The life cycle of a star

The life cycle of a star depends on its mass. Every star begins in a cloud of dust and gas called a nebula, which collapses under gravity into a protostar and then becomes a main sequence star, fusing hydrogen into helium. A star about the size of the Sun then becomes a red giant, then a white dwarf, and finally a black dwarf. A star many times more massive than the Sun becomes a red supergiant, explodes as a supernova, and leaves behind a neutron star or a black hole.

The stages in a star's life

AQA GCSE Physics says a star's life cycle is determined by its size, and asks you to describe the life cycle of a star the size of the Sun and of a star much more massive than the Sun.

How every star begins

1. Nebula

A nebula is a huge cloud of dust and gas, mostly hydrogen. Gravity pulls parts of it together into denser clumps. NASA describes stars forming in large, cold clouds of gas and dust where dense pockets collapse under gravity.

2. Protostar

As a clump collapses, it gets denser and its gravitational potential energy is transferred to thermal energy, so it heats up. This hot, dense ball is a protostar. It is not yet a true star, because nuclear fusion has not started.

3. Main sequence star

When the core is hot and dense enough, hydrogen nuclei start to fuse into helium, releasing huge amounts of energy. The star is now a main sequence star. Two forces balance: gravity pulls the star inwards, and the energy from fusion pushes outwards. AQA describes this as an equilibrium between gravitational collapse and expansion due to fusion energy. This balance can last billions of years, and it is the longest stage of a star's life. NASA says the Sun is roughly midway through its main sequence stage.

Life cycle of a star like the Sun

4. Red giant

Once most of the hydrogen in its core has been used up, fusion in the core slows and the balance is lost. The core shrinks and heats up, while the outer layers swell and cool, so the star glows red. Heavier elements such as carbon are made by fusing helium.

5. White dwarf

Eventually the outer layers drift away into space. The hot, very dense core that is left is a white dwarf. No fusion happens in it; it simply shines from its stored heat.

6. Black dwarf

Over an extremely long time it loses its heat and grows dimmer, until it no longer gives out significant light or heat. It is then called a black dwarf.

Life cycle of a massive star

4. Red supergiant

A very massive star has a shorter, brighter main sequence stage, because it uses its hydrogen much faster. It then swells into a red supergiant. Its core is hot enough to fuse heavier and heavier elements, up to iron. NASA notes that the largest stars fuse silicon into iron before they explode.

5. Supernova

Fusing iron does not release energy, so once the core is iron, nothing holds it up against gravity. The core collapses in a fraction of a second and the outer layers are blasted out in a supernova: an explosion so bright it can briefly outshine a whole galaxy.

6. Neutron star or black hole

The core left behind is either a neutron star, an incredibly dense ball made mostly of neutrons, or, if the remaining core is massive enough, a black hole: a region where gravity is so strong that not even light can escape. See what is a black hole for more.

Put the stages in order in this question from the Black Holes course, then press Mark.

Comparing the two paths

How stars make the elements

AQA says fusion processes in stars produce all of the naturally occurring elements. It works in steps:

• Main sequence: hydrogen fuses into helium.
• Red giants and supergiants: helium fuses into carbon, and in massive stars heavier elements form, up to iron.
• Supernovae: AQA says elements heavier than iron are produced in a supernova, and the explosion distributes the elements throughout the universe.

Those scattered elements end up in new nebulae, which form new stars and planets. The carbon in your body and the iron in your blood were made inside stars. For where the hydrogen came from in the first place, see the Big Bang.

Check your understanding.

Common mistakes

• Giving a Sun-sized star a supernova. Only stars much more massive than the Sun explode as supernovae.
• Calling a protostar a star. Fusion has not started yet.
• Saying fusion happens in a white dwarf. It shines only because it is still hot.
• Mixing up red giant and red supergiant. Red giant: Sun-sized star. Red supergiant: massive star.
• Saying elements up to iron are made in supernovae. Elements up to iron form by fusion in cores; heavier ones form in supernovae.
• Missing the forces. On the main sequence, gravity inwards balances fusion energy outwards.

Practise with a short timed paper

Space physics comes up in GCSE Physics. The daily GCSE Physics Higher paper is a short timed paper of 8 questions, with a new one every day. To explore what happens when the biggest stars collapse, try the Black Holes course.

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