Cambridge IGCSE Physics 0625: Space physics – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover topic 6, Space physics, of the Cambridge IGCSE Physics 0625 syllabus for examination in 2026, 2027 and 2028: sections 6.1 (The Earth and the Solar System) and 6.2 (Stars and the Universe). Where needed, use: speed of light = 3.0 × 10⁸ m/s, 1 light-year = 9.5 × 10¹⁵ m, H₀ = 2.2 × 10⁻¹⁸ per second, 1 day = 86 400 s.

Tier note: questions 1 to 7 and question 13 use Core content only, and every candidate should do them. Questions 8 to 12, marked (Extended), include Supplement content of the 0625 syllabus (6.1.1 Supplement 4; 6.1.2 Supplement 7–10; 6.2.1 Supplement 2; 6.2.2 Supplement 2–3; 6.2.3 Supplement 5–11), which is examined on Papers 2 and 4 only. Core candidates do not need them. The marks shown are indicative only.

Related: study guide, revision notes, course hub, printable checklist.

Questions

1. (a) Name the planet between the Earth and Jupiter. (b) State where most asteroids are found. (c) State what type of body Pluto is. [3]

2. This question is about the Earth.

(a) State the approximate time the Earth takes to rotate once on its axis. [1] (b) Explain why a town has day and night. [2] (c) Explain why the northern hemisphere has summer and winter. [3]

3. A space probe is 1.35 × 10¹² m from the Earth. Calculate the time, in minutes, for its radio signal (travelling at the speed of light) to reach the Earth. [3]

4. This question is about the Moon.

(a) State the approximate time the Moon takes to orbit the Earth. [1] (b) Explain why the Moon appears to change shape over this time. [3]

5. Use the accretion model to explain why the four inner planets are small and rocky but the four outer planets are large and gaseous. [4]

6. This question is about gravity in the Solar System.

(a) Planet X has a greater gravitational field strength at its surface than planet Y. Suggest a reason. [1] (b) A spacecraft moves away from the Earth. Describe how the gravitational field strength of the Earth acting on it changes. [1] (c) Explain why the planets orbit the Sun. [2]

7. This question is about the Sun and the Milky Way.

(a) Name the two elements that make up most of the Sun. [1] (b) Name the three regions of the electromagnetic spectrum in which the Sun radiates most of its energy. [1] (c) Name the galaxy that contains the Sun. [1] (d) State what is meant by a light-year. [1] (e) State the approximate diameter of this galaxy in light-years. [1]

8. (Extended) A planet orbits a distant star with an average orbital radius of 2.4 × 10¹¹ m and an orbital period of 500 days.

(a) Calculate the average orbital speed of the planet. [3] (b) A second planet orbits the same star at a greater distance. State and explain how its orbital speed compares. [2]

9. (Extended) The table gives data for five planets around a star.

Planet	Orbital distance / million km	Orbital period / days	Density / g/cm³	Surface temperature / °C	Surface g / N/kg
P	45	60	5.6	350	3.1
Q	95	185	5.0	120	8.2
R	180	480	4.1	−40	5.6
S	640	3200	1.2	−130	21
T	1600	12 800	0.8	−190	9.4

(a) Describe the relationship between orbital distance and orbital period. [1] (b) Using the data, state which planets are likely to be rocky and give a reason. [2] (c) Suggest why planet S has the largest surface gravitational field strength. [1] (d) Calculate the average orbital speed of planet R in m/s. [2]

10. (Extended) A comet moves round the Sun in a very stretched elliptical orbit.

(a) State where on its orbit the comet travels fastest. [1] (b) Use the conservation of energy to explain your answer to (a). [3] (c) State how the Sun’s position in this orbit differs from its position in a circular orbit. [1]

11. (Extended) This question is about the life cycle of stars.

(a) Describe how a stable star forms from an interstellar cloud of gas and dust. [3] (b) State the nuclear reaction that powers a stable star. [1] (c) Describe what happens to a much more massive star once most of the hydrogen in its centre is converted to helium. [4]

12. (Extended) Light from a distant galaxy is observed to be redshifted.

