Space Physics GCSE — Stars, Galaxies and the Universe

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Space physics sits at the end of the GCSE Physics specification and is often under-revised — students run out of time before they get to it. That's a mistake, because space physics questions are very predictable and reward straightforward recall of specific sequences and definitions. This guide covers the whole topic clearly.

The Scale of the Universe

Understanding the enormous scales involved in space physics is the starting point. From smallest to largest:

• Planets: Bodies orbiting a star. Earth has a diameter of about 12,700 km.
• Stars: Massive balls of plasma undergoing nuclear fusion. The Sun has a diameter of about 1.4 million km.
• Solar systems: A star and all the objects orbiting it.
• Galaxies: Collections of billions of stars, held together by gravity. The Milky Way contains around 200–400 billion stars and is about 100,000 light-years across.
• The Universe: Everything that exists — all matter, energy, space and time. Contains hundreds of billions of galaxies.

Distances in space are so large that kilometres become impractical. A light-year is the distance light travels in one year — approximately 9.46 × 10¹⁵ metres. The nearest star to the Sun (Proxima Centauri) is about 4.2 light-years away.

The Life Cycle of Stars

Stars go through predictable stages in their lives. The path depends on the star's initial mass.

For Stars Like Our Sun (Medium Mass)

1. Nebula: A cloud of gas and dust in space. Gravity causes the cloud to contract and pull together.
2. Protostar: As the cloud contracts, it heats up. A protostar forms — not yet hot enough for nuclear fusion.
3. Main sequence star: When the core is hot enough for hydrogen nuclei to fuse into helium, nuclear fusion begins. The outward pressure from fusion balances the inward gravitational force — the star is stable. Our Sun has been in this stage for about 5 billion years and will remain here for another 5 billion.
4. Red giant: When hydrogen in the core runs out, the core contracts and heats further while the outer layers expand and cool. The star becomes a red giant — much larger and cooler at its surface.
5. Planetary nebula: The outer layers are shed, forming a glowing shell of gas around the dying star.
6. White dwarf: The hot, dense core that remains — no longer undergoing fusion. It slowly cools over billions of years. Eventually becomes a cold black dwarf (though the universe is not yet old enough for any black dwarfs to have formed).

For Very Massive Stars

1. Nebula → Protostar → Main sequence star (same as above, but more massive stars burn through their fuel much faster)
2. Red supergiant: Expands to a much larger red giant than a medium star would form.
3. Supernova: The core collapses catastrophically and then explodes outward in a supernova — one of the most energetic events in the universe. Elements heavier than iron are formed during the supernova explosion.
4. Neutron star or black hole: If the remaining core is between about 1.4 and 3 solar masses, it becomes a neutron star — incredibly dense (a teaspoon would weigh about a billion tonnes). If more massive, it collapses further into a black hole — a region where gravity is so strong that nothing, not even light, can escape.

Orbital Motion

Planets orbit stars, moons orbit planets, and satellites orbit planets — all due to gravity. For a circular orbit, gravity provides the centripetal force that keeps the object moving in a circle rather than flying off in a straight line.

The further an object is from what it orbits, the slower its orbital speed and the longer its orbital period. The planets closer to the Sun (Mercury, Venus) orbit faster than those further out (Neptune, Uranus). This is explained by the decreasing gravitational force with distance.

Red-Shift and the Expanding Universe

When astronomers observe the light from distant galaxies, the spectral lines (characteristic wavelengths produced by specific elements) are shifted toward the red end of the spectrum — they have longer wavelengths than expected. This is called red-shift.

Red-shift occurs because the galaxies are moving away from us. Just as the pitch of a sound drops when a source moves away (the Doppler effect), the wavelength of light increases when a light source moves away — shifting toward the red end of the spectrum.

Crucially, galaxies in all directions show red-shift — they are all moving away from us. The further a galaxy is from us, the greater its red-shift and the faster it is moving away. This is Hubble's Law.

The Big Bang Theory

If galaxies are currently moving apart, then in the past they must have been closer together. Extrapolating backwards, all matter and energy in the universe was once concentrated at a single point. The Big Bang was the rapid expansion from this initial state approximately 13.8 billion years ago.

Evidence supporting the Big Bang:

• Red-shift of galaxies: Consistent with the universe expanding from a single point.
• Cosmic Microwave Background (CMB) radiation: A faint, uniform microwave radiation detected in all directions — the afterglow of the hot early universe. Predicted by Big Bang theory and detected in 1965.

The AQA space physics specification is at the AQA GCSE Physics specification page.