Evolution of a Low-Mass Star (OCR A Level Physics)

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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

Lifecycle of Solar mass stars, downloadable IGCSE & GCSE Physics revision notes

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.

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

Exam Tip

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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Katie M

Author: Katie M

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