Cambridge IGCSE Physics 0625: Space physics – Revision Notes

These notes condense 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). Everything not marked is Core and can appear on any paper. Items marked Extended only are Supplement content, examined on Papers 2 and 4 only. For full explanations and worked examples, go to the space physics study guide.

Other links: practice questions for this topic, the IGCSE Physics hub and the printable IGCSE Physics checklist.

Numbers to know

Quantity	Value	Tier
Earth: one rotation on its tilted axis	about 24 hours	Core
Earth: one orbit of the Sun	about 365 days	Core
Moon: one orbit of the Earth	about one month	Core
Speed of light in a vacuum	3.0 × 10⁸ m/s	Recall is Extended only (syllabus 3.3); Core still calculates light travel times
Diameter of the Milky Way	about 100 000 light-years	Core
One light-year	9.5 × 10¹⁵ m	Extended only
Hubble constant H₀	2.2 × 10⁻¹⁸ per second	Extended only

Equations

Equation	Meaning	Tier
t = d / v	time for light to travel distance d (v = 3.0 × 10⁸ m/s)	Core
v = 2πr / T	average orbital speed; r = average orbit radius, T = orbital period	Extended only (recall)
H₀ = v / d	Hubble constant; v = recession speed, d = distance of galaxy	Extended only (recall)
d / v = 1 / H₀	estimate of the age of the Universe	Extended only

6.1.1 The Earth

• Day and night: the Earth spins on its axis once in about 24 h. The side facing the Sun has day.
• Sun’s daily path: the Sun appears to rise in the east and set in the west because the Earth rotates.
• Seasons: the axis is tilted. As the Earth orbits (about 365 days), each hemisphere leans towards the Sun for part of the year: Sun higher, days longer, more energy per square metre, so summer.
• Moon’s phases: the Moon reflects sunlight. As it orbits (about one month) you see different amounts of its lit half.

6.1.2 The Solar System

• Members: one star (the Sun); eight planets; minor planets (dwarf planets such as Pluto, asteroids in the asteroid belt); moons orbiting planets; smaller bodies such as comets and natural satellites.
• Order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
• Inner four: rocky and small. Outer four: gaseous and large.
• Gravity: surface field strength depends on the planet’s mass; field strength falls with distance from the planet.
• Why planets orbit the Sun: the Sun holds most of the Solar System’s mass. Its gravitational attraction keeps objects in orbit.
• Extended only: orbits of planets, minor planets and comets are elliptical. The Sun is not at the centre unless the orbit is nearly circular.
• Extended only: further from the Sun, its field is weaker and planets orbit more slowly.
• Extended only: an object in an elliptical orbit is fastest when nearest the Sun.

Method in steps: the accretion model

1. Interstellar cloud of gas and dust, containing many elements.
2. Gravity pulls it together; the rotating cloud forms an accretion disc around the young Sun.
3. Near the Sun: hot, only rock and metal stay solid, so small rocky planets.
4. Far out: cold, gas and ice available; big cores pull in gas, so large gaseous planets.

Method in steps: fastest near the Sun (Extended only)

1. Moving towards the Sun, the object loses gravitational potential energy.
2. This is transferred to kinetic energy.
3. More kinetic energy means higher speed. Total energy is conserved.

Method in steps: v = 2πr / T (Extended only)

1. Convert r to metres (1 million km = 10⁹ m; 1 km = 1000 m).
2. Convert T to seconds (1 day = 86 400 s).
3. Substitute and give units.

Small reminder: r = 1.5 × 10¹¹ m, T = 3.15 × 10⁷ s gives v = 3.0 × 10⁴ m/s.

6.2.1 The Sun as a star

• Medium-sized star, mostly hydrogen and helium.
• Radiates most energy as infrared, visible light and ultraviolet.
• Extended only: powered by nuclear reactions; in stable stars, fusion of hydrogen into helium.

6.2.2 Stars

• A galaxy contains many billions of stars. The Sun is in the Milky Way.
• Other stars in the Milky Way are much further away than the Sun.
• A light-year is the distance light travels through space in one year. Extended only: 9.5 × 10¹⁵ m.

