GCSE Physics Space and the universe revision

Our solar system is the Sun plus everything bound to it by gravity: eight planets, dwarf planets (Pluto among them), moons orbiting the planets, asteroids (rocky bodies, mostly between Mars and Jupiter) and comets (ice and dust on long elliptical journeys). Zoom out: the Sun is one ordinary star among billions in the Milky Way galaxy. Zoom out again: the universe contains billions of galaxies, each with billions of stars. That sense of scale is the foundation of the whole topic.

Stars are born in nebulae — vast clouds of dust and gas. Gravity pulls the material inward; the core grows hotter and denser until nuclear fusion ignites: hydrogen nuclei fuse into helium, releasing enormous energy. The star settles onto the main sequence, where it stays for billions of years because of a perfect tug-of-war: gravity pulls in, the outward pressure from fusion energy pushes out — balanced, the star is stable.

When the hydrogen runs low, the balance tips. A Sun-sized star swells into a red giant, puffing off its outer layers and ending as a white dwarf — a hot, dense core with no fusion left, slowly cooling. A much more massive star becomes a red supergiant and then dies in a supernova — an explosion that outshines galaxies — leaving either a neutron star (crushed to nuclear density) or, for the most massive, a black hole whose gravity allows nothing, not even light, to escape.

Fusion in main-sequence stars turns hydrogen into helium; in later stages and in massive stars, fusion builds larger nuclei — carbon, oxygen, iron. Here is where you come from: the carbon in your cells and the oxygen you breathe were fused inside stars that died long ago. Elements heavier than iron cannot be made efficiently by fusion (it costs energy instead of releasing it) — they form mainly in the chaos of supernovae, which then scatter these elements through space, seeding the nebulae of future stars and planets.

Every orbit — planet round the Sun, Moon round the Earth, satellite round the planet — works the same way: gravity provides the centripetal force, pulling the object towards the centre so it keeps turning instead of flying off in a straight line. The object is perpetually falling, but its sideways motion means it keeps missing.

Artificial satellites come in two classic flavours. A geostationary satellite orbits at just the right height above the equator with a period of 24 hours, staying fixed over one point of the Earth — ideal for TV and communications dishes that never need to move. Low polar orbits circle much lower, passing over the poles while the Earth rotates beneath — great for mapping, weather and spying, since every part of the planet comes into view within a day.

When astronomers take the light of distant galaxies and compare it with the light of familiar elements, the familiar patterns appear but shifted towards the red end — longer wavelengths. This red-shift means the galaxies are moving away from us. Crucially, the further away a galaxy is, the greater its red-shift — the faster it recedes. There is nothing special about our position: from any galaxy, all the others recede. The picture is not galaxies flying through space, but space itself stretching, carrying the galaxies apart like currants in rising dough.

Run the film backwards: if everything is separating now, everything was once together — the whole universe compressed into a single, extremely small, hot and dense point. That beginning is the Big Bang. Its second pillar of evidence is the cosmic microwave background (CMB): faint microwave radiation arriving from every direction in the sky, the stretched remnant of the early universe’s fierce glow. Red-shift shows the expansion; the CMB shows the afterglow — together they make the Big Bang the accepted theory of origins.