§ 6.1 The Earth and the Solar System
Key ideas
- The Earth spins once on its tilted axis every 24 hours (day and night) and orbits the Sun in about 365 days; the Moon orbits the Earth in about a month. The tilt of the axis, not distance, causes the seasons.
- The Solar System: four small rocky inner planets, four large gaseous outer planets, plus moons, the asteroid belt between Mars and Jupiter, and comets on long elliptical orbits.
- Gravitational attraction keeps planets and satellites in orbit; the Sun's pull weakens with distance, so the outer planets travel more slowly and take far longer to orbit.
- The Solar System formed by accretion: a rotating cloud of gas and dust (a nebula) collapsed under gravity into a disc; the Sun took almost all the mass and the planets grew from the rest.
- Extended: orbits are ellipses with the Sun not at the centre. A comet speeds up as it falls towards the Sun (potential energy → kinetic energy) and slows as it climbs away, keeping its total energy constant.
Equations
v = 2πr / Torbital speed = circumference ÷ orbital period (Extended)m/s
Fig. 1 · One year and one month: the Earth circles the Sun in about 365 days while the Moon circles the Earth in about a month, both held by gravitational attraction.
Fig. 2 · The seasons: the Earth's axis stays tilted as it orbits, so the hemisphere leaning towards the Sun gets summer and, six months later, winter.
Fig. 3 · Orbital speed (Extended): the planet covers the whole circumference 2πr once every period T, and its velocity always points along the tangent.
Watch out: the seasons come from the tilt of the Earth's axis, not from the Earth being nearer the Sun in summer. Both hemispheres are the same distance away; one simply leans into the sunlight.
§ 6.2 Stars and the Universe
Key ideas
- The Sun is a medium-sized star of mostly hydrogen and helium, powered by fusion of hydrogen into helium, radiating mostly visible light and infrared.
- A galaxy is a collection of billions of stars; the Sun is one star in the Milky Way. Astronomical distances are measured in light-years: the distance light travels in one year.
- Star life cycle: nebula → protostar → main-sequence star, where the outward pressure of fusion balances gravity. A Sun-sized star then becomes a red giant → planetary nebula → white dwarf; a massive star becomes a red supergiant → supernova → neutron star or black hole.
- Light from distant galaxies is redshifted (stretched to longer wavelengths): the galaxies are moving away, and the most distant recede fastest. The Universe is expanding from the Big Bang, about 13.8 billion years ago.
- Extended: the cosmic microwave background (CMB) is the stretched afterglow of the early Universe, strong evidence for the Big Bang; 1/H0 gives an estimate of the Universe's age.
Equations
H0 = v / dHubble constant = recession speed ÷ distance (Extended)1/s
1 light-year = 9.5 × 10¹⁵ mthe distance light travels in one yearm
Fig. 4 · Two endings: a stable main-sequence star follows the top branch if it is Sun-sized, the bottom branch if it is much more massive; the remnant depends on the mass left behind.
Fig. 5 · Redshift: the galaxy's spectral lines keep their pattern but slide to longer wavelengths; the bigger the shift, the faster the galaxy is receding.
Watch out: a light-year measures distance, not time. It is how far light travels in a year, about 9.5 × 10¹⁵ m, so "4 light-years away" describes how far, never how long.