IGCSE Physics · Unit 6: Space Physics · Core + Extended

Space Physics CoreExtended adds orbital mechanics, stellar life cycle & the Hubble constant

The final unit of the syllabus: the Earth’s motion and its place in the Solar System, what keeps planets in orbit, how stars (including our Sun) generate energy and evolve over their lifetimes, and the evidence that the Universe is expanding from a single point — the Big Bang.

Why this matters This unit brings together ideas from right across the syllabus — gravitational field strength (from Forces), energy conservation (from Energy), and the electromagnetic spectrum (from Waves) — and applies them on a cosmic scale. If a concept here feels unfamiliar, it’s often worth checking whether it’s really a callback to an earlier unit.

1. The Earth

The Earth is a planet that rotates on its axis, which is tilted, once in approximately 24 hours. This rotation explains the apparent daily motion of the Sun across the sky, and the periodic cycle of day and night (whichever side of the Earth faces the Sun experiences day; the far side experiences night).

The Earth orbits the Sun once in approximately 365 days, and this — combined with the Earth’s tilted axis — explains the periodic nature of the seasons (different parts of the Earth’s surface receive more or less direct sunlight at different points in the orbit).

The Moon orbits the Earth once in approximately one month, which explains the periodic cycle of the Moon’s phases (the changing amount of the Moon’s sunlit side visible from Earth as their relative positions change).

Average orbital speed Extended v = 2πr / T
where r is the average radius of the orbit and T is the orbital period.
Worked example — orbital speed Find the average orbital speed of a satellite orbiting the Earth at an average radius of 7 × 10⁶ m, with an orbital period of 6000 s.
v = 2πr / T = (2 × π × 7 × 10⁶) / 6000 = (4.4 × 10⁷) / 6000 ≈ 7333 m/s
Try it — The Earth’s Motion
  1. Explain why different parts of the Earth experience day and night at any given moment.
  2. Explain why the Earth’s tilted axis, combined with its orbit around the Sun, causes the seasons.
  3. A moon orbits a planet at an average radius of 4 × 10⁵ m, with an orbital period of 3600 s. Calculate its average orbital speed.
Show answers
  1. The Earth rotates on its axis once approximately every 24 hours. At any moment, the side of the Earth facing the Sun experiences daylight, while the side facing away experiences night — as the Earth rotates, different regions move between these two states.
  2. Because the Earth’s axis is tilted, different parts of the Earth’s surface receive more direct (concentrated) sunlight at different points in its orbit around the Sun, and the length of daylight also varies — this is what produces the yearly, repeating cycle of seasons.
  3. v = 2πr/T = (2 × π × 4×10⁵)/3600 = (2.51×10⁶)/3600 ≈ 698 m/s

2. The Solar System

The Solar System contains: one star (the Sun); the eight named 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; moons, which orbit the planets; and smaller Solar System bodies, including comets and other natural satellites.

Why the inner planets differ from the outer planets The four planets nearest the Sun (Mercury, Venus, Earth, Mars) are rocky and small; the four furthest from the Sun (Jupiter, Saturn, Uranus, Neptune) are gaseous and large. This is explained by the accretion model of Solar System formation: the model depends on gravity pulling material together; it starts from an interstellar cloud of gas and dust containing many different elements; and the rotation of this cloud causes it to flatten into a spinning accretion disc, within which material gradually clumps together under gravity. Closer to the young, hot Sun, only rocky/metallic material could survive and accumulate (lighter gases were blown away by the Sun’s radiation), forming small, dense, rocky planets; further out, where it was colder, gases could also condense and accumulate in huge quantities, forming the large gas planets.

Gravitational Field Strength Around Planets

The strength of the gravitational field at the surface of a planet depends on the mass of the planet (a more massive planet has a stronger surface gravity). The gravitational field strength around a planet decreases as distance from the planet increases.

Why the planets orbit the Sun The Sun contains most of the mass of the entire Solar System, and this is why the planets orbit the Sun rather than the other way around. The force that keeps a planet in orbit around the Sun is the gravitational attraction of the Sun acting on the planet, constantly pulling it toward the Sun and curving its path into an orbit rather than a straight line.
Calculating light travel time Since light travels at a known, finite (though very fast) speed, the time it takes to cross a significant distance — such as between two objects in the Solar System — can be calculated using time = distance / speed, with the speed of light (3.0 × 10⁸ m/s) from the Waves unit.
Worked example — light travel time The average distance from the Sun to Mars is about 2.28 × 10¹¹ m. Find how long light takes to travel this distance.
time = distance / speed = (2.28 × 10¹¹) / (3.0 × 10⁸) = 760 s (about 12.7 minutes)
Elliptical orbits Extended Planets, minor planets, and comets actually travel in elliptical (oval-shaped) orbits, not perfect circles — and the Sun is not at the centre of this ellipse (except in the special case where the orbit happens to be almost circular). As distance from the Sun increases, the Sun’s gravitational field strength decreases, and correspondingly the orbital speeds of the planets decrease too (more distant planets orbit more slowly).
An object in an elliptical orbit travels faster when closer to the Sun and slower when further away. This can be explained using the conservation of energy: as the object moves closer to the Sun, it loses gravitational potential energy, and by conservation of energy, this is transferred into kinetic energy, increasing its speed — and vice versa as it moves further away.
Try it — The Solar System
  1. Explain, using the accretion model, why the planets closest to the Sun are small and rocky.
  2. Explain why the planets orbit the Sun rather than the Sun orbiting the planets.
  3. Explain, using conservation of energy, why a comet in an elliptical orbit speeds up as it approaches the Sun.
Show answers
  1. Close to the young, hot Sun, lighter gases could not condense or survive (being blown away by the Sun’s radiation), so only rocky and metallic material was able to accumulate there — forming small, dense, rocky planets. Further from the Sun, where it was colder, gases could also condense and accumulate in large quantities, forming much larger gas planets.
  2. The Sun contains most of the mass of the entire Solar System, and its much stronger gravitational pull dominates the system — the planets, having far less mass, are held in orbit around the Sun by its gravitational attraction, rather than the reverse.
  3. As the comet approaches the Sun, it moves to a lower gravitational potential energy (closer to the Sun). By conservation of energy, this lost gravitational potential energy is transferred into kinetic energy, increasing the comet’s speed.

3. The Sun as a Star

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

How the Sun produces energy Extended Stars are powered by nuclear reactions that release energy. In stable stars like the Sun, this nuclear reaction is fusion — specifically, the fusion of hydrogen into helium, in the star’s extremely hot, high-pressure core.
Try it — The Sun
  1. State the two elements that make up most of the Sun’s composition.
  2. Name the three regions of the electromagnetic spectrum in which the Sun radiates most of its energy.
  3. State the nuclear process that powers the Sun, and the two elements involved.
Show answers
  1. Hydrogen and helium.
  2. Infrared, visible light, and ultraviolet.
  3. Nuclear fusion — the fusion of hydrogen into helium.

4. Stars

A galaxy is made up of many billions of stars. The Sun is one star within the galaxy known as the Milky Way. Other stars in the Milky Way are much further away from the Earth than the Sun is — distances so vast that a more convenient unit than the metre is used: the light-year, the distance travelled by light (in a vacuum) in one year (1 light-year = 9.5 × 10¹⁵ m, Extended).

Worked example — light-years Extended A star is 4.2 light-years from Earth. Find this distance in metres.
distance = 4.2 × (9.5 × 10¹⁵) ≈ 4.0 × 10¹⁶ m
The life cycle of a star Extended 1. Formation: a star forms from an interstellar cloud of gas and dust containing hydrogen.
2. Protostar: the cloud collapses inward under its own gravitational attraction, increasing in temperature as it does so.
3. Stable star: the protostar becomes a stable star once the inward force of gravitational attraction is balanced by an outward force due to the very high temperature (and pressure) at the star’s centre.
4. Running out of fuel: eventually, every star runs out of hydrogen fuel for its nuclear fusion reactions.
5. Expansion: once most of the hydrogen at the centre has been converted to helium, most stars expand into red giants; more massive stars expand into red supergiants.
6a. Less massive stars: a red giant sheds its outer layers, forming a planetary nebula, leaving behind a small, dense white dwarf at its centre.
6b. More massive stars: a red supergiant explodes violently as a supernova, forming a nebula containing hydrogen and newly created heavier elements, and leaving behind either a neutron star or a black hole at its centre (depending on the original star’s mass).
7. The cycle continues: the nebula produced by a supernova may itself go on to form new stars, potentially with orbiting planets.
Try it — Stars and Their Life Cycle
  1. Define a light-year.
  2. Describe what happens to a protostar as it forms, and explain what causes it to become a stable star.
  3. Describe the difference in the eventual fate of a star like the Sun compared to a much more massive star, once each runs out of hydrogen.
Show answers
  1. A light-year is the distance travelled by light (in a vacuum) in one year.
  2. A protostar forms as an interstellar cloud of gas and dust collapses inward under its own gravitational attraction, causing its temperature to rise. It becomes a stable star once the inward force of gravity is exactly balanced by an outward force resulting from the very high temperature at its centre.
  3. A star like the Sun (less massive) will expand into a red giant, then shed its outer layers to form a planetary nebula, leaving a white dwarf behind. A much more massive star will instead expand into a red supergiant, then explode as a supernova, leaving behind either a neutron star or a black hole.

5. The Universe

The Milky Way is just one of many billions of galaxies making up the Universe, with a diameter of approximately 100000 light-years.

Redshift is an increase in the observed wavelength of electromagnetic radiation emitted from receding (moving away) stars and galaxies. Light from distant galaxies appears redshifted compared to the same light emitted here on Earth — and this redshift is evidence that the Universe is expanding, supporting the Big Bang Theory (the idea that the Universe began from an extremely hot, dense point and has been expanding ever since).

