Space physics — physics only (4.8)

Stars are born, burn and die; galaxies race away from each other; and the light they send us carries the news. This reference covers AQA GCSE Physics 8463, topic 4.8 Space physics.

How the exam treats this topic:

• The whole topic is physics only, sitting on Paper 2.
• The three orbital-motion statements are HT only (circular orbits, changing velocity at constant speed, stable-orbit radius changes).
• Facts must be exact: the life-cycle sequence, the fusion story of the elements, and the red-shift chain.

The solar system: one star (the Sun), eight planets, the dwarf planets orbiting the Sun, and natural satellites (moons) orbiting planets. Our solar system is a small part of the Milky Way galaxy.

The Sun's formation: a cloud of dust and gas (a nebula 星云) was pulled together by gravitational attraction. As it collapsed:

1. the dense centre heated until fusion began — a star lit;
2. fusion's outward pressure balances gravity's inward pull — an equilibrium that lasts the star's main-sequence life.

The life cycle is determined by the star's size.

Sun-sized star: nebula → protostar → main sequence (fusion of hydrogen; equilibrium) → red giant (hydrogen runs out; helium and heavier elements fuse; the star swells) → white dwarf (fusion stops; the core shrinks and cools) → eventually a black dwarf.

Massive star (much more massive than the Sun): nebula → protostar → main sequence → red supergiant → supernova 超新星 (explosion) → neutron star, or — for the most massive — a black hole.

Where the elements come from (a favourite sequence):

• Fusion in stars makes elements up to iron.
• Elements heavier than iron form in a supernova.
• The supernova distributes the elements throughout the universe — the stuff of planets and people.

Gravity provides the centripetal force keeping planets and satellites in circular orbits.

• Planets: orbit the Sun. Moons: natural satellites orbiting planets. Artificial satellites: made by us, orbiting the Earth. All held by gravity, and distinguished only by what they orbit and who made them.
• (HT) A circular orbit has changing velocity but unchanged speed — velocity is a vector, and the direction changes continuously; the gravitational force acts at right angles to the motion, changing direction but not speed.
• (HT) For a stable orbit at a different speed, the radius must change: move faster and the orbit must be smaller (or the star pulls you off course); move slower and it must be larger.

Light from most distant galaxies shows an increase in wavelength — a shift towards the red end: red-shift 红移.

• The further away the galaxy, the faster it is receding and the bigger the red-shift.
• Red-shift means the universe is expanding. Played backwards, everything was once in a very small, extremely hot and dense region — the Big Bang.

The credited chain: observed red-shift → galaxies receding → further = faster → space itself expanding → Big Bang. And the scientific-method point: observations (red-shift surveys, and since 1998 supernovae showing galaxies receding ever faster) are the evidence from which the theory is built; much remains unknown, e.g. dark matter and dark energy.

• List the solar system's contents and the Sun's formation from a nebula by gravity.
• Draw or order both life cycles; state where every element forms.
• Explain orbits with gravity; (HT) explain changing velocity at constant speed and radius changes for stable orbits.
• State the red-shift chain and its Big Bang conclusion, the 1998 supernova observation, and one open unknown.