7.18P Describe the evolution of stars with a mass larger than the Sun

The idea

A star more massive than the Sun follows a faster, more violent route. Strong gravity makes its core hotter, so it fuses hydrogen quickly and leaves the main sequence sooner. It swells into a red supergiant and fuses heavier elements in shells, building up an iron core. Iron fusion does not release energy, so the outward pressure fails and the core collapses suddenly. The collapse triggers a supernova, which blows off the outer layers and can leave a neutron star or, for the most massive stars, a black hole. The key contrast with a Sun-like star is that a massive star ends in a supernova rather than shedding layers as a planetary nebula and leaving a white dwarf.

What earns the marks

• States that a star more massive than the Sun has stronger gravity and a hotter core, so it uses its hydrogen fuel faster.
• Describes the star expanding into a red supergiant as heavier elements fuse in shells around the core.
• Explains that an iron core cannot release energy by fusion, so the outward pressure fails and the core collapses.
• Describes the collapse producing a supernova that ejects the outer layers.
• Identifies that the remnant is a neutron star, or a black hole for the most massive stars, rather than a white dwarf.

Common mistakes and how to put them right

• Says a massive star ends as a white dwarf.

  Put it right by stating that it ends in a supernova leaving a neutron star or black hole.

• Claims iron fusion releases large amounts of energy.

  Put it right by explaining that iron fusion does not release energy, so the core collapses.

• Describes a planetary nebula as the end of a massive star.

  Put it right by reserving that outcome for lower-mass stars.

• Ignores mass when describing the outcome.

  Put it right by linking greater mass to a hotter core and a faster, more violent end.

Exam tips

• Compare the massive-star route with the Sun-like route so the contrast in the final stage is clear.
• Name the sequence in order: main sequence, red supergiant, core collapse, supernova, remnant.
• Explain why the iron core causes collapse rather than simply stating that it does.
• State the remnant as neutron star or black hole and link the choice to the star's mass.