The key idea
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.
Understand it · 01
Separate rotation, orbit and changing viewpoint
Earth’s rotation produces the daily cycle of sunlight and darkness. Its orbit around the Sun takes approximately one year. Seasons arise mainly because Earth’s tilted axis changes the height of the Sun in the sky and the duration of daylight during the year. Explaining summer only as Earth being nearer the Sun cannot explain why the northern and southern hemispheres experience opposite seasons at the same time.
The Moon reflects sunlight. As it orbits Earth, we see different fractions of its sunlit half, producing its phases. Ordinary phases are not caused by Earth’s shadow; that is relevant to a lunar eclipse. The Solar System contains one star, the Sun, together with planets and smaller bodies. A galaxy is a much larger collection of stars, gas and other matter; the Sun belongs to the Milky Way.
Understand it · 02
Describe an orbit using speed and direction
Gravity provides the inward resultant force that continually changes an orbiting object’s direction. In an ideal circular orbit the speed can remain constant while the velocity changes because velocity includes direction. The object is therefore accelerating. There is no need to invent an additional outward force that balances gravity in the usual inertial-frame explanation: a zero resultant force would produce straight-line motion instead.
For a circular orbit, v = 2πr/T uses the orbit’s radius measured from the centre of the body, not just height above its surface. Convert the period into seconds for an answer in metres per second. In an elliptical orbit around the Sun, the Sun is at one focus. The object moves faster nearer the Sun as gravitational potential energy decreases and kinetic energy increases.
Understand it · 03
Use initial mass to choose the star’s later path
A star forms as gravity pulls together gas and dust. As the cloud contracts, its interior heats. A main-sequence star is maintained by energy released in nuclear fusion, while an outward pressure gradient balances the inward effect of gravity. “Balanced” does not mean the star has no gravity or no moving particles. It describes the large-scale stability of its structure over that stage of its life.
The later path depends strongly on the star’s initial mass. A lower-mass star such as the Sun eventually expands into a red giant, loses outer material and leaves a white dwarf. A sufficiently massive star develops into a red supergiant and undergoes a supernova; the remnant can be a neutron star or, for a sufficiently massive remnant, a black hole. Do not put all stars through every branch of the diagram.
Understand it · 04
Connect observations to the expansion model
A distant galaxy’s spectral lines can appear at longer wavelengths than the corresponding laboratory lines. This redshift is evidence used to investigate recession and the expansion of the Universe. On sufficiently large scales, more distant galaxies tend to have larger recession speeds. The relationship v = H₀d models this trend. Nearby galaxies can have additional motions due to local gravity, so the overall trend does not require every individual galaxy to be redshifted.
The cosmic microwave background provides separate evidence for an early hot, dense Universe. Expansion has stretched that ancient radiation to much longer wavelengths. An estimate t ≈ 1/H₀ treats the expansion history in a simplified way and should not be called an exact age. Convert H₀ into compatible units first. A light-year, meanwhile, is a distance travelled by light in a year, not a unit of age.
Key definitions
- 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.
Equations, units & conditions
Light-travel time
t = d ÷ c
Units: s
Use d in metres and c = 3.0 × 10⁸ m/s.
Circular orbital speed
v = 2πr ÷ T
Units: m/s
Extended: r is the orbital radius; T is the period in seconds.
Hubble relationship
v = H₀d
Units: m/s
Extended: H₀ in s⁻¹ and d in m give v in m/s.
Estimated cosmic age
t ≈ 1 ÷ H₀
Units: s
Extended: an estimate based on a simplified expansion model.
Worked examples, step by step
Example 1
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.
Follow the method
- Use v = H₀d. Both inputs are already in SI units: v = (2.2 × 10⁻¹⁸)(2.0 × 10²⁴) = 4.4 × 10⁶ m/s.
- For the simplified age estimate, t ≈ 1/H₀ = 1/(2.2 × 10⁻¹⁸) = 4.55 × 10¹⁷ s.
- 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).
Practice questions with worked solutions
Try each question on paper before opening the solution. These are original academy questions; the suggested marks are a self-checking guide, not an official exam-board mark scheme. Difficulty labels describe this practice set, not predicted grades.
Starter · 3 suggested marks
1. Convert light time to distance
Light from an object takes 420 s to reach an observer. Calculate the distance travelled using c = 3.0 × 10⁸ m/s.
Show solution and self-check: question 1
- Use d = ct.
- d = (3.0 × 10⁸) × 420 = 1.26 × 10¹¹ m.
1.26 × 10¹¹ m.
Self-check · one mark per point
- Use distance = speed × time.
- Substitute the supplied light speed and 420 s.
- Calculate 1.26 × 10¹¹ m.
Build · 4 suggested marks
2. Calculate an orbital speed
A satellite travels in a circular orbit of radius 7.2 × 10⁶ m in 96 minutes. Calculate its speed and explain why it is accelerating despite constant speed.
Show solution and self-check: question 2
- T = 96 × 60 = 5760 s.
- v = 2πr/T = 2π × 7.2 × 10⁶/5760 ≈ 7854 m/s.
- Its direction of motion changes, so its velocity changes.
Speed ≈ 7.9 × 10³ m/s. It accelerates because its velocity changes direction.
Self-check · one mark per point
- Convert 96 minutes to 5760 s.
- Use circular circumference divided by period.
- Calculate approximately 7.9 × 10³ m/s.
- Explain changing velocity direction.
Common mistakes & how to avoid them
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
Understand the why
Questions, explained
Start with the short answer, then follow the reasoning. Each explanation has a permanent link you can share with a classmate or return to when revising.
Is space physics in Cambridge IGCSE Physics 0625?
Yes. Cambridge IGCSE Physics 0625 includes Space Physics in its 2026–2028 syllabus. It covers Earth, the Solar System, stars and the Universe, with additional Supplement requirements for Extended students. Always use the specification for your examination year and route to determine the required detail.
The topic name alone does not identify everything a student must calculate or explain. Core and Supplement columns distinguish requirements within Cambridge, while Pearson Edexcel organises overlapping ideas under its own astrophysics specification. A revision library can explain shared physics without claiming that every paragraph is required for every course. Start with your qualification code, examination year and Core or Extended route, then use the official syllabