Cambridge IGCSE Physics 0625: Space physics – Study Guide

This study guide teaches topic 6, Space physics, of the Cambridge IGCSE Physics 0625 syllabus for examination in 2026, 2027 and 2028. It covers sections 6.1 (The Earth and the Solar System) and 6.2 (Stars and the Universe). Core content is examined for every candidate. Content labelled Extended only is Supplement content: it is tested on Paper 2 (Multiple Choice, Extended) and Paper 4 (Theory, Extended), not on the Core Papers 1 and 3.

See also the revision notes, the practice questions, the Cambridge IGCSE Physics hub and the printable IGCSE Physics checklist.

What this topic covers

Section	Core	Extended only
6.1.1 The Earth	Day and night, the Sun’s daily motion, seasons, Moon’s phases	v = 2πr / T
6.1.2 The Solar System	Members; accretion model; gravity; light travel times	Elliptical orbits; planetary data; speed and distance
6.2.1 The Sun as a star	Size, composition, radiation emitted	Fusion of hydrogen into helium
6.2.2 Stars	Galaxies, Milky Way, light-year	9.5 × 10¹⁵ m; life cycle of a star
6.2.3 The Universe	Milky Way size; redshift; Big Bang	CMBR; H₀ = v / d; age of the Universe

6.1.1 The Earth

Day and night. The Earth is a planet that spins on its axis once in about 24 hours. The axis is tilted. The half facing the Sun has day; the other half has night. As the Earth turns, each place passes into and out of the lit half.

The Sun’s apparent daily motion. The Sun seems to rise in the east, reach its highest point around midday and set in the west. It is the Earth’s rotation, not the Sun, that moves.

The seasons. The Earth orbits the Sun once in about 365 days. Because the axis is tilted, for part of each orbit a hemisphere leans towards the Sun: the Sun is higher, days are longer and each square metre receives more energy, so it is summer there. Half an orbit later that hemisphere leans away: winter. The cycle repeats every orbit. Seasons are not caused by the Earth being closer to the Sun.

The Moon’s phases. The Moon orbits the Earth in about one month. It reflects sunlight. Half of it is always lit, but as it orbits you see different amounts of that lit half (new, crescent, half, gibbous, full), so the cycle of phases repeats about once a month.

Average orbital speed (Extended only)

Average orbital speed is defined by

v = 2πr / T

where r is the average orbit radius and T is the orbital period (distance round one orbit ÷ time). You must recall it.

Worked example. The Earth’s average orbital radius is 1.5 × 10¹¹ m and its period is 365 days. Calculate its average orbital speed.

T = 365 × 24 × 60 × 60 = 3.1536 × 10⁷ s
v = 2πr / T = (2 × π × 1.5 × 10¹¹) / (3.1536 × 10⁷)
v = 2.99 × 10⁴ m/s  ≈ 3.0 × 10⁴ m/s (about 30 km/s)

6.1.2 The Solar System

Members of the Solar System

The Solar System contains:

• one star, the Sun
• eight planets. In order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
• minor planets that orbit the Sun, including dwarf planets such as Pluto, and asteroids in the asteroid belt (between Mars and Jupiter)
• moons, which orbit the planets
• smaller Solar System bodies, including comets and natural satellites.

Rocky and gaseous planets: the accretion model

The four inner planets are small and rocky; the four outer planets are large and gaseous. The accretion model explains this.

1. The Solar System formed from an interstellar cloud of gas and dust containing many elements.
2. Gravity pulled the material together. The cloud was rotating, so as it collapsed it flattened into a spinning accretion disc around the young Sun.
3. Near the Sun it was hot. Only rock and metal stayed solid, so particles stuck together into small rocky planets.
4. Further out it was cold, so gases and ices were also available. Large cores formed and their strong gravity pulled in gas, making large gaseous planets.

Gravity in the Solar System

• The gravitational field strength at the surface of a planet depends on the planet’s mass.
• The field strength around a planet decreases as the distance from the planet increases.
• The Sun contains most of the mass of the Solar System (well over 99%). That is why the planets orbit the Sun.
• The force that keeps an object in orbit around the Sun is the gravitational attraction of the Sun. It always acts towards the Sun.

Light travel time

Light travels at 3.0 × 10⁸ m/s in a vacuum, so use time = distance ÷ speed.

Worked example. The Sun is 1.5 × 10¹¹ m from the Earth. How long does its light take to reach us?

t = d / v = (1.5 × 10¹¹) / (3.0 × 10⁸) = 500 s  (8 minutes 20 seconds)

Elliptical orbits (Extended only)

Planets, minor planets and comets move in elliptical orbits. The Sun is not at the centre of the ellipse, except when the orbit is nearly circular (as most planetary orbits are). Comets often have very stretched orbits.

An object in an elliptical orbit travels faster when closer to the Sun. Explain it with conservation of energy: as the object moves towards the Sun, it loses gravitational potential energy. That energy is transferred to kinetic energy, so it speeds up. Moving away, kinetic energy is transferred back to gravitational potential energy and it slows down. The total energy stays the same.

Distance from the Sun (Extended only)

The Sun’s gravitational field strength and the planets’ orbital speeds both decrease as distance from the Sun increases.

Analysing planetary data (Extended only)

You may be given data on orbital distance, orbital duration, density, surface temperature and surface gravitational field strength. Here is an imaginary system around a Sun-like star.

