Space Physics: Exam Practice

This lesson provides exam-focused revision for the entire AQA GCSE Physics Space Physics topic (4.8), consolidating key concepts, common exam questions, mark schemes, and strategies for achieving top marks. This is a Physics-only topic — it does not appear in Combined Science Trilogy. This lesson brings together everything you have learned across the previous nine lessons and focuses on how to apply your knowledge in the exam.

---

Key Definitions You Must Know

The following definitions appear frequently in exam questions. Learn them precisely — examiners look for specific scientific terminology.

Term	Definition
Solar system	The Sun and all the objects held in orbit around it by gravity (planets, moons, asteroids, comets, dwarf planets)
Planet	A large body that orbits a star, is roughly spherical, and has cleared the neighbourhood around its orbit
Dwarf planet	A body that orbits the Sun, is roughly spherical, but has NOT cleared its orbital neighbourhood
Moon (natural satellite)	A natural body that orbits a planet due to gravitational attraction
Artificial satellite	A human-made object placed into orbit around a celestial body
Main sequence star	A stable star in which the inward pull of gravity is balanced by the outward radiation pressure from fusion
Red giant	A large, cool, red star formed when a sun-like star exhausts its hydrogen fuel and expands
Red supergiant	A very large, cool, red star formed when a massive star exhausts its hydrogen fuel
White dwarf	The hot, dense remnant core of a sun-like star after its outer layers have been ejected as a planetary nebula
Neutron star	An extremely dense remnant of a massive star after a supernova; composed almost entirely of neutrons
Black hole	A region of space where gravity is so strong that nothing, not even light, can escape
Supernova	A massive explosion at the end of a massive star's life; produces elements heavier than iron
Nebula	A cloud of gas (mainly hydrogen) and dust in space; the birthplace of stars
Red-shift	The shift of spectral lines towards longer (redder) wavelengths, indicating the source is moving away
Blue-shift	The shift of spectral lines towards shorter (bluer) wavelengths, indicating the source is moving towards the observer
Doppler effect	The change in observed frequency (and wavelength) of a wave due to relative motion between source and observer
Big Bang	The theory that the universe began from an extremely hot, dense point approximately 13.8 billion years ago
CMBR	Cosmic Microwave Background Radiation — faint microwave radiation from all directions; remnant heat from the Big Bang
Dark matter	Invisible matter that exerts gravitational force but does not emit, absorb, or reflect electromagnetic radiation
Dark energy	An unknown form of energy causing the expansion of the universe to accelerate
Geostationary orbit	An orbit above the equator with a period of 24 hours; the satellite appears stationary relative to the ground
Polar orbit	A low orbit that passes over or near the poles; used for Earth observation and mapping

Exam Tip: Many students lose marks by giving vague or imprecise definitions. For example, defining a supernova as "a big explosion" is not sufficient. A better definition is: "A supernova is the violent explosion of a massive star at the end of its life, during which elements heavier than iron are formed and ejected into space." Precision matters.

---

Common Exam Questions and Model Answers

Question Type 1: Describe the Life Cycle of a Star

Typical question: "Describe the life cycle of a star that is much more massive than the Sun." (6 marks)

Model answer structure:

1. The star forms from a nebula — a cloud of gas and dust that collapses under gravitational attraction.
2. As the material collapses, it heats up, forming a protostar.
3. When the core temperature is high enough, nuclear fusion begins (hydrogen fusing to form helium), and the protostar becomes a main sequence star.
4. The main sequence star is stable because the inward force of gravity is balanced by the outward radiation pressure from fusion.
5. When the hydrogen fuel is exhausted, the core contracts and the outer layers expand. The star becomes a red supergiant.
6. The red supergiant eventually explodes as a supernova, ejecting material into space and forming elements heavier than iron.
7. The remnant core becomes either a neutron star (if the core mass is between about 1.4 and 3 solar masses) or a black hole (if the core mass exceeds about 3 solar masses).

Exam Tip: For 6-mark "describe" questions, aim for six clear, sequential points. Use correct terminology at every stage. Do not skip steps — for example, always include the protostar stage, and always specify what happens to the outer layers versus the core.