Life cycle of a star (Extended only)

interstellar cloud of gas and dust (hydrogen)
        ↓ gravity pulls it in, it heats up
protostar
        ↓ gravity inwards balanced by outward force from high core temperature
stable star (fusing hydrogen into helium)
        ↓ hydrogen in the centre mostly converted to helium
   ┌────────────────────────┴───────────────────────┐
red giant (less massive star)              red supergiant (more massive star)
   ↓                                                 ↓
planetary nebula + white dwarf        supernova → nebula (hydrogen + heavier elements)
                                              + neutron star or black hole
                                              nebula may form new stars with planets

6.2.3 The Universe

• The Milky Way is one of many billions of galaxies; diameter about 100 000 light-years.
• Redshift: an increase in the observed wavelength of electromagnetic radiation from receding stars and galaxies.
• Light from distant galaxies is redshifted compared with light emitted on Earth, so the Universe is expanding. This supports the Big Bang Theory.
• Extended only: CMBR is microwave radiation of a specific frequency observed at all points in space around us. It was produced shortly after the Universe formed and has been expanded into the microwave region as the Universe expanded.
• Extended only: recession speed v comes from the change in wavelength (redshift). Distance d of a far galaxy comes from the brightness of a supernova in it.
• Extended only: 1 / H₀ estimates the age of the Universe, which is evidence that all its matter was once at a single point.

Must-know distinctions

• Rotation vs orbit: rotation (spin, 24 h) gives day and night; orbit (365 days) with tilt gives seasons.
• Red giant vs red supergiant: red giant comes from a less massive star and ends as a white dwarf in a planetary nebula; red supergiant comes from a more massive star and ends in a supernova.
• Neutron star or black hole vs white dwarf: the first two are left after a supernova; a white dwarf is not.
• Light-year vs year: one is a distance, the other a time.
• Redshift vs CMBR: redshift shows galaxies are receding now; CMBR is radiation left from shortly after the Universe formed. Both support the Big Bang.

Quick self-test

1. How long does the Earth take to spin once on its axis?
2. Give the reason for the seasons in one sentence.
3. Name the planet between Saturn and Neptune.
4. The Moon is 3.8 × 10⁸ m from the Earth. How long does light take to travel this distance?
5. What force keeps a comet in orbit around the Sun?
6. (Extended) A planet has an average orbital radius of 7.8 × 10¹¹ m and a period of 4330 days. Calculate its average orbital speed.
7. Name the three regions of the electromagnetic spectrum in which the Sun radiates most of its energy.
8. (Extended) State where a comet travels fastest in its orbit and give the energy transfer that explains it.
9. (Extended) What does a red supergiant form, and what can be left at the centre?
10. (Extended) A galaxy recedes at 4.4 × 10⁶ m/s. Calculate its distance in metres.
11. (Extended) Convert 2.0 × 10⁶ light-years into metres.
12. (Extended) Why is radiation produced shortly after the Universe formed now observed as microwaves?

Answers

1. About 24 hours.
2. The Earth’s axis is tilted, so as it orbits the Sun each hemisphere in turn leans towards the Sun and gets more energy per square metre.
3. Uranus.
4. t = 3.8 × 10⁸ ÷ 3.0 × 10⁸ = 1.3 s.
5. The gravitational attraction of the Sun.
6. T = 4330 × 86 400 = 3.74 × 10⁸ s; v = 2π × 7.8 × 10¹¹ ÷ 3.74 × 10⁸ = 1.3 × 10⁴ m/s.
7. Infrared, visible light and ultraviolet.
8. Nearest the Sun; gravitational potential energy is transferred to kinetic energy as it approaches.
9. It explodes as a supernova, forming a nebula of hydrogen and heavier elements; a neutron star or black hole is left.
10. d = v / H₀ = 4.4 × 10⁶ ÷ 2.2 × 10⁻¹⁸ = 2.0 × 10²⁴ m.
11. 2.0 × 10⁶ × 9.5 × 10¹⁵ = 1.9 × 10²² m.
12. As the Universe expanded, the radiation was expanded (its wavelength stretched) into the microwave region.

Where marks are usually lost

• Seasons explained by distance from the Sun instead of the tilted axis.
• Moon phases blamed on the Earth’s shadow.
• Days left unconverted in v = 2πr / T, giving an answer in m/day while the question wants m/s.
• Million km not converted to metres (a factor of 10⁹).
• “Gravity is stronger” given alone when the question says “use conservation of energy”.
• The Sun drawn or described at the centre of an elliptical orbit.
• Stable-star answers that name only one force: you need gravity inwards and the outward force from high core temperature, and the word “balanced”.
• Redshift described as a colour change rather than an increase in observed wavelength.
• Age of the Universe left in seconds when years are asked for.
• Light-year used as a time unit.

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.