Earth and the Solar System
The Earth spins on a tilted axis once a day (day and night) and orbits the Sun once a year; the tilt gives the seasons. The Moon orbits the Earth about once a month, showing phases as we see different amounts of its sunlit half. Eight planets, minor planets and comets orbit the Sun, held by gravity. Orbital speed is .
≈ 25 minStars and the HR diagram
The Hertzsprung–Russell diagram plots luminosity against surface temperature (hot on the left). Most stars lie on the main sequence; red giants and supergiants are top right; white dwarfs bottom left. Nearby distances come from parallax: (parsecs) (arcseconds). AQA also uses magnitudes: .
≈ 45 minStars and the Universe
Stars form from collapsing clouds of gas and dust and shine by fusing hydrogen. Sun-like stars swell into red giants and end as white dwarfs; much more massive stars explode as supernovae, leaving neutron stars or black holes. Light from distant galaxies is redshifted: the further away, the faster they recede. The Universe is expanding from a Big Bang about billion years ago.
≈ 30 min