Life Cycle of a Star (Cambridge (CIE) IGCSE Physics): Revision Note

Life cycle of low-mass stars

Extended Tier Only

• A low-mass star is one with a mass of up to about eight times that of the Sun

• After the main sequence, a low-mass star finishes its life cycle in the following evolutionary stages:

red giant → planetary nebula → white dwarf

Red giant

• After several billion years, the hydrogen fuel used for nuclear reactions begins to run out

• Once this happens, the rate of fusion decreases, which causes the core to shrink and heat up

  • As the energy produced by fusion decreases, the inward force due to gravity becomes greater than the outward force due to the thermal pressure

• Eventually, the star becomes a red giant when the core becomes hot enough for helium to fuse into carbon

• The energy released by re-ignited fusion reactions causes the outer layers of the star to expand and cool

Planetary nebula

• Once the helium in the core runs out, fusion reactions cannot continue

• The star becomes unstable and the core collapses under its own gravity

• The outer layers are ejected into space as a planetary nebula

White dwarf

• The collapsed core of the red giant is called a white dwarf

• The white dwarf cools down over time and as a result, the amount of energy it emits decreases

The life cycle of a low-mass star

Life cycle of high-mass stars

Extended Tier Only

• A high-mass star is one with a mass of more than about eight times that of the Sun

• After the main sequence, a high-mass star finishes its life cycle in the following evolutionary stages:

red supergiant → supernova → neutron star (or black hole)

Red supergiant

• After several million years, the hydrogen in the core begins to run out

• Similar to a low-mass star, the rate of fusion decreases and the core shrinks and heats up

• The star becomes a red supergiant when the core becomes hot enough for helium fusion to start

• This causes the outer layers of the star to expand and cool

• In the core of the star, helium fuses into carbon

  • This is followed by further fusion reactions in which successively heavier elements, such as nitrogen and oxygen, are formed

• During this stage, the core collapses and expands repeatedly as fusion reactions start and stop

Supernova

• Eventually, fusion reactions inside the red supergiant cannot continue once iron is formed

• The core of the star will collapse rapidly and initiate a gigantic explosion called a supernova

• At the centre of this explosion, a dense body called a neutron star will form

• The outer layers of the star are ejected into space forming new clouds of dust and gas (nebula)

• The nebula from a supernova may form new stars with orbiting planets

  • The heaviest elements (elements heavier than iron) are formed during a supernova and are ejected into space

  • These nebulae may form new planetary systems

Neutron star (or black hole)

• In the case of the most massive stars, the neutron star that forms at the centre will continue to collapse under the force of gravity until it forms a black hole

• A black hole is an extremely dense point in space that not even light can escape from

The life cycle of a high-mass star