Evolution of a Massive Star (OCR A Level Physics)
Revision Note
Evolution of a Massive Star
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 more than 10 times the mass of the Sun (> 10 MSun)
Massive stars become red supergiants and then either a neutron star or a black hole
Massive stars have more fuel, but they use it up faster, so they spend less time on the main sequence
Lifecycle of massive stars
1. Red Supergiant
The star follows the same process as the formation of a red giant
Hydrogen in the core runs out
Nuclear fusion slows
Radiation pressure decreases so the inward and outward acting forces are no longer in equilibrium
The core collapses
Fusion in the core stops and the outer layers expand and cool
The shell burning and core burning cycle in massive stars goes beyond that of low-mass stars, fusing elements up to iron
There is still fuel in areas outside the core
Temperatures generated by the collapsing core are high enough to fuse nuclei in the shell (shell burning)
Contraction of the core generates temperatures high enough to fuse heavier elements in the core (core burning)
This cycle continues, fusing heavier and heavier elements at successively higher temperatures and pressures
In each stable fusion phase, electron degeneracy pressure and radiation pressure balance the gravitational force and prevent the core from collapsing
Eventually, an iron core is formed
The Chandrasekhar limit determines if the core is stable enough to remain as a white dwarf
If the mass of the core is less than 1.4 times the mass of the sun, then it will remain as a white dwarf
If the mass of the core is greater than 1.4 times the mass of the sun, then the electron degeneracy pressure is not enough to prevent the core from collapsing
2. Supernova
Once the iron core forms, it becomes unstable and begins to collapse as no more fusion reactions can occur
The gravitational potential energy transferred in the collapse produces intense heating
Gravitational pressure forces protons and electrons in the iron atoms to combine to form neutrons, releasing huge amounts of energy
The outer shell is blown out in an explosive supernova
The outer layers fall inwards and rebound off the core causing shockwaves
The shockwaves cause the star to explode in a supernova
The supernova generates temperatures great enough to fuse heavy nuclei with neutrons to form all the known elements beyond iron
A Type II supernova: a bright and powerful explosion which happens at the end of a high-mass star's life. A shockwave ejects the materials in the outer shells of the star into space, and the core collapses
3. Neutron Stars & Black Holes
After the supernova explosion, the collapsed neutron core can remain intact
This is known as a neutron star
If the neutron core mass is greater than 3 times the solar mass (3 MSun), the pressure on the core becomes so great that the core collapses even further
In this case, the gravitational forces are so strong that the escape velocity of the core is greater than the speed of light, hence, photons are unable to escape
This is known as a black hole
Worked Example
Describe the evolution of a star much more massive than our Sun from its formation to its eventual death.
Answer:
Step 1: Underline the command words ‘describe’ and ‘explain’
A describe question does not need you to explain why the processes happen, but you do need to go into detail about what happens in each stage
Step 2: Plan the answer
Use the white space around the question to 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
Nuclear fusion
Main sequence
Red super giant
Supernova
Neutron star/black hole
Step 3: Add to the list any important points or key words that need to be included for each stage
Nebula – gravitational collapse
Protostar – heats up and glows
Nuclear fusion – H to He generates energy
Main sequence – stable, forces balanced
Red super giant – expands and cools
Supernova – core collapses
Neutron star/black hole – remnant
Step 4: Begin writing the answer using words from the question stem to begin
A star more massive than our Sun will form from…
Step 5: 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
Nuclear fusion of hydrogen nuclei to helium nuclei generates massive amounts of energy
The outward radiation pressure and gas pressure balance the inward gravitational pressure and the star becomes stable entering the main sequence stage
When the hydrogen runs out, the outer layers of the star expand and cool forming a red supergiant
Eventually, the core collapses and the star explodes in a supernova
The remnant core either remains intact forming a neutron star, or the core collapses further resulting in a black hole
Examiner Tips and Tricks
When revising the life cycles of stars, it's useful to draw a flow diagram showing the life cycles of low-mass and high-mass stars together to ensure you are comfortable with the stages which both go through and which stages differ
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