---

Question Type 2: Explain Red-Shift and the Big Bang

Typical question: "Explain how red-shift provides evidence for the Big Bang theory." (4 marks)

Model answer:

1. Light from distant galaxies is red-shifted — the spectral lines are shifted towards longer wavelengths.
2. This means the galaxies are moving away from us (Doppler effect).
3. More distant galaxies show greater red-shift, meaning they are receding faster (Hubble's Law).
4. This shows the universe is expanding. Tracing the expansion backwards suggests all matter was once concentrated at a single point — the Big Bang, approximately 13.8 billion years ago.

Question Type 3: Evidence for the Big Bang

Typical question: "State two pieces of evidence that support the Big Bang theory." (2 marks)

Model answer:

1. The red-shift of distant galaxies — showing the universe is expanding.
2. The cosmic microwave background radiation (CMBR) — faint microwave radiation from all directions in space, which is the remnant heat from the hot, dense early universe.

---

Question Type 4: Compare Satellite Orbits

Typical question: "Compare polar orbits and geostationary orbits." (4 marks)

Model answer:

Feature	Polar Orbit	Geostationary Orbit
Altitude	Low (200–2,000 km)	High (~35,786 km)
Period	Short (~90 minutes)	24 hours
Position	Passes over poles; scans different areas	Stays above same point on equator
Uses	Earth observation, weather, mapping	Communications, TV broadcasting

---

Question Type 5: Dark Matter

Typical question: "Explain what dark matter is and describe one piece of evidence for its existence." (4 marks)

Model answer:

1. Dark matter is matter that does not emit, absorb, or reflect electromagnetic radiation — it is invisible to telescopes.
2. It has mass and exerts gravitational force on visible matter.
3. Evidence: Galaxy rotation curves show that stars at the edges of galaxies orbit faster than predicted by the visible mass alone.
4. This higher-than-expected orbital speed can only be explained by the presence of additional unseen mass (dark matter) in a halo around the galaxy.

Exam Tip: For dark matter questions, always state what dark matter does (exerts gravitational force) and what it does not do (emit, absorb, or reflect EM radiation). Then give specific evidence — galaxy rotation curves are the best example for GCSE level.

---

Equations You Must Know

Equation	Meaning	Units
W = m x g	Weight = mass x gravitational field strength	N = kg x N/kg
v = H x d	Recession velocity = Hubble constant x distance	km/s = (km/s/Mpc) x Mpc
change in wavelength / wavelength = v / c	Red-shift = recession velocity / speed of light	No units (ratio)

Worked Example: Weight on Different Bodies

An astronaut has a mass of 75 kg. Calculate their weight on:

(a) Earth (g = 9.8 N/kg): W = 75 x 9.8 = 735 N

(b) The Moon (g = 1.6 N/kg): W = 75 x 1.6 = 120 N

(c) Mars (g = 3.7 N/kg): W = 75 x 3.7 = 277.5 N

Worked Example: Red-Shift Calculation

A hydrogen spectral line has a rest wavelength of 656 nm. In a distant galaxy, this line is observed at 670 nm.

(a) Calculate the change in wavelength: Change = 670 - 656 = 14 nm

(b) Calculate the red-shift (z): z = 14 / 656 = 0.0213

(c) Calculate the recession velocity: v = z x c = 0.0213 x 3 x 10 to the power 8 = 6.4 x 10 to the power 6 m/s

Exam Tip: Always show your working in calculation questions, even if you can do it in your head. Write the equation, substitute the values, then calculate. Include units in your final answer. Marks are awarded for method as well as the final answer — so even if you get the wrong number, you can still gain marks for correct method.

---

Command Words

Understanding command words is essential for structuring your answers correctly:

Command Word	What It Means	How to Answer
State	Give a brief, factual answer	No explanation needed — just the fact
Describe	Say what happens, in order	Give a sequence of events or features; no need to explain why
Explain	Say what happens AND why	Must include reasoning using scientific concepts
Compare	Identify similarities and differences	Use comparative language ("whereas," "but," "both")
Evaluate	Weigh up evidence and form a judgement	Consider strengths, weaknesses, and reach a conclusion
Calculate	Work out a numerical answer	Show working, use correct equation, include units
Suggest	Apply your knowledge to an unfamiliar context	There may be more than one acceptable answer

---

Common Mistakes to Avoid