Space physics
The Earth, Moon and Solar System, orbits, the Sun and the life cycle of stars, galaxies and the expanding Universe.
📌 Key points
- The Earth rotates on its axis about once every 24 hours, giving day and night. It orbits the Sun in about 365 days; its tilted axis causes the seasons.
- The Moon orbits the Earth in about one month. Its phases are caused by how much of its sunlit half we can see.
- The eight planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The inner four are small and rocky; the outer four are large and mostly gas.
- The Solar System formed from a cloud of gas and dust that collapsed under gravity (the accretion model). Comets have very elliptical orbits.
- The Sun’s gravity keeps the planets in orbit. Planets further from the Sun feel a weaker pull, move more slowly and have longer orbital periods. Orbital speed v = 2πrT.
- The Sun is a medium-sized star made mostly of hydrogen and helium. It releases energy by nuclear fusion and radiates mainly infrared, visible light and ultraviolet.
- A light-year is the distance light travels in one year, about 9.5 × 1015 m.
- Stars form from clouds of gas and dust (nebulae). A stable star lasts while the outward force from its hot core balances the inward pull of gravity.
- A star like the Sun becomes a red giant, then a planetary nebula with a white dwarf. A much more massive star becomes a red supergiant, explodes as a supernova and leaves a neutron star or black hole.
- The Milky Way is one of billions of galaxies. Redshift of light from distant galaxies and the cosmic microwave background radiation (CMBR) are evidence for the Big Bang and an expanding Universe. The Hubble constant H₀ = vd ≈ 2.2 × 10−18 per second. The age of the Universe is about 1H₀.
📖 Key terms
- Light-year
- The distance travelled by light in a vacuum in one year.
- Galaxy
- A huge group of billions of stars held together by gravity.
- Redshift
- An increase in the observed wavelength of light from a galaxy moving away from us.
- Nebula
- A cloud of gas and dust in space where stars form.
- Supernova
- The explosion of a massive star at the end of its life.
- CMBR
- Cosmic microwave background radiation: microwaves from all directions, left over from the early Universe.
- Orbital period
- The time taken for one complete orbit.
✏️ Worked example
The Moon orbits the Earth at an average radius of 3.84 × 108 m, with an orbital period of 27.3 days. Find its average orbital speed.
- Convert the period to seconds: T = 27.3 × 24 × 3600 = 2 358 720 s.
- Distance in one orbit = 2πr = 2 × π × 3.84 × 108 = 2.413 × 109 m.
- v = 2πrT = 2.413 × 1092 358 720.
- Answer: v = 1020 m/s (about 1.0 km/s).
Convert the orbital period into seconds before using v = 2πrT if you want the speed in m/s.
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