Stellar Evolution » Life Cycle of Massive Stars
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 8.10
- Name the stages in the life of a star much bigger than the Sun
- Explain why massive stars use up their fuel faster
- Describe a supernova and what it can leave behind
- Compare the life cycles of massive stars and Sun-like stars
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Unlock This CourseBigger stars, bigger changes
In the lesson Life Cycle of Stars Like the Sun you followed a star of similar mass to the Sun. A star's mass decides how its life goes. A star with a mass larger than the Sun starts in the same way, but its life is shorter and it ends far more dramatically.
A massive star goes through these stages:
- nebula
- main sequence star
- red supergiant
- supernova explosion
- neutron star, or a black hole for the most massive stars
Key terms:
- Red supergiant: an enormous, cool, red star. It is much bigger than a red giant.
- Supernova: a huge explosion that happens when a red supergiant collapses and blows off its outer layers.
- Neutron star: a tiny, extremely dense star left behind after a supernova.
- Black hole: an object so dense and with such a strong gravitational pull that not even light can escape from it.
Stage 1: the nebula
A massive star begins just like the Sun. A nebula is a cloud of dust and hydrogen gas. Gravity pulls it together, it heats up, and fusion starts. The difference is that a massive star forms from a nebula with more mass in it.
Stage 2: the main sequence star
A massive star is stable on the main sequence for the same reason as the Sun. The outward pressure from fusion is balanced by the inward pull of gravity. But a massive main sequence star is different from the Sun in three ways:
🔥 Hotter
Its surface is much hotter than the Sun's. Many massive stars are blue or white.
🌟 Brighter
It gives out far more light and energy every second.
⏳ Shorter-lived
It stays on the main sequence for a much shorter time than the Sun.
Why massive stars use up their fuel faster
You might expect a bigger star to last longer because it has more hydrogen. In fact it lasts for a shorter time. A massive star has a much stronger gravitational pull, so its core is squeezed harder and gets hotter. At this higher temperature and pressure, fusion happens much faster. The star releases energy at a huge rate, which is why it is so bright, and it burns through its hydrogen far more quickly than the Sun does.
Learn this
Massive stars use up their fuel faster than the Sun because fusion goes on at a much higher rate in their hotter, more squeezed cores. So they live for a shorter time.
Stage 3: the red supergiant
When the hydrogen in the core of a massive star runs out, the balance between pressure and gravity is lost. The star swells and its surface cools, just as a red giant does. But because the star was massive to begin with, it swells far more and becomes a red supergiant. It is red because its surface is cool.
Stage 4: the supernova
The red supergiant does not slowly fade away like a red giant. Its core collapses and the star blows itself apart in a huge explosion called a supernova. For a short time, a supernova can shine as brightly as a whole galaxy. The explosion throws the star's outer layers out into space.
Stage 5: neutron star or black hole
What is left in the middle after the supernova depends on the mass of the star:
- Massive stars that are not the very biggest leave behind a neutron star. This is a very small, very dense object.
- The most massive stars leave behind a black hole. Its gravity is so strong that nothing can escape from it, not even light.
Comparing the two life cycles
| Stage | Star like the Sun | Star more massive than the Sun |
|---|---|---|
| 1 | Nebula | Nebula |
| 2 | Main sequence star | Main sequence star (hotter, brighter, shorter-lived) |
| 3 | Red giant | Red supergiant |
| 4 | Outer layers drift away | Supernova explosion |
| 5 | White dwarf | Neutron star, or black hole for the most massive stars |
Worked examples
Worked example 1: put the stages in order
Put these in order for a massive star: supernova, nebula, red supergiant, neutron star, main sequence star.
Step 1: all stars begin as a nebula.
Step 2: fusion starts and the star becomes a main sequence star.
Step 3: the core hydrogen runs out and the star swells into a red supergiant.
Step 4: the star explodes in a supernova.
Step 5: the core that is left is a neutron star.
Answer: nebula, main sequence star, red supergiant, supernova, neutron star.
Worked example 2: explain a shorter life
Explain why a star with a mass much larger than the Sun has a shorter life.
Step 1: the gravitational pull is stronger, so the core is hotter and under higher pressure.
Step 2: fusion happens at a much higher rate.
Answer: the star uses up its hydrogen much faster than the Sun, so it stays on the main sequence for a shorter time.
Worked example 3: which end point?
Two massive stars both explode as supernovae. Star A is massive and star B is one of the most massive stars known. What is left behind?
Answer: star A leaves a neutron star. Star B, being the most massive, leaves a black hole.
Common mistakes
Common mistakes
- Saying a massive star lasts longer because it has more fuel. It burns its fuel much faster, so it lasts for a shorter time.
- Writing red giant for a massive star. A massive star becomes a red supergiant.
- Saying a massive star becomes a white dwarf. That is the end of a Sun-like star.
- Forgetting the supernova. It comes between the red supergiant and the neutron star or black hole.
- Saying every massive star becomes a black hole. Only the most massive stars do. The others leave a neutron star.
Exam-style question
A star has a mass much larger than the Sun.
(a) Name the stage that comes after the main sequence star. [1 mark]
(b) Explain why this star spends less time on the main sequence than the Sun does. [2 marks]
(c) Describe what happens to the star after this stage until it becomes a black hole. [3 marks]
Model answer
(a) Red supergiant (1).
(b) The star has a stronger gravitational pull, so its core is hotter and fusion happens at a higher rate (1). It uses up its hydrogen faster than the Sun (1).
(c) The star explodes in a supernova (1). The outer layers are thrown into space (1). The core that is left is so dense that nothing, not even light, can escape from it, so it is a black hole (1).
Exam tip
For a "why do massive stars live for a shorter time" question, link the mass to the rate of fusion. Saying only "they are bigger" will not get the marks.