Solar system, life cycle of stars and orbital motion (physics only) – GCSE Physics exam questions with mark scheme

1Grade 1–3Triple only3 marks

(a) Name the galaxy that contains our Solar System. [1]

(b) What name is given to natural satellites that orbit planets? [1]

(a) the Milky Way(1)
(b) moons(1)
(c) any one from: communications / weather monitoring / satellite navigation (GPS) / observing Earth or space(1)
Answer: (a) Milky Way (b) moons (c) e.g. communications
Examiner tip: The Moon is a natural satellite; satellites launched by people are artificial.

2Grade 4–5Triple only3 marks

Describe how the Sun was formed. [3]

a cloud of dust and gas (nebula)(1)
pulled together by gravity (forming a protostar)(1)
temperature (and density) increases until (hydrogen) nuclei join / nuclear fusion starts (allow hydrogen fuses to form helium; do not accept fission)(1)
Answer: A nebula of dust and gas is pulled together by gravity into a protostar; it heats up until fusion of hydrogen into helium begins.
Examiner tip: Write 'fusion' (light nuclei joining), not 'fission' — and name gravity as the force pulling the dust and gas together.

3Grade 4–5Triple only5 marks

The stages in the life cycle of a star about the same size as the Sun are listed in the wrong order.

white dwarf main sequence star nebula red giant black dwarf protostar

(a) Write the stages in the correct order. [3]

(b) Explain why a main sequence star is stable for a long time. [2]

(a) nebula → protostar first(1)
(a) main sequence star → red giant next(1)
(a) white dwarf → black dwarf last(1)
(b) the inward force due to gravity(1)
(b) is balanced by the outward force (pressure) due to fusion energy in the core(1)
Answer: (a) nebula, protostar, main sequence star, red giant, white dwarf, black dwarf (b) gravity inwards is balanced by the outward pressure from fusion
Examiner tip: Stability needs BOTH forces named and the word 'balanced' (equilibrium).

4Grade 6–7Triple only5 marks

Describe what happens to a star much more massive than the Sun after its main sequence stage. Explain how elements heavier than iron came to be found in the Solar System. [5]

it becomes a red super giant (allow red supergiant; do not accept red giant)(1)
it explodes as a supernova(1)
(the remaining core) becomes a neutron star or a black hole (allow either)(1)
elements heavier than iron are formed (by fusion) in a supernova(1)
the explosion distributes these elements through space / the universe (allow: the Solar System formed from a nebula / cloud of dust and gas containing material from an earlier supernova)(1)
Answer: Red super giant → supernova → neutron star or black hole. Elements heavier than iron are made in the supernova, spread through space, and the Solar System formed from this material.
Examiner tip: Fusion in a normal star makes elements only up to iron — heavier ones need a supernova.

5Grade 6–7HigherTriple only4 marks

A satellite moves at a constant speed in a circular orbit around the Earth.

(a) Explain why the satellite is accelerating even though its speed is constant. [3]

(b) A second satellite is in a stable orbit with a smaller radius. Compare its speed with the speed of the first satellite. [1]

(a) the direction of motion is continuously changing(1)
(a) so its velocity is changing (velocity is a vector)(1)
(a) the resultant force (gravity) acts towards the centre of the Earth, so it accelerates towards the centre(1)
(b) it moves faster (a smaller orbit needs a higher speed to be stable)(1)
Answer: (a) Direction changes so velocity changes; gravity acts towards the centre, giving an acceleration. (b) Faster.
Examiner tip: Acceleration is a change in VELOCITY — a change of direction counts even at constant speed.

6Grade 4–5Triple only5 marks

The graph shows the orbital speed and the mean orbital radius of six planets in our Solar System.

(a) Describe how the orbital speed of a planet changes as its orbital radius increases. [2]

(b) Ceres is a dwarf planet. Ceres orbits the Sun at a mean orbital radius of 414 million km.

