Edexcel IGCSE Physics · Spec 8.2
Stellar Evolution
The life cycle of stars, from nebula to their final stages.
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Stellar Evolution
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Explained
The life of a star, and the fork in the road
Every star begins in a nebula, a vast cloud of gas and dust. Gravity pulls the cloud together, and as it collapses the material in the centre becomes hotter and denser. That contracting centre is a protostar.
When the core is hot enough, hydrogen nuclei begin to fuse into helium, releasing enormous amounts of energy. At that moment the protostar becomes a star.
The main sequence
A star spends most of its life on the main sequence, and it is stable there for a specific reason worth understanding rather than memorising.
Gravity is pulling the star inwards. The outward pressure from the energy released by fusion is pushing outwards. The two balance, so the star neither collapses nor expands. That balance lasts as long as the hydrogen does, which for our Sun is about ten thousand million years.
What happens when the hydrogen runs out
Fusion slows, the outward pressure drops, and the balance is lost. The core contracts while the outer layers expand and cool, so the star becomes a red giant, or a red supergiant if it is much more massive than the Sun.
From here the path depends entirely on mass, and this is where the two routes separate.
Stars like the Sun
A red giant sheds its outer layers, leaving the hot dense core behind as a white dwarf. No fusion takes place in a white dwarf. It simply cools and fades over an enormous span of time.
Much more massive stars
A red supergiant ends violently, exploding as a supernova. What remains at the centre depends on how massive the star was: a neutron star, which is extraordinarily dense, or for the most massive stars a black hole, whose gravity is so strong that not even light escapes.
Supernovae matter beyond the star itself. The elements heavier than iron are formed in them and scattered into space, which is where the material for later stars and planets comes from.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme asks for the remaining stages in the life of a massive star and lists supernova, then neutron star, then black hole. Its note instructs the examiner to deduct a mark if the order of the stages is incorrect or if incorrect stages are named. It allows pulsar as an alternative to neutron star.
Order is being marked as well as content, so a correct list in the wrong sequence loses credit, and adding an extra stage that does not belong costs you rather than covering you.
On the following part, about why two stars end differently, the mark scheme wants the idea that evolution depends on mass, and awards both marks at once for expressing that mass in solar masses relative to the Sun.
A later paper compares a white dwarf with the Sun and credits, among other points, that fusion does not take place in the white dwarf but does in the Sun. That single sentence is what separates a dead star from a living one.
Colour and temperature
A star's colour tells you its surface temperature, and the order is the opposite of the everyday association with hot and cold taps. Red stars are the coolest, then orange, yellow and white, with blue and blue white the hottest.
So our yellow Sun sits in the middle, red supergiants such as Betelgeuse are relatively cool despite their size, and a blue star is far hotter than either.
Spec 8.2
What you need to know
- Describe how a star forms
- Explain the main sequence
- Describe how stars end their lives
Active recall
Quick check
Answer each question before opening the answer.
Outline the life cycle of a star like the Sun.
Nebula → main sequence star → red giant → white dwarf.
What happens to a star much more massive than the Sun at the end of its life?
Red supergiant → supernova → neutron star or black hole.
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