5.10.3 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, 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 M_Sun − 10 M_Sun)
  • 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

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

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