3.8 · Space physics

Space physics

Gravity gathers dust into stars, stars fuse the elements, and the red-shift of distant galaxies tells us the whole universe is expanding from a beginning 14 thousand million years ago. OxfordAQA's final topic closes the course with stellar life cycles, orbits and the Big Bang evidence.

OxfordAQA International GCSE Physics 9203, topic 3.8 (Version 5.1). Statements marked (P) are Paper-2-only.

Life cycle of a star

Stars form when gravity pulls dust and gas (mainly hydrogen and helium) together — smaller lumps become planets. In the long main sequence, hydrogen fuses to helium in the core and gravity balances the outward forces. The more massive a star, the hotter its core and the heavier the nuclei it can build by fusion.

When hydrogen runs out, mass decides the ending. A Sun-like star swells to a red giant, sheds its layers, and the exposed core cools from white dwarf to black dwarf. A massive star swells to a red supergiant, fuses ever-heavier elements (carbon, nitrogen, oxygen), then blasts apart as a supernova — scattering the elements (those heavier than iron are made in the explosion itself) — leaving a neutron star or black hole. Fusion in stars and supernovae made almost every naturally occurring element heavier than helium: the early universe held almost only hydrogen and helium.

Solar system and orbital motion

The solar system: eight planets orbiting the Sun, plus moons, asteroids, dwarf planets and comets. The Milky Way is one of thousands of millions of galaxies, each with thousands of millions of stars. Artificial satellites orbit Earth in geostationary or low polar orbits.

Gravity supplies the centripetal force: it acts towards the centre, changing the direction of motion (velocity) without changing speed. Greater separation means weaker gravity, so lower orbital speed: to stay at a particular radius, a satellite must move at the matching speed — change the speed and the orbit changes radius.

Worked example (comparing orbits). A low polar satellite orbits about 800 km up and goes round about every 100 minutes; a geostationary satellite is about 36 000 km up and takes 24 hours, so it stays above one point on the equator. The higher satellite feels weaker gravity, so it moves more slowly and has further to go, so its period is much longer. If a satellite is slowed down, gravity pulls it into a lower orbit.

Red shift and the expanding universe (P)

A moving wave source changes the observed wavelength (the Doppler effect): receding sources stretch waves — longer wavelength, lower frequency. The further a galaxy, the faster it recedes and the bigger the red-shift 红移 — space itself is expanding, pointing back to a very small beginning: the Big Bang 大爆炸. The cosmic microwave background 宇宙微波背景 — radiation filling the universe from shortly after that beginning — is the second pillar; the Big Bang theory is currently the only theory that explains it.

Worked example (P) (reading red-shift). The same hydrogen line is shifted by 2 nm in galaxy A and by 8 nm in galaxy B. B's shift is four times larger, so B is moving away about four times faster, and so it is further away. A line shifted to a shorter wavelength (blue-shift) would mean the source is moving towards us.

Common mistakes

• Saying a satellite in a circular orbit has no acceleration. Its speed is constant but its direction changes, so its velocity changes: gravity accelerates it towards the centre.
• Saying a higher orbit needs a higher speed. Further out, gravity is weaker, so the orbital speed is lower.
• Saying every star ends as a black hole. A Sun-like star ends as a white dwarf; only the most massive stars become neutron stars or black holes.
• Saying stars make all elements by fusion in their cores. Elements heavier than iron are made in supernovae.
• (P) Saying red-shifted galaxies look red. Every line in the spectrum moves to a longer wavelength.
• (P) Saying the galaxies move through space away from one centre. Space itself is expanding, so every distant galaxy moves away from every other.

Check yourself

1. What pulls dust and gas into stars?
2. What happens in a main-sequence star's core?
3. List the three stages a Sun-like star passes through after its main sequence.
4. Which stars become supernovae?
5. Where are elements heavier than iron made?
6. Name the two artificial-satellite orbit types.
7. Why does a satellite in a higher orbit move more slowly?
8. (P) What happens to a wave's wavelength when its source recedes?
9. (P) State the two pieces of evidence for the Big Bang.
10. (P) What is the CMBR?

Answers: 1 gravitational attraction; 2 hydrogen fuses to helium, releasing energy that balances gravity; 3 red giant, white dwarf, black dwarf; 4 stars far more massive than the Sun; 5 in supernovae; 6 geostationary and low polar; 7 gravity is weaker farther out, so a smaller centripetal force and speed suffice; 8 (P) it increases (red-shift); frequency falls; 9 (P) red-shift increasing with distance; the CMBR; 10 (P) radiation filling the universe, left from soon after the Big Bang.