Cosmic microwave background radiation (CMBR) Extended Microwave radiation of a specific frequency is observed coming from all points in space around us, known as cosmic microwave background radiation (CMBR). This radiation was produced shortly after the Universe was formed (in the very early, extremely hot Universe), and has since been stretched into the microwave region of the electromagnetic spectrum as the Universe has expanded — it is essentially “the afterglow” of the very early, hot Universe, redshifted by billions of years of cosmic expansion.
Measuring recession speed and distance Extended The speed, v, at which a galaxy is moving away from the Earth can be found from the change in wavelength of its starlight due to redshift (a larger redshift indicates a faster recession speed). The distance, d, of a far galaxy can be determined using the brightness of a supernova within that galaxy (since supernovae of a particular type have a known, consistent actual brightness, comparing this to how bright it appears from Earth reveals the distance).
The Hubble constant Extended The Hubble constant, H₀, is defined as the ratio of the speed at which a galaxy is moving away from the Earth to its distance from the Earth.
H₀ = v / d
The current estimate for H₀ is 2.2 × 10⁻¹⁸ per second.
Worked example — using the Hubble constant A galaxy is moving away from the Earth at a speed of 3.3 × 10⁶ m/s. Using H₀ = 2.2 × 10⁻¹⁸ /s, find its distance from Earth.
H₀ = v/d → d = v/H₀ = (3.3 × 10⁶) / (2.2 × 10⁻¹⁸) = 1.5 × 10²⁴ m
Estimating the age of the Universe Extended d/v = 1/H₀
This equation represents an estimate for the age of the Universe. Since d/v has units of time (distance ÷ speed), and this equals a constant (1/H₀) regardless of which galaxy’s data is used, this supports the idea that all the matter in the Universe was once present at a single point (i.e., that the Big Bang happened a specific, calculable time ago) — every galaxy’s motion, traced backward, points to the same starting moment.
Worked example — estimating the age of the Universe Using H₀ = 2.2 × 10⁻¹⁸ per second, estimate the age of the Universe in seconds.
age ≈ 1/H₀ = 1 / (2.2 × 10⁻¹⁸) ≈ 4.5 × 10¹⁷ s
Try it — The Expanding Universe
  1. Explain what redshift is, and why it is evidence that the Universe is expanding.
  2. A galaxy is 4.0 × 10²⁴ m from Earth. Using H₀ = 2.2 × 10⁻¹⁸ /s, calculate its recession speed.
  3. Explain what the equation d/v = 1/H₀ tells us about the history of the Universe.
Show answers
  1. Redshift is an increase in the observed wavelength of light from a receding star or galaxy. Almost all distant galaxies show redshift when observed from Earth, meaning almost all of them are moving away from us — this is exactly what would be expected if space itself is expanding in all directions, supporting the Big Bang Theory.
  2. H₀ = v/d → v = H₀ × d = (2.2×10⁻¹⁸) × (4.0×10²⁴) = 8.8 × 10⁶ m/s
  3. Since d/v gives the same value (1/H₀) no matter which galaxy’s data is used, it suggests that, tracing every galaxy’s motion backward in time, they would all have been at the same place at the same moment in the past — providing evidence that all the matter in the Universe was once concentrated at a single point, consistent with the Big Bang Theory.
Common mistakes to watch for
  • Mixing up the Earth’s rotation (causes day/night, ~24 hours) with its orbit around the Sun (causes seasons, ~365 days).
  • Thinking the Sun is at the exact centre of every planet’s orbit — this is only true for a perfectly circular orbit; for an elliptical orbit, the Sun is off-centre.
  • Forgetting that an object in an elliptical orbit moves FASTER when closer to the Sun, not slower.
  • Confusing a light-year (a unit of distance) with a unit of time — despite containing the word “year,” it measures how far light travels, not how long something takes.
  • Mixing up which type of star remnant forms from which mass of star — white dwarf (via planetary nebula) for less massive stars; neutron star or black hole (via supernova) for more massive ones.
  • Forgetting to rearrange H₀ = v/d correctly when solving for d or v — a very common place to lose marks through simple algebra slips with such large/small numbers.

6. Quick-Fire Challenge Round

True or False?
  1. The Earth’s seasons are caused by its rotation on its axis.
  2. The gaseous, large planets in the Solar System are found closest to the Sun.
  3. Redshift is evidence that the Universe is expanding.
  4. A more massive star is more likely to end its life as a white dwarf than a less massive star.
Show answers
  1. False — the Earth’s rotation causes day and night; the seasons are caused by the Earth’s ORBIT around the Sun, combined with its tilted axis.
  2. False — the small, rocky planets are closest to the Sun; the large, gaseous planets are furthest from the Sun.
  3. True — redshift shows that light from distant galaxies has stretched to longer wavelengths, consistent with those galaxies (and space itself) moving apart, i.e. an expanding Universe.
  4. False — it’s the opposite: a LESS massive star (like the Sun) is more likely to end as a white dwarf; a MORE massive star is more likely to explode as a supernova, leaving a neutron star or black hole.
Spot the Error
  1. “A comet in an elliptical orbit moves at a constant speed throughout its orbit, since orbital speed doesn’t depend on distance from the Sun.”
  2. “A light-year is the time it takes light to travel from the Sun to the Earth.”
  3. “H₀ = v/d, so if a galaxy’s distance d is very large, its recession speed v must be very small.”
Show answers
  1. An object in an elliptical orbit does NOT move at constant speed — it moves faster when closer to the Sun and slower when further away, which can be explained using conservation of energy (gravitational potential energy converting to kinetic energy and back).
  2. A light-year is a unit of DISTANCE (the distance light travels in one year), not a unit of time, despite its name. The time for light to travel from the Sun to Earth is only about 8 minutes, which is a completely different (much smaller) quantity.
  3. Rearranging H₀ = v/d gives v = H₀ × d — since H₀ is a constant, a LARGER distance d actually corresponds to a LARGER recession speed v, not a smaller one. More distant galaxies recede faster, according to Hubble’s law.