Planet	Orbital distance / million km	Orbital period / days	Density / g/cm³	Surface temperature / °C	Surface g / N/kg
A	75	130	5.3	300	3.5
B	120	260	5.0	90	9.1
C	200	560	3.8	−50	4.0
D	900	5400	1.3	−150	24
E	2100	19 000	1.1	−210	11

Patterns: period increases and surface temperature falls with distance; A, B and C are dense (rocky), D and E are not (gaseous); D has the largest surface g, so it is probably the most massive.

Worked example. Compare the speeds of A and E.

A: v = 2π × 75 × 10⁹ / (130 × 86 400) = 4.2 × 10⁴ m/s
E: v = 2π × 2100 × 10⁹ / (19 000 × 86 400) = 8.0 × 10³ m/s

The nearer planet is faster, as expected. Remember 1 million km = 10⁹ m.

6.2.1 The Sun as a star

The Sun is a medium-sized star. It consists mostly of hydrogen and helium. It radiates most of its energy in the infrared, visible light and ultraviolet regions of the electromagnetic spectrum.

Extended only. Stars are powered by nuclear reactions that release energy. In stable stars these reactions are the fusion of hydrogen into helium.

6.2.2 Stars

• A galaxy is made up of many billions of stars.
• The Sun is a star in the galaxy called the Milky Way.
• The other stars in the Milky Way are much further from the Earth than the Sun is.
• Astronomical distances can be measured in light-years. One light-year is the distance travelled in (the vacuum of) space by light in one year. It is a distance, not a time.

Extended only. One light-year = 9.5 × 10¹⁵ m. For example, a star 4.2 light-years away is 4.2 × 9.5 × 10¹⁵ = 4.0 × 10¹⁶ m away.

Life cycle of a star (Extended only)

1. A star forms from an interstellar cloud of gas and dust containing hydrogen.
2. A protostar is the cloud collapsing and getting hotter because of its own internal gravitational attraction.
3. It becomes a stable star when the inward force of gravity is balanced by an outward force due to the high temperature in its centre.
4. All stars eventually run out of hydrogen as fuel for the nuclear reaction.
5. When most of the hydrogen in the centre has become helium, most stars expand into red giants; more massive stars expand into red supergiants.
6. A red giant from a less massive star forms a planetary nebula with a white dwarf at its centre.
7. A red supergiant explodes as a supernova. This forms a nebula containing hydrogen and new heavier elements, and leaves a neutron star or a black hole at its centre.
8. The nebula from a supernova may form new stars with orbiting planets.

6.2.3 The Universe

The Milky Way is one of many billions of galaxies in the Universe. Its diameter is about 100 000 light-years.

Redshift is an increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies. Light from distant galaxies appears redshifted compared with light emitted on the Earth. So distant galaxies are moving away from us. This is evidence that the Universe is expanding, and it supports the Big Bang Theory.

Extended only: CMBR, speed, distance and H₀

• Microwave radiation of a specific frequency is observed at all points in space around us. It is the cosmic microwave background radiation (CMBR).
• The CMBR was produced shortly after the Universe was formed. As the Universe expanded, this radiation was expanded (stretched) into the microwave region.
• The speed v at which a galaxy moves away from the Earth is found from the change in wavelength of its starlight due to redshift. A bigger change means a faster galaxy.
• The distance d of a far galaxy can be found using the brightness of a supernova in that galaxy.
• The Hubble constant H₀ is the ratio of the speed at which a galaxy moves away from the Earth to its distance from the Earth: H₀ = v / d. You must recall this.
• The current estimate is H₀ = 2.2 × 10⁻¹⁸ per second.
• d / v = 1 / H₀ is an estimate of the age of the Universe. This supports the idea that all the matter in the Universe was once at a single point.

Worked example. A galaxy is moving away at 1.1 × 10⁷ m/s. Find its distance in metres and light-years, and estimate the age of the Universe in years.

d = v / H₀ = (1.1 × 10⁷) / (2.2 × 10⁻¹⁸) = 5.0 × 10²⁴ m
in light-years: (5.0 × 10²⁴) / (9.5 × 10¹⁵) = 5.3 × 10⁸ light-years
age = 1 / H₀ = 1 / (2.2 × 10⁻¹⁸) = 4.5 × 10¹⁷ s
one year = 365 × 24 × 3600 = 3.15 × 10⁷ s
age = (4.5 × 10¹⁷) / (3.15 × 10⁷) = 1.4 × 10¹⁰ years (about 14 billion years)

Common errors

• Saying seasons happen because the Earth is nearer the Sun in summer. Use the tilt of the axis.
• Blaming the Moon’s phases on the Earth’s shadow.
• Treating a light-year as a time.
• Forgetting to convert days to seconds, or million km to metres, in v = 2πr / T.
• Putting the Sun at the centre of an elliptical orbit.
• Answering “gravity is stronger” when asked to use conservation of energy: name the GPE to KE transfer.
• Writing that stars “burn” hydrogen. Say nuclear fusion of hydrogen into helium.
• Mixing up white dwarf (from a red giant) and neutron star or black hole (from a supernova).

Next steps

Use the revision notes self-test, then the practice questions. For the whole course, try the free Core diagnostic or Extended diagnostic.

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 The Earth and the Solar System and 6.2 Stars and the Universe.