" question asking how a main-sequence star is formed from a nebula.

Write (three marks): (1) Gravity pulls the gas and dust in the nebula together, causing the cloud to collapse. (2) As the cloud contracts, the temperature rises (kinetic energy of the particles increases). (3) When the core temperature is high enough, nuclear fusion of hydrogen begins — the star is now on the main sequence.

Watch out: do not say the cloud "explodes" or "ignites". The examiner credits the idea that fusion starts, not a sudden explosion. The term protostar can be included for the collapsing hot ball before fusion begins and the mark scheme accepts it, but the key credited step is fusion starting.