(a) Describe what is meant by redshift. [1] (b) State what redshift in the light from distant galaxies suggests about the Universe. [1] (c) State how astronomers find (i) the speed at which the galaxy moves away and (ii) the distance of the galaxy. [2] (d) The galaxy moves away from the Earth at 6.6 × 10⁶ m/s. Calculate its distance from the Earth in metres. [2] (e) Convert this distance into light-years. [1] (f) Use the Hubble constant to estimate the age of the Universe in years. [3] (g) Explain why cosmic microwave background radiation is observed in the microwave region. [2]

13. This question is about galaxies and the Universe.

(a) State what a galaxy is made up of. [1] (b) Compare the distance from the Earth of the other stars in the Milky Way with the distance from the Earth to the Sun. [1] (c) Describe what is meant by redshift. [1] (d) Light from distant galaxies appears redshifted compared with light emitted on the Earth. State what this shows about the Universe, and name the theory that it supports. [2]

Answers

1. (a) Mars [1] (b) In the asteroid belt, between Mars and Jupiter [1] (c) A dwarf planet (minor planet) [1] [3] Examiner insight: One mark per part; “Pluto is a planet” earns nothing in (c).

2. (a) About 24 hours [1] (b) The Earth rotates on its axis [1]; the town moves between the side facing the Sun (day) and the side facing away (night) [1]. (c) The Earth’s axis is tilted [1]; as the Earth orbits the Sun, the northern hemisphere leans towards the Sun for part of the orbit and away for another part [1]; leaning towards the Sun, it has a higher Sun, longer days and more energy per square metre, so summer [1]. Examiner insight: A (c) answer based on the Earth being closer to the Sun scores zero for that point.

3. t = d / v [1]; t = 1.35 × 10¹² ÷ 3.0 × 10⁸ = 4500 s [1]; 4500 ÷ 60 = 75 minutes [1] [3] Examiner insight: The question asks for minutes, so 4500 s left unconverted loses the final mark; the first two marks still stand.

4. (a) About one month [1] (b) The Moon does not give out light; it reflects light from the Sun [1]; half of the Moon is always lit [1]; as it orbits the Earth we see different amounts of the lit half [1]. Examiner insight: “The Earth’s shadow covers part of the Moon” describes an eclipse, not phases, and earns no credit.

5. Gravity pulled the interstellar cloud of gas and dust together [1]; the rotating cloud formed an accretion disc around the young Sun [1]; near the Sun it was hot, so only rocky and metallic material could form small planets [1]; further out it was cold, so gases were available and large cores attracted large amounts of gas [1]. [4] Examiner insight: Four separate points are needed; repeating the question’s “rocky and small” earns nothing.

6. (a) Planet X has a greater mass [1] (b) It decreases as the distance from the Earth increases [1] (c) The Sun contains most of the mass of the Solar System [1]; the gravitational attraction of the Sun keeps the planets in orbit [1]. Examiner insight: In (c) the force must be named as gravitational attraction of the Sun; “gravity” alone, with no body named, usually loses the second mark.

7. (a) Hydrogen and helium [1] (b) Infrared, visible light and ultraviolet [1] (c) The Milky Way [1] (d) The distance travelled by light in (the vacuum of) space in one year [1] (e) About 100 000 light-years [1] Examiner insight: Part (b) needs all three regions for the mark, and in (d) calling a light-year a time scores zero.

8. (a) T = 500 × 86 400 = 4.32 × 10⁷ s [1]; v = 2πr / T = (2π × 2.4 × 10¹¹) ÷ 4.32 × 10⁷ [1]; v = 3.5 × 10⁴ m/s (34 900 m/s) [1] (b) Its orbital speed is lower [1]; the star’s gravitational field is weaker further from the star [1]. Examiner insight: Using T = 500 unconverted gives 3.0 × 10⁹ m/day; the equation mark stays but the final mark is lost.