7. Mixed Practice — Bringing It All Together

  1. State the approximate time taken for the Earth to complete one full rotation on its axis.
  2. List the eight planets of the Solar System in order from the Sun.
  3. State the nuclear process that powers stable stars like the Sun, and the elements involved.
  4. Describe the final stage in the life cycle of a star with a mass similar to the Sun.
  5. A galaxy is 2.0 × 10²⁵ m from Earth. Using H₀ = 2.2 × 10⁻¹⁸ /s, calculate its recession speed.
Show answers
  1. Approximately 24 hours.
  2. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
  3. Nuclear fusion — specifically, the fusion of hydrogen into helium.
  4. It expands into a red giant, sheds its outer layers to form a planetary nebula, and leaves behind a white dwarf at its centre.
  5. v = H₀ × d = (2.2×10⁻¹⁸) × (2.0×10²⁵) = 4.4 × 10⁷ m/s

Practice at Home

Parts C and D include Extended-only content.

Part A — The Earth and the Solar System
  1. Explain how the Moon’s orbit around the Earth relates to the Moon’s phases.
  2. List the bodies contained within the Solar System, other than the Sun and the eight planets.
  3. Explain why the Sun, rather than any planet, is at the centre of the Solar System’s orbits.
Part B — The Sun and stars
  1. State the two main elements the Sun is composed of.
  2. Explain what a galaxy is, and name the galaxy that contains the Sun.
  3. Explain why astronomers use light-years rather than metres to measure distances between stars.
Part C — Orbits and stellar life cycle (Extended)
  1. A planet orbits its star at an average radius of 1.5 × 10¹¹ m with an orbital period of 3.15 × 10⁷ s. Calculate its average orbital speed.
  2. Describe the difference between the eventual fate of a red giant and a red supergiant.
  3. Explain, using conservation of energy, why an object in an elliptical orbit slows down as it moves away from the Sun.
Part D — The expanding Universe (Extended)
  1. Explain what CMBR is, and how it relates to the early Universe.
  2. A galaxy recedes from Earth at 6.6 × 10⁶ m/s. Using H₀ = 2.2 × 10⁻¹⁸ /s, calculate its distance from Earth.
  3. Explain why the equation d/v = 1/H₀ is described as an estimate for the age of the Universe.

Key Vocabulary Recap

Accretion model — the theory that the Solar System formed from a collapsing, rotating cloud of gas and dust
Light-year — the distance travelled by light in one year (a unit of distance, not time)
Protostar — a collapsing cloud of gas and dust, heating up under gravity, before becoming a stable star
Redshift — an increase in the observed wavelength of light from a receding star or galaxy
Big Bang Theory — the theory that the Universe began at a single point and has been expanding ever since
Hubble constant (H₀) — the ratio of a galaxy’s recession speed to its distance (H₀ = v/d)

Syllabus Reference

This note covers Cambridge IGCSE Physics (0625) section 6, Space physics, in full — 6.1 The Earth and the Solar System (6.1.1 The Earth, 6.1.2 The Solar System) and 6.2 Stars and the Universe (6.2.1 The Sun as a star, 6.2.2 Stars, 6.2.3 The Universe). Core and Supplement content are both included, with Extended-only material (orbital speed, the accretion model’s detail, elliptical orbits and orbital speed variation, light-years in metres, the stellar life cycle, CMBR, and the Hubble constant with the age-of-the-Universe equation) clearly flagged throughout. This completes the full set of notes for the Cambridge IGCSE Physics (0625) syllabus.

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