AQA GCSE Physics 8463: Space physics – Revision Notes

These notes condense Topic 8, Space physics (sections 4.8.1 and 4.8.2), of the AQA GCSE Physics (8463) specification, for first teaching 2016 with exams from June 2018 (version 1.1). The topic is examined on Paper 2 at both Foundation and Higher tier, every May/June. Two statements on circular orbits are Higher tier only and are marked below.

For full explanations and worked examples, use the Space physics study guide. Test yourself with the Space physics practice questions. The course hub is AQA GCSE Physics and the printable checklist lists every statement. The free diagnostics help you spot gaps across the course.

4.8.1.1 The solar system – key facts

• One star: the Sun.
• Eight planets and the dwarf planets orbit the Sun.
• Natural satellites = moons, which orbit planets. They are part of the solar system.
• The solar system is a small part of the Milky Way galaxy.
• The Sun formed from a nebula (a cloud of dust and gas) pulled together by gravitational attraction.

Method in steps: how a star starts to shine

1. Gravity pulls dust and gas together.
2. The material becomes denser; the core gets hotter.
3. Hot, dense enough → light nuclei join: nuclear fusion.
4. Fusion releases energy; the star shines.

Method in steps: why a main sequence star is stable

1. Gravity pulls material inwards (collapse).
2. Energy from fusion pushes material outwards (expansion).
3. The two are balanced, so the star is in equilibrium and stays the same size.

4.8.1.2 Life cycle of a star

The mass (size) of the star decides its life cycle.

Stage	Star about the size of the Sun	Star much more massive than the Sun
1	nebula	nebula
2	protostar	protostar
3	main sequence star	main sequence star
4	red giant	red super giant
5	white dwarf	supernova
6	black dwarf	neutron star or black hole

Memory hook: the Sun-sized path ends cold and dark (black dwarf); the massive path ends in an explosion (supernova), leaving something very dense (neutron star or black hole).

Where the elements come from

Where	What forms
Main sequence core	hydrogen fuses to helium
Later fusion in stars	heavier elements, up to iron
Supernova	elements heavier than iron
Supernova explosion	distributes the elements throughout the universe

Fusion processes in stars produce all the naturally occurring elements.

4.8.1.3 Orbits and satellites

• Gravity provides the force that keeps planets and satellites (natural and artificial) in circular orbits.

	Planet	Moon	Artificial satellite
Orbits	the Sun	a planet	usually the Earth
Natural?	yes	yes	no, made by people
Held by	Sun’s gravity	planet’s gravity	Earth’s gravity

Similarities: all orbit something; all kept in orbit by gravity. Distinctions: what they orbit; natural or artificial; size.

Higher tier only – the two orbit rules

Rule 1: constant speed, changing velocity. Direction changes all the time → velocity (a vector) changes → the object is accelerating → the resultant force is gravity, towards the centre. Speed (a scalar) stays the same.

Rule 2: speed and radius are linked. For a stable orbit, if the speed changes, the radius must change. Smaller orbit ↔ faster speed. Larger orbit ↔ slower speed.

Useful equation from Forces

For an orbit, distance in one orbit = circumference = 2πr.

Equation	Symbols	Units
distance travelled = speed × time	s = v t	m, m/s, s
circumference of a circle	2πr	m

Worked reminder: a moon moves in an orbit of radius 4.0 × 10⁸ m and takes 2.4 × 10⁶ s per orbit. v = 2π × 4.0 × 10⁸ / 2.4 × 10⁶ = 1050 m/s (3 s.f.).

4.8.2 Red-shift

Definitions

Term	Meaning
Red-shift	the observed increase in the wavelength of light from most distant galaxies
Receding	moving away from us
Big Bang theory	the universe began from a very small region that was extremely hot and dense
Dark mass (dark matter)	bends light and holds galaxies together but does not emit electromagnetic radiation
Dark energy	the unknown cause of the universe expanding ever faster

The pattern

• The further away a galaxy is, the faster it is moving away.
• The faster it recedes, the bigger the increase in wavelength.

Method in steps: red-shift → Big Bang

1. Light from distant galaxies is red-shifted, so they are moving away.
2. Further galaxies move away faster → space itself is expanding.
3. An expanding universe must have been smaller in the past.
4. Traced back, it began from a very small, extremely hot and dense region → supports the Big Bang.