9. (a) The greater the orbital distance, the longer the orbital period [1] (b) P, Q and R [1]; they have high densities (about 4 to 6 g/cm³), much higher than S and T [1] (c) S has the greatest mass [1] (d) v = (2π × 180 × 10⁹) ÷ (480 × 86 400) [1] = 2.7 × 10⁴ m/s [1] Examiner insight: The reason in (b) must quote the data (density values); in (d), leaving million km unconverted loses the accuracy mark.

10. (a) At the point closest to the Sun [1] (b) As the comet moves towards the Sun it loses gravitational potential energy [1]; this is transferred to kinetic energy [1]; so its speed increases, and total energy is conserved [1]. (c) The Sun is not at the centre of the elliptical orbit [1] Examiner insight: “Gravity is stronger near the Sun” scores nothing in (b); the named transfer GPE → KE earns the marks.

11. (a) The cloud collapses under its own gravitational attraction, forming a protostar [1]; its temperature increases as it collapses [1]; it becomes stable when inward gravity is balanced by an outward force due to the high temperature at its centre [1]. (b) Nuclear fusion of hydrogen into helium [1] (c) It expands to form a red supergiant [1]; the red supergiant explodes as a supernova [1]; this forms a nebula containing hydrogen and new heavier elements [1]; a neutron star or a black hole is left at the centre [1]. Examiner insight: A red giant or white dwarf in (c) is the path of a less massive star and earns no credit.

12. (a) An increase in the observed wavelength of electromagnetic radiation from receding stars or galaxies [1] (b) The Universe is expanding (supporting the Big Bang Theory) [1] (c) (i) From the change in wavelength of the galaxy’s starlight due to redshift [1]; (ii) from the brightness of a supernova in the galaxy [1] (d) d = v / H₀ = 6.6 × 10⁶ ÷ 2.2 × 10⁻¹⁸ [1] = 3.0 × 10²⁴ m [1] (e) 3.0 × 10²⁴ ÷ 9.5 × 10¹⁵ = 3.2 × 10⁸ light-years [1] (f) age = 1 / H₀ = 1 ÷ 2.2 × 10⁻¹⁸ = 4.5 × 10¹⁷ s [1]; one year = 365 × 86 400 = 3.15 × 10⁷ s [1]; age = 4.5 × 10¹⁷ ÷ 3.15 × 10⁷ = 1.4 × 10¹⁰ years [1] (g) The CMBR was produced shortly after the Universe was formed [1]; as the Universe expanded, the radiation was expanded into the microwave region [1]. Examiner insight: Error carried forward is normally allowed from (d) into (e), so a wrong distance can still earn the conversion mark.

13. (a) Many billions of stars [1] (b) The other stars in the Milky Way are much further away from the Earth than the Sun is [1] (c) An increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies [1] (d) The Universe is expanding [1]; it supports the Big Bang Theory [1]. Marking tip: In (c), “the light turns red” earns nothing; the mark is for an increase in the observed wavelength.

Where marks are usually lost

• Explaining seasons by distance from the Sun rather than the tilted axis.
• Confusing rotation (day and night) with orbit (seasons).
• Leaving days, or million km, unconverted in v = 2πr / T.
• Giving only one force for a stable star; you need both forces and the word “balanced”.
• Sending a massive star to a white dwarf, or a Sun-like star to a supernova.
• Leaving the age of the Universe in seconds when years are asked for.
• Describing redshift as light “turning red” rather than an increase in observed wavelength.

Next steps

• Space physics revision notes and study guide.
• Cambridge IGCSE Physics hub and printable checklist.
• Free 0625 Core diagnostic or 0625 Extended diagnostic.
• Book a free trial class.

Official syllabus

Cambridge IGCSE Physics 0625 syllabus, for examination in 2026, 2027 and 2028 (Version 2), Cambridge University Press & Assessment. Topic 6, Space physics: sections 6.1 and 6.2.