Life Cycle of a High Mass Star (AQA A Level Physics): Revision Note
Exam code: 7408
Evolution of a massive star
Next stages for high mass stars
The fate of a star beyond the main sequence depends on its mass
A star is classed as a high-mass star if it has a mass greater than eight times the mass of the Sun
A high-mass star will become a red supergiant before exploding as a supernova
Moderately massive stars eventually become neutron stars
The most massive stars in the Universe become black holes
4. Red supergiant
The star follows the same process as the formation of a red giant
The shell-burning and core-burning cycle in massive stars goes beyond that of low-mass stars, fusing elements up to iron
5. Supernova
The iron core collapses
The outer shell is blown out in an explosive supernova
6. Neutron star (or black hole)
After the supernova explosion, the collapsed neutron core can remain intact having formed a neutron star
If the neutron core mass is greater than three times the solar mass, the pressure on the core becomes so great that the core collapses and produces a black hole
Worked Example
Describe the evolution of a star much more massive than our Sun from its formation to its eventual death.
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Answer:
Step 1: Plan your answer
List the stages that a massive star goes through, this will help you form your answer in a logical sequence of events
Nebula | Protostar | Main sequence | Red supergiant | Supernova | Neutron star/black hole |
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Step 2: Use the plan to keep the answer concise and logically sequenced
A star more massive than our Sun will form from clouds of gas and dust called a nebula
The gravitational collapse of matter increases the temperature of the cloud causing it to glow - this is a protostar [1 mark]
Nuclear fusion of hydrogen nuclei to helium nuclei generates massive amounts of energy
The outward radiation and gas pressure balance the inward gravitational pressure allowing the star to become stable as it enters the main sequence stage [1 mark]
When the hydrogen runs out, the outer layers of the star expand and cool to form a red supergiant [1 mark]
The core becomes hot enough for helium fusion
Once helium fusion ends, successive cycles of expansion and collapse occur as heavier elements are fused in the core, up to iron [1 mark]
Eventually, once iron has formed in the core and fusion reactions can no longer continue, the outward layers of the star collapse and the star undergoes a shockwave explosion known as a supernova [1 mark]
The remnant of the core collapses further and forms either a neutron star or a black hole [1 mark]
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