Stars and their life cycleIB MYP Physics: Revision notes
Section 1
How a star is born
Stars form in a nebula, a huge cloud of gas (mostly hydrogen) and dust. Gravity pulls the material together into clumps. As the gas falls inwards it gets hotter and denser, forming a protostar. When the core is hot enough (millions of degrees), hydrogen nuclei begin to fuse, and a star is born.
Section 2
The main sequence: fusion balances gravity
A star spends most of its life on the main sequence. In the core, nuclear fusion joins hydrogen nuclei to make helium and releases huge amounts of energy.
This gives an outward pressure that balances the inward pull of gravity. The star stays the same size, so it is stable. Our Sun has been on the main sequence for about 5 billion years and will stay there for about another 5 billion.
Fusion joins nuclei together. Fission splits them. Stars run on fusion, not fission, and not on burning (a chemical reaction).
Section 3
A star like the Sun: red giant to white dwarf
When the hydrogen in the core runs low, the balance is lost. The core shrinks and heats up, while the outer layers expand and cool. The star becomes a red giant and fuses helium into heavier elements.
Eventually the outer layers drift away into space, leaving the hot, small core: a white dwarf. A white dwarf slowly cools and fades.
Order: nebula, protostar, main sequence, red giant, white dwarf.
Section 4
Massive stars: supernova and beyond
A star much more massive than the Sun lives a shorter life because it fuses hydrogen faster. It becomes a red supergiant. When fusion in its core can no longer support it, the core collapses and the star explodes as a supernova.
What remains depends on the mass:
- a very dense neutron star, or
- for the most massive stars, a black hole, which has gravity so strong that not even light can escape.
Order: nebula, protostar, main sequence, red supergiant, supernova, neutron star or black hole.
Section 5
Where heavy elements come from
The early Universe had almost only hydrogen and helium. Inside stars, fusion makes heavier elements: helium joins to make carbon and oxygen, and in massive stars fusion goes on up to iron.
Elements heavier than iron are made in the energy of a supernova. The explosion scatters all these elements into space, where they join a new nebula. New stars and planets, and eventually living things, form from this material. The iron in your blood was made in a star.
If asked how heavy elements get to Earth, give the chain: made in star, thrown out in supernova, new nebula, gravity forms new star and planets.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Stars and their life cycle
- A planetarium presenter in Dubai explains that every star begins life inside a vast cloud of gas and dust called a nebula, and that the Sun is now about halfway through a long, stable period of its life.Explain why the Sun stays the same size for billions of years.2 marks
- Astronomers compare the future of the Sun with that of Betelgeuse, a red supergiant star that is about 15 times more massive than the Sun. They predict very different endings for the two stars.Compare the end of the Sun's life with the end of Betelgeuse's life.2 marks
- Chemists have found that the iron in blood, the calcium in bones and the oxygen we breathe were not present in the early Universe, which contained almost only hydrogen and helium. Astronomers say that we are made of the material of old stars.Describe how elements heavier than hydrogen and helium are made.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).