Evolution of a Low-Mass Star (OCR A Level Physics)
Revision Note
Evolution of a Low Mass Star
Once the internal forces within a star become unbalanced, then they will no longer be in equilibrium causing the star to expand or contract
This happens when fusion in the core of stars stops, and hence thermal expansion ceases at the end of the star's life
The fate of a star beyond the main sequence depends on its mass
A star is classed as a low-mass star if it has a mass between 0.5 and 10 times the mass of the Sun (0.5 MSun − 10 MSun)
A low-mass star will become a red giant before turning into a white dwarf
The lifecycle of a low-mass star
1. Red Giant
The hydrogen fuelling the star begins to run out, nuclear fusion stops, the star shrinks and then swells and cools to form a red giant
Most of the hydrogen nuclei in the core of the star have been fused into helium and so nuclear fusion slows and the energy released by fusion decreases
The radiation pressure caused by the fusion reaction also decreases, so the inward gravitational force becomes greater than the outward force from the gas pressure and radiation pressure
The core collapses, leading to an increase in temperature as it compresses under the weight of the star
Fusion in the core stops
The outer layers of the star expand and then cool forming a red giant
Fusion continues in the shell around the core
There are still hydrogen nuclei in the areas outside of the core
The heat generated by the collapsing core provides temperatures high enough for this hydrogen to fuse in a process called shell hydrogen burning
Contraction of the core continues, providing temperatures high enough to fuse helium into carbon and oxygen in a process called core helium burning
2. Planetary Nebula
The outer layers of the star are released
Helium burning in the core releases massive amounts of energy in the fusion reactions
The outward radiation pressure increases balancing the inward and outward forces
When the helium in the core runs out, the core contracts again producing temperatures high enough to fuse the helium in the areas outside the core in a process called helium shell burning
The carbon-oxygen core is not hot enough to fuse the heavy elements, the star becomes unstable and begins to collapse again
The outer layers of gas are ejected back into space forming a planetary nebula
3. White Dwarf
The solid core collapses under its own mass, leaving a very hot, dense core called a white dwarf
No further fusion reactions take place
White dwarfs continue to radiate energy in the form of photons that were produced in previous fusion reactions
Eventually, the white dwarf will cool to a few degrees Kelvin and will no longer emit any significant heat or light (black dwarf)
Worked Example
Stars less massive than our Sun will leave the main sequence and become red giants.
Describe and explain the next stages of evolution for such stars.
Answer:
Step 1: Underline the command words ‘describe’ and ‘explain’
Describe questions require details of the processes occurring
Explain questions require details of how and why those processes occur
This question requires both
Step 2: Understand what the question is asking for
The stars in the question are less massive than the Sun, therefore it is referring to low-mass stars
The question asks for the next stage of evolution after becoming a red giant, so assume that it requires an explanation of the processes during the red giant phase
Step 3: Plan the answer
Make a list of the remaining stages in the evolution of a low-mass star
Red giant
Planetary nebula
White dwarf
Add to the list any important points or keywords that need to be included in the answer
Red giant
Fuel runs out
Forces no longer balanced
Expands and cools
Fusion continues in shell
Planetary nebula
Carbon-oxygen core not hot enough for further fusion
Outer layers released
White dwarf
Core collapses leaving a remnant core
Step 4: Begin writing the answer using words from the question stem
Low-mass stars will leave the main sequence and become red giants…
Step 5: Use the plan to keep the answer concise and logically sequenced
Low mass stars will leave the main sequence and become red giants when the hydrogen in the core runs out
There is a reduction in the energy released by fusion, so the radiation pressure decreases
The radiation pressure and gas pressure no longer balance the gravitational pressure and the core collapses
Fusion no longer takes place inside the core
The outer layers expand and cool to form a red giant
Temperatures generated by the collapsing core are high enough for fusion to occur in the shell around the core
Contraction of the core produces temperatures great enough for the fusion of helium into carbon and oxygen inside the core
The carbon-oxygen core is not hot enough for further fusion, so the core collapses
The outer layers are ejected forming a planetary nebula
The remnant core remains intact leaving a hot, dense, solid core called a white dwarf
Examiner Tips and Tricks
If an exam question asks you to describe the evolution of a low-mass star, refer to the main steps in the process.
For example:
Hydrogen runs out and nuclear fusion stops
The star shrinks and swells into a red giant
Fusion continues in the shell around the core
The outer layers are released and the core collapses into a white dwarf
But if the question asks to you explain, you must also include details of why those events take place
Always read the question carefully and take a moment to plan your answer!
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