Evolution of a Massive Star (OCR A Level Physics)

• 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 M_Sun) 
  • 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 M_Sun), 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

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