Method in steps: comparing red-shift data

1. Increase in wavelength = observed − laboratory value.
2. Percentage increase = increase ÷ laboratory value × 100.
3. Bigger increase → faster recession → further away.

Worked reminder: a line at 500 nm in the lab is observed at 515 nm. Increase = 15 nm; percentage = 15 ÷ 500 × 100 = 3.0 %.

How observations become theories

• Scientists observe (measure red-shifts of many galaxies).
• They look for a pattern (speed increases with distance).
• They propose a model that explains it (an expanding universe from a Big Bang).
• New data can change the model: since 1998 onwards, supernova observations suggest distant galaxies are receding ever faster.

There is still much not understood, for example dark mass and dark energy.

Must-know distinctions

• Red giant vs red super giant: Sun-sized stars become red giants; much more massive stars become red super giants.
• White dwarf vs black dwarf: a white dwarf is hot and shines; a black dwarf is one that has cooled.
• Neutron star vs black hole: both are left after a supernova; a black hole forms from the most massive stars.
• Speed vs velocity (Higher tier only): in a circular orbit, speed is constant, velocity changes.
• Moon vs artificial satellite: a moon is a natural satellite; an artificial one is made and launched by people.
• Red-shift vs “turning red”: red-shift is an increase in wavelength, not a change in the colour of the whole galaxy.
• Galaxy vs solar system: the solar system is one star and what orbits it; the Milky Way galaxy contains a huge number of stars.

Quick self-test

1. How many stars are in the solar system? Name it.
2. What is a natural satellite?
3. What two effects are balanced in a main sequence star?
4. Give the stage after “red super giant” in the life cycle of a massive star.
5. Where are elements heavier than iron produced?
6. A spectral line measured at 434 nm in the lab is observed at 447 nm in light from a galaxy. Calculate the percentage increase in wavelength.
7. A satellite orbits at radius 2.70 × 10⁷ m with a period of 4.40 × 10⁴ s. Calculate its speed.
8. Galaxy P is 2.0 × 10²⁴ m away and recedes at 4600 km/s. Speed is proportional to distance. Estimate the speed of galaxy Q, 3.0 × 10²⁴ m away.
9. (Higher tier only) A satellite in a stable orbit is moved to a smaller orbit. How must its speed change?
10. (Higher tier only) Why is an object in a circular orbit accelerating even though its speed is constant?
11. What did observations of supernovae since 1998 suggest?
12. Name two things about the universe that are still not understood.

Answers

1. One: the Sun.
2. A moon; a natural object that orbits a planet.
3. Inward pull of gravity (collapse) and outward push from fusion energy (expansion).
4. Supernova.
5. In a supernova.
6. Increase = 13 nm; 13 ÷ 434 × 100 = 3.0 % (2.995…).
7. Distance = 2π × 2.70 × 10⁷ = 1.70 × 10⁸ m; v = 1.70 × 10⁸ ÷ 4.40 × 10⁴ = 3860 m/s (3 s.f.).
8. 4600 × 3.0 ÷ 2.0 = 6900 km/s.
9. It must increase: smaller orbit, faster speed.
10. Its direction changes, so its velocity changes; a change in velocity is an acceleration (caused by gravity towards the centre).
11. Distant galaxies are receding ever faster (the expansion is speeding up).
12. Dark mass (dark matter) and dark energy.

Where marks are usually lost

• Writing “the gas cloud explodes to form the Sun” instead of “gravity pulls the dust and gas together”.
• Explaining stability with one force only; you need gravity inwards and fusion energy outwards, balanced.
• Mixing up the two life cycles: red giant with supernova, or red super giant with white dwarf.
• Leaving out “protostar” or “main sequence” when asked for the whole sequence.
• Claiming every element is made in a supernova; only elements heavier than iron are.
• Forgetting the second half of the supernova point: it distributes the elements through the universe.
• (Higher tier) Saying the “speed changes” in a circular orbit; it is the velocity that changes.
• (Higher tier) Reversing the orbit rule; faster means a smaller radius.
• Dividing a wavelength change by the observed value instead of the laboratory value.
• Describing red-shift without stating that the wavelength increases and that further galaxies show a bigger increase.

Official syllabus

AQA GCSE Physics (8463) specification, for first teaching 2016, exams from June 2018, version 1.1, published by AQA – section 4.8 Space physics (4.8.1 Solar system; stability of orbital motions; satellites, and 4.8.2 Red-shift).