Use the graph to estimate the orbital speed of Ceres. [1]

Orbital speed = ____________ km/s

Name the force that keeps each planet in its orbit and give the direction in which this force acts. [2]

(a) the (orbital) speed decreases
allow the further from the Sun, the slower the planet moves
do not accept ‘inversely proportional’(1)
(a) the decrease is less / the curve flattens at larger radii
allow speed falls quickly for small radii and slowly for large radii
allow quoted pair of values showing this, eg Mercury → Mars speed falls by about 23 km/s but Jupiter → Saturn falls by only about 3 km/s(1)
(b) 18 (km/s)
allow any value from 17 to 19 (km/s)(1)
(c) gravity / gravitational force (of attraction between the planet and the Sun)(1)
(c) towards the (centre of the) Sun
allow towards the centre of the orbit / circle
ignore downwards / inwards on its own(1)
Answer: (a) Speed decreases as radius increases, but by less and less (curve flattens). (b) about 18 km/s (17–19). (c) Gravity (of the Sun), acting towards the centre of the Sun.
Examiner tip: For a 2-mark ‘describe’ on a curve, give the trend and how the gradient changes – ‘speed decreases’ alone is only 1 mark.

7Grade 6–7HigherTriple only7 marks

The diagram shows a communications satellite in a circular orbit around the Earth. The satellite is moving anticlockwise at a constant speed. The diagram is not drawn to scale.

(a) Which arrow, A, B, C or D, shows the direction of the resultant force on the satellite? [1]

(b) Which arrow, A, B, C or D, shows the direction of the velocity of the satellite at the instant shown? [1]

The radius of the orbit is 42 200 km.

The distance travelled in one orbit $= 2\pi \times \text{radius}$

Calculate the orbital speed of the satellite in m/s. [3]

(d) The satellite is only in a stable orbit if it moves at exactly this speed.

Explain what would happen to the satellite if its speed were increased while it was at this radius. [2]

(a) A(1)
(b) B(1)
(c) distance $= 2\pi \times 4.22\times10^{7}$ m $= 2.65\times10^{8}$ m
allow $2\pi \times 42\,200 = 265\,150$ km(1)
(c) time $= 24\times60\times60 = 86\,400$ s
allow correct substitution into $v = s/t$ with both values in consistent units, eg $265\,150 \div 24$(1)
(c) $v = 3070$ m/s
allow 3069 / 3100 / 3.07 × 10³ m/s
allow 11 000 km/h or 3.07 km/s for 2 marks only (answer not in m/s)(1)
(d) the gravitational force (on the satellite) would not be large enough to keep it moving in a circle of this radius
allow the force of gravity stays the same but a bigger force would be needed for the faster speed(1)
(d) so the satellite moves (further) away from the Earth / its orbit radius increases
allow it would move into a higher orbit
allow it would escape from orbit
do not accept it moves closer to the Earth(1)
Answer: (a) A (b) B (c) $v = \dfrac{2\pi\times4.22\times10^{7}}{86\,400} = 3070$ m/s (d) Gravity is no longer big enough to keep it in that circle, so it moves further from the Earth (larger radius).
Examiner tip: Convert both km → m and hours → s before dividing: answers of 11 000 or 3.07 lose the final mark because they are not in m/s.

8Grade 6–7Triple only6 marks

A star with the same mass as the Sun changes size several times during its life cycle.

Explain, in terms of the forces acting, how and why the size of the star changes from the time it forms until the end of its life cycle. [6]

Formation: the star forms from a nebula / cloud of dust and gas; gravity pulls the dust and gas together so the cloud collapses / gets smaller; it becomes a protostar which gets hotter (as it collapses)(1)
Fusion starts: when the temperature and density are high enough, hydrogen nuclei fuse to form helium; fusion releases energy which produces an outward force / pressure(1)
Main sequence: the inward force of gravity is balanced by the outward force due to fusion energy / the star is in equilibrium; so the size stays the same (for billions of years)(1)
Red giant: when the hydrogen (in the core) begins to run out, heavier elements (eg helium) start to fuse; the outward force becomes greater than gravity so the star expands into a red giant(1)
White dwarf: when fusion stops there is no longer an outward force (big enough) to balance gravity; gravity makes the star collapse into a small, hot, dense white dwarf(1)
Black dwarf: the white dwarf cools down and becomes a black dwarf (when it no longer emits significant energy); its size does not change further
ignore references to red super giants / supernovae / neutron stars / black holes; do not credit them as stages of this star(1)

Level	Marks	What it looks like
Level 3	5–6	A detailed, logically sequenced account from nebula to final stage in which each change of size is explained by whether the inward gravitational force or the outward force from fusion energy is bigger, including a clear explanation of the main-sequence equilibrium.
Level 2	3–4	Most stages in a correct order with some explanation in terms of forces; the balance on the main sequence is stated, but links between the forces and the expansion or collapse in later stages may be missing or unclear.
Level 1	1–2	Simple statements naming some stages or forces, with little or no linking of force to change of size.
No relevant content	0	Nothing worthy of credit.

