The life cycle of stars: how a star forms, the main sequence and the balance of forces, and the different fates of stars depending on their mass (separate physics).

A top-band level-of-response answer describes the sequence in order: a star forms when gravity pulls together a cloud of dust and gas (a nebula) into a protostar, which heats up as it contracts until nuclear fusion of hydrogen into helium begins. During the stable main sequence phase, the inward force of gravity is balanced by the outward pressure produced by the energy released in fusion, so the star stays the same size for a long time. When the hydrogen runs low, a massive star swells into a red supergiant, then explodes as a supernova. The remnant left behind is a dense neutron star, or a black hole if the star was massive enough. Markers reward the correct ordered sequence (nebula, protostar, main sequence, red supergiant, supernova, neutron star or black hole) and a clear explanation of the gravity-versus-fusion-pressure balance during the main sequence.

Nuclear fusion in main sequence stars and red giants/supergiants builds up heavier elements from lighter ones, producing elements up to iron (1 mark). Elements heavier than iron cannot be made by ordinary fusion and are instead created during the extreme conditions of a supernova explosion (1 mark). The supernova explosion also scatters all of these elements out into space (1 mark). This scattered dust and gas can later be pulled together by gravity to form new stars and planets, which is why the Earth and living things contain these heavier elements (1 mark). Markers reward fusion forming elements up to iron, supernovae forming the heaviest elements and distributing them, and the link to the formation of new planets.