The Life Cycle of Solar Mass Stars (AQA GCSE Physics): Revision Note

The Life Cycle of Stars

• Stars go through a sequence of evolutionary stages, known as the life cycle of a star

• All stars follow the same initial stages:

nebula → protostar → main sequence star

• However, the final stages of a star's life cycle depend on its size

• The diagram below shows the life cycles of stars which are:

  • about the same size as the Sun

  • much bigger than the Sun

Summary of the life cycles of stars

The life cycle of a star depends on whether it is similar in size to the Sun or much larger

Solar Mass Stars

• After the main sequence, stars which are similar in size to the Sun go through 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 heavier elements, such as carbon and oxygen

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

White dwarf

• Once the helium in the core begins to run out, fusion reactions cannot continue

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

• The remnant of the core is known as a white dwarf

• The white dwarf will be cooling down, and as a result, the amount of energy it emits will decrease

Black dwarf

• Once the star has lost a significant amount of energy, it becomes a black dwarf

• It will continue to cool until it eventually disappears from sight

Life cycle of a solar mass star

The life cycle of a star about the same size as the Sun