Answer: Nebula collapses under gravity → protostar heats up → fusion of hydrogen to helium starts; on the main sequence the inward pull of gravity is balanced by the outward force from fusion energy so the size is constant; when hydrogen runs out heavier elements fuse, the outward force exceeds gravity and the star expands to a red giant; when fusion stops gravity wins and it collapses to a white dwarf, which cools to a black dwarf.
Examiner tip: Every change of size needs a ‘because’: name which force is bigger at that stage – listing the stages in order only reaches Level 1.

9Grade 8–9HigherTriple only8 marks

The table gives approximate data for four main sequence stars. Star B has the same mass as the Sun.

Star	Mass compared with the Sun	Time spent as a main sequence star in years
A	0.8	$1.7\times10^{10}$
B	1.0	$1.0\times10^{10}$
C	15	$1.5\times10^{7}$
D	25	$7.0\times10^{6}$

(a) Describe the relationship between the mass of a star and the time it spends as a main sequence star. [1]

(b) Calculate how many times longer star B spends as a main sequence star than star D.

Give your answer to 2 significant figures. [2]

Suggest why. [1]

(d) The universe is about $1.4\times10^{10}$ years old.

Explain why scientists think that no star with the same mass as star A has ever stopped being a main sequence star. [2]

(e) Stars A and D will both end their life cycles as small, very dense objects.

Name the final stage of the life cycle of star A and a possible final stage of star D. [2]

(a) the greater the mass, the shorter the time (as a main sequence star)
allow converse
do not accept inversely proportional(1)
(b) $\dfrac{1.0\times10^{10}}{7.0\times10^{6}}$
allow 1428.6 / 1429 / 1430 for this mark(1)
(b) 1400 (times)
answer must be given to 2 s.f. for this mark(1)
(c) (the hydrogen in) star D fuses (much) faster / at a (much) higher rate (so it uses up its hydrogen sooner)
allow star D uses up its hydrogen (fuel) more quickly
ignore star D is bigger / hotter on its own(1)
(d) star A spends (about) $1.7\times10^{10}$ years / longer than $1.4\times10^{10}$ years on the main sequence / its main-sequence time is longer than the age of the universe(1)
(d) so even a star of this mass that formed (soon after the universe began) has not had enough time to use up its hydrogen / finish the main sequence stage
do not accept ‘star A is older than the universe’(1)
(e) star A: black dwarf
allow white dwarf(1)
(e) star D: neutron star or black hole
do not accept black dwarf / white dwarf / supernova(1)
Answer: (a) Greater mass → shorter main-sequence time. (b) $1.0\times10^{10} \div 7.0\times10^{6} = 1429 \approx 1400$ times. (c) Hydrogen in D fuses at a much higher rate, so it is used up sooner. (d) A’s main-sequence time (1.7 × 10¹⁰ yr) is longer than the age of the universe, so even the earliest such stars have not had time to finish. (e) A: black dwarf (via white dwarf); D: neutron star or black hole.
Examiner tip: In (b) the 2 s.f. instruction is worth a mark: 1430 or 1428.6 gets 1 of 2.

10Grade 1–3FoundationTriple only6 marks

Some objects found in space are listed below.

artificial satellite dwarf planet galaxy moon planet star

(a) Use words from the list to complete each sentence. Each word may be used once or not at all.

(i) Pluto is a ____________________ . [1]

(ii) The Hubble Space Telescope was launched from Earth. It is a type of ____________________ . [1]

(iii) The Sun is a ____________________ . [1]

(b) How many planets are there in our Solar System? [1]

(d) Mars and the Moon both move in orbits.

Give one difference between the orbit of Mars and the orbit of the Moon. [1]

(a)(i) dwarf planet(1)
(a)(ii) artificial satellite(1)
(a)(iii) star(1)
(b) 8 / eight(1)
(c) gravity
allow gravitational (force / attraction) / weight
ignore magnetic(1)
(d) Mars orbits the Sun but the Moon orbits the Earth / a planet
allow Mars orbits a star and the Moon orbits a planet
allow Mars has a (much) larger orbit / takes (much) longer to complete one orbit
do not accept the Moon orbits the Sun as the only statement(1)
Answer: (a)(i) dwarf planet (ii) artificial satellite (iii) star (b) 8 (c) gravity (d) Mars orbits the Sun; the Moon orbits the Earth.
Examiner tip: In (d) say what each object orbits – ‘the Moon is smaller’ compares the objects, not their orbits, and scores nothing.

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