Life Cycle of Stars
- All stars, including the Sun, began as a cloud of dust and gas
- Once a star has formed, it will spend its life going through a sequence of evolutionary stages, known as the life cycle of a star
Summary of the Life Cycles of Stars
Flow diagram showing the life cycle of a star which is the same size as the Sun (solar mass) and the lifecycle of a star which is much more massive than the Sun
Star Formation
- All stars follow the same initial stages:
Nebula → protostar → main sequence star
1. Nebula
- Stars form from a giant interstellar cloud of gas and dust called a nebula
2. Protostar
- The force of gravity within a nebula pulls the particles closer together until a hot ball of gas forms, known as a protostar
- As the particles are pulled closer together the density of the protostar will increase
- This results in more frequent collisions between the particles which causes the temperature to increase
3. Main Sequence Star
- Once the protostar becomes hot enough, nuclear fusion reactions occur within its core
- Once a star initiates fusion, it is known as a main-sequence star
- During the main sequence, the star is in equilibrium and said to be stable
The Life Cycle of a Low Mass Star
- After the main sequence, a low-mass star finishes its life cycle in the following evolutionary stages:
Red giant → planetary nebula → white dwarf
4. Red Giant
- After several billion years, the hydrogen causing the fusion reactions in the star will begin to run out
- Once this happens, the fusion reactions in the core will start to die down
- The star will begin to fuse helium which causes the outer part of the star to expand
- As the star expands, its surface cools and it becomes a red giant
5. Planetary Nebula
- Once the helium fusion reactions have finished, the star will become unstable and eject the outer layer of dust and gas
- The layer of dust and gas which is ejected is called a planetary nebula
6. White Dwarf
- The core which is left behind will collapse completely, due to the pull of gravity, and the star will become a white dwarf
- The white dwarf will be cooling down and as a result, the amount of energy it emits will decrease
The Life Cycle of a High Mass Star
- 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)
- The key differences between a lower mass and higher mass star at this stage are:
- A higher mass star will stay on the main sequence for a shorter time before it becomes a red supergiant
- A lower mass star fuses helium into heavy elements, such as carbon, whereas a higher mass star fuses helium into even heavier elements, such as iron
4. Red Supergiant
- After several million years, the hydrogen causing the fusion reactions in the star will begin to run out
- Once this happens, the fusion reactions in the core will start to die down
- The star will begin to fuse helium which causes the outer part of the star to expand
- As the star expands, its surface cools and it becomes a red supergiant
5. Supernova
- Once the fusion reactions inside the red supergiant cannot continue, the core of the star will collapse suddenly and cause a gigantic explosion called a supernova
- At the centre of this explosion, a dense body called a neutron star will form
- The outer remnants of the star are ejected into space forming new clouds of dust and gas (nebula)
- The heaviest elements are formed during a supernova, and these are ejected into space
- These nebulae may form new planetary systems
6. 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
