IGCSE Physics revision · Space physics
Space physics
Space physics connects familiar ideas about motion, gravity, energy and waves to observations of planets, stars and galaxies. Start each answer by identifying which scale you are discussing: a planet's orbit, a star's evolution or the expansion of the Universe.
These notes follow section 6 of Cambridge IGCSE Physics 0625 for exams in 2026–2028. Core and Extended students share the foundations; stellar evolution, orbital-speed calculations and quantitative Hubble work include Supplement content for Extended. Check your own specification before using the notes for another qualification.
Dr Desouky Physics Academy · Meet your physics tutor
What the syllabus demands
- —Connect Earth's rotation and orbit to day, night and seasons, and the Moon's orbit to its phases.
- —Recognise Solar System bodies, the order of the planets, and how gravitational accretion forms a rotating planetary system.
- —Compare planetary data and explain how gravity keeps planets in orbit; calculate light-travel times.
- —Extended: calculate orbital speeds and explain the changing speed of an object along an elliptical orbit.
- —Place the Sun within the Milky Way, use light-years, and distinguish stars, galaxies and the Universe.
- —Extended: explain fusion in stable stars and the different evolutionary paths of lower-mass and massive stars.
- —Explain cosmological redshift; Extended students also use the microwave background, Hubble's relationship and an estimated age of the Universe.
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Light-year
- A distance: how far light travels through a vacuum in one year, approximately 9.5 × 10¹⁵ m.
- Galaxy
- A large gravitationally bound system containing stars, gas, dust and dark matter. Our Solar System is within the Milky Way.
- Protostar
- A forming star whose collapsing gas cloud becomes hotter as gravity transfers energy into the material's internal energy.
- Nuclear fusion
- A reaction in which light nuclei join to form heavier nuclei. Hydrogen fusion supplies energy during a star's main-sequence stage.
- Redshift
- An increase in the observed wavelength of radiation compared with its emitted wavelength. Cosmological redshift is associated with the expansion of space.
- Hubble constant
- The proportionality constant between a distant galaxy's recession speed and its distance in Hubble's relationship: H₀ = v/d.
The equations
More equations to practise: the IGCSE formula sheet.
Where the marks die
Common mistakes to check
- 01
Confusing rotation with revolution: one rotation gives a day, one orbit around the Sun gives a year. Seasons come from Earth's tilted axis changing the Sun's angle and daylight duration, not simply Earth moving nearer to the Sun.
- 02
Explaining ordinary Moon phases using Earth's shadow. The Moon reflects sunlight; its orbit changes the fraction of its illuminated half visible from Earth. Earth's shadow is relevant to a lunar eclipse.
- 03
Putting the Sun at the centre of an elongated elliptical orbit. It lies at one focus. Nearer the Sun, an orbiting object has lower gravitational potential energy and greater kinetic energy, so it moves faster.
- 04
Leaving out a stage in stellar evolution. A lower-mass star develops into a red giant, sheds a planetary nebula and leaves a white dwarf. A sufficiently massive star becomes a red supergiant and undergoes a supernova, leaving a neutron star or black hole.
- 05
Treating a light-year as a duration or a galaxy as one star. Convert light-years to metres before using SI equations; the Sun is one star among the many in our galaxy.
- 06
Saying that every nearby galaxy must recede. The expansion trend concerns large distances; local gravitational motion can produce blueshift. Explain the overall redshift-distance pattern when discussing evidence for expansion.
- 07
Reporting 1/H₀ as an exact age. Its units are seconds if H₀ is in s⁻¹, and the estimate assumes a simplified expansion history. Divide by seconds per year only after calculating the time.
One worked example, done properly
Question
Original practice question: a distant galaxy is 2.0 × 10²⁴ m away. Use H₀ = 2.2 × 10⁻¹⁸ s⁻¹ to estimate its recession speed and the age of the Universe in years. Take one year as 3.16 × 10⁷ s.
Method
- 1.Use v = H₀d. Both inputs are already in SI units: v = (2.2 × 10⁻¹⁸)(2.0 × 10²⁴) = 4.4 × 10⁶ m/s.
- 2.For the simplified age estimate, t ≈ 1/H₀ = 1/(2.2 × 10⁻¹⁸) = 4.55 × 10¹⁷ s.
- 3.Convert seconds to years: (4.55 × 10¹⁷)/(3.16 × 10⁷) = 1.44 × 10¹⁰ years. The value is an estimate from the supplied model and constant.
Recession speed = 4.4 × 10⁶ m/s; estimated age ≈ 1.4 × 10¹⁰ years (14 billion years).
Official syllabus references
These notes and worked examples are original revision material. Check the current specification for your exam board and exam year.