The Life Cycle of Stars (Oxford AQA IGCSE Physics)
Revision Note
Life Cycle & Mass
Stars have a life-cycle which means they change over time
The exact stages of the life cycle are determined by the mass of the star
All stars start in a stellar nebula, become a protostar and then a main sequence star
During the main sequence
Hydrogen nuclei combine in nuclear fusion reactions to produce light and heat
Stars can maintain their energy output for millions of years because of the large amounts of hydrogen available in space
Once all of the hydrogen has reacted, the star moves onto the next stage in its life cycle
This is dependent on the mass of the star
Summary of the life cycles of stars
Life Cycle of High-Mass Stars
After the main sequence, a larger star finishes its life cycle in the following evolutionary stages:
Red supergiant → supernova → neutron star (or black hole)
The life cycle of a high-mass star
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 to form carbon
This is followed by further fusion reactions in which nitrogen and oxygen are formed
Heavier elements up to iron are also formed
This causes the outer part of the star to expand
As the star expands, its surface cools and it becomes a red supergiant
Supernova
Once the fusion reactions inside the red supergiant cannot continue, the core of the star will collapse suddenly
The outer layers are blown away in a gigantic explosion
This is 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 during the supernova explosion, 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
Life Cycle of Low-Mass Stars
After the main sequence, a smaller mass star, like our Sun, finishes its life cycle in the following evolutionary stages:
Red giant → planetary nebula → white dwarf → black dwarf
The life cycle of a low-mass star
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
This causes the core to shrink and heat up
The core will shrink because the inward force due to gravity will become greater than the outward force due to the pressure of the expanding gases as the fusion dies down
A new series of reactions will then occur around the core, for example, helium nuclei will undergo fusion to form heavier elements, such as carbon
These reactions will cause the outer part of the star to expand to become a red giant
It is red because the outer surface starts to cool
Planetary nebula
Once this second stage of fusion reactions has 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
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
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
Worked Example
Stars can be categorised into groups based on their surface temperature and relative luminosity, as shown in the diagram below. The position of a star on the diagram is dependent on these factors.
A star with a relative luminosity of 1 emits the same amount of energy as the Sun each second.
Alpha Centauri A is a star in the main sequence period of its life cycle and it has a similar mass to the Sun.
Using the diagram, explain what changes will occur in the temperature and luminosity of Alpha Centauri A after it moves out of its main sequence.
Answer:
Step 1: Identify the sequence of stages Alpha Centauri A will go through beyond the main sequence
Alpha Centauri A has a mass similar to the Sun so it will undergo the life cycle of a smaller-mass star after its main sequence
Red giant → planetary nebula → white dwarf → black dwarf
Step 2: Locate red giant stars on the diagram and identify the temperature change and luminosity change as Alpha Centauri A becomes a red giant
Red giant stars are found to the right of Alpha Centauri A on the diagram
The temperature of Alpha Centauri A will decrease as it changes to a red giant
Red giant stars are found above Alpha Centauri A on the diagram
The luminosity of Alpha Centauri A will increase as it changes to a red giant
Step 3: Locate white dwarf stars on the diagram and identify the temperature change and luminosity change as Alpha Centauri A changes from a red giant to a white dwarf
White dwarf stars are found to the left of red giant stars
The temperature of Alpha Centauri A will increase as it changes from a red giant to a white dwarf
White dwarf stars are found below red giant stars
The luminosity of Alpha Centauri A will decrease as it changes from a red giant to a white dwarf
Examiner Tip
Stars can be categorised by their luminosity and surface temperature. Whilst you are not expected to know this, you may be asked to interpret graphs showing this relationship.
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