Section 1 of 9Beginning like a smaller star
A star with a mass well above the Sun’s (more than about 8 solar masses) begins its life in the same way: a nebula collapses to form a protostar, which becomes a main sequence star when hydrogen fusion starts.
- and live only a few million to tens of millions of years rather than billions.
Section 2 of 9Red supergiant and heavier elements
When core hydrogen is used up, the star expands to become a red supergiant, far bigger than a red giant.
- then to oxygen, neon, silicon and finally iron.
Fusion of iron does not release energy: iron nuclei are the most stable, so using them as fuel absorbs energy instead. Once the core is iron, the star can no longer generate outward pressure and the core collapses.
Section 3 of 9Supernova
When the iron core collapses, it does so in less than a second.
- and in our own bodies, came from: it is said that we are made of "star stuff".
Section 4 of 9Neutron star or black hole
After the supernova, the core that remains can be in one of two forms depending on its mass.
- It is made almost entirely of neutrons packed together, with a diameter of only about 20 km, but a mass greater than the Sun’s.
- It is so dense that a teaspoon would have a mass of billions of tonnes.
- Gravity is so strong that the material collapses to a very small region,
- and nothing, not even light, can escape from within a certain distance.
- (the event horizon)
- and nothing, not even light, can escape from within a certain distance.
- Black holes can not be seen directly; their presence is deduced from the way they affect the motion of nearby stars and gas,
- and from X-rays emitted by gas falling in.
Section 5 of 9Comparison of life cycles
| Stage | Sun-like star | Massive star |
|---|---|---|
| Start | Nebula, protostar | Nebula, protostar |
| Long stable stage | Main sequence (billions of years) | Main sequence (millions of years) |
| After hydrogen runs out | Red giant | Red supergiant |
| Ending | Planetary nebula | Supernova |
| Remnant | White dwarf | Neutron star or black hole |
The mass of the star at its birth determines which path it follows.
Section 6 of 9Why the mass matters
The starting mass of a star decides the entire sequence of its life.
- so its core reaches higher temperatures,
- allowing heavier nuclei to fuse and releasing energy at a much greater rate.
- from neutron star to black hole.
Section 7 of 9Worked examples
Explain why a massive star has a shorter main-sequence life than the Sun, even though it contains more hydrogen.
A massive star has a much greater gravitational pressure, so its core is hotter and denser and fusion proceeds at a much higher rate. Its luminosity is therefore very large, and the fuel is used up rapidly, in millions rather than billions of years.
Why does the production of energy by fusion stop at iron?
Iron nuclei are among the most stable nuclei, so fusing them does not release energy; it would need an input of energy. Once the core is iron, no more outward pressure can be produced to balance gravity, and the core collapses, leading to a supernova.
A supernova remnant has a core of 2.5 solar masses. Another has a core of 5 solar masses. What is each likely to become?
The 2.5 solar mass core is below about 3 solar masses, so it becomes a neutron star.
The 5 solar mass core is above the limit, so it collapses into a black hole.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Red supergiant | Extremely large, cool and luminous star formed from a massive star after the main sequence. |
| Supernova | Explosion of a massive star at the end of its life. |
| Neutron star | Extremely dense remnant made mostly of neutrons. |
| Black hole | Region where gravity is so strong that nothing can escape. |
| Heavy elements | Elements heavier than iron, formed in supernovae. |