Astronomy and Space
The Solar System
The solar system consists of the Sun (a star) at the centre, with 8 planets orbiting it, plus dwarf planets, moons, asteroids and comets. Planets are kept in orbit by gravitational attraction between the planet and the Sun.
| Body | Description |
|---|---|
| Terrestrial planets | Mercury, Venus, Earth, Mars — rocky, small, dense |
| Gas giants | Jupiter, Saturn — large, low density, mostly hydrogen/helium |
| Ice giants | Uranus, Neptune — contain water, ammonia, methane ices |
| Dwarf planets | Pluto, Eris — too small to clear their orbital path |
Orbital period: the time for one complete orbit. Planets further from the Sun have longer orbital periods (Kepler's third law).
Light-year: the distance light travels in one year ≈ 9.46 × 10¹⁵ m. Used to measure astronomical distances.
Life Cycle of Stars
Similar mass to the Sun
- Nebula — cloud of gas (mostly hydrogen) and dust contracts under gravity.
- Protostar — temperature rises; not yet fusing.
- Main sequence star — hydrogen fuses to helium in core; stable for billions of years (Sun has ~5 billion years left).
- Red giant — hydrogen depleted; outer layers expand; helium fuses in shell.
- White dwarf — outer layers shed as a planetary nebula; dense core remains (Earth-sized); cools slowly.
Much more massive than the Sun
After the red giant stage: 4. Red supergiant — even larger; heavier elements fuse (up to iron). 5. Supernova — catastrophic explosion; elements heavier than iron formed; visible as bright flash. 6. Neutron star (moderately massive) or Black hole (very massive).
The heavy elements created in supernovae (carbon, iron, gold, etc.) are scattered through space and eventually form new stars, planets — and ultimately, us.
The Big Bang and Expansion of the Universe
Evidence for the Big Bang theory:
- Cosmic Microwave Background CMB radiation: uniform microwave radiation in all directions — thermal remnant of the hot early universe.
- Redshift of galaxies: light from distant galaxies is shifted to longer (redder) wavelengths, indicating they are moving away from us.
- Hubble's observations: nearly all galaxies are receding; more distant galaxies recede faster.
Redshift: when a light source moves away, wavelengths are stretched → shifted toward red end of the spectrum. More redshift = moving faster = further away. This shows the universe is expanding.
Hubble's law: recession speed = H₀ × distance, where H₀ is Hubble's constant (~70 km/s/Mpc).
Tracing back the expansion implies all matter started at a single point approximately 13.8 billion years ago — the Big Bang.
Gravity and Orbits
All objects with mass attract each other by gravity. The Sun's gravity keeps planets in (approximately) elliptical orbits. Planets closer to the Sun orbit faster and have shorter periods.
Satellites (artificial) orbit Earth: low Earth orbit (ISS, ~400 km) has short period (~90 min); geostationary orbit (~36 000 km) has period of 24 hours so the satellite appears stationary over one point.
⚠Common mistakes
- Stars don't become black holes automatically: only the most massive stars end as black holes. Sun-like stars become white dwarfs.
- Redshift means moving away, not just "red": the wavelength shifts toward red end; the object is receding, not necessarily red.
- Light-year is a distance, not a time: 1 ly ≈ 9.46 × 10¹⁵ m.
- Big Bang was not an explosion in space: it was an expansion of space itself from a single point of infinite density.
AI-generated · claude-opus-4-7 · v3-wjec-physics