The structure of the Solar System, the life cycle of stars, red-shift of light from distant galaxies, and how red-shift provides evidence for an expanding universe and the Big Bang theory.

A P8 Describe question worth four marks. A star forms when a cloud of dust and gas (a nebula) is pulled together by gravity, heating up until nuclear fusion of hydrogen into helium begins; this stable phase is the main sequence (2 marks for the nebula collapsing under gravity and fusion starting). When the hydrogen runs low, a Sun-sized star swells into a red giant (1 mark). Finally it sheds its outer layers (forming a planetary nebula) and the core is left as a hot, dense white dwarf (1 mark). Markers reward the nebula-to-main-sequence formation, the red giant stage, and the white dwarf end point for a Sun-sized star. A common error is to give the high-mass route (supernova, neutron star or black hole) for a Sun-sized star.

A P8 question worth four marks. Red-shift is the increase in the wavelength of light from distant galaxies, shifting it towards the red end of the spectrum, which happens because the galaxies are moving away from us (2 marks for the increase in wavelength caused by galaxies moving away). Observations show that more distant galaxies have a greater red-shift, so they are moving away faster, which means the whole universe is expanding (1 mark). Tracing this expansion backwards suggests everything began from a single, very hot, dense point: this is the Big Bang theory (1 mark). Markers reward red-shift as increasing wavelength from receding galaxies, the link to an expanding universe, and the Big Bang as the implied origin. A common error is to describe red-shift as a colour change of the galaxy itself rather than a stretch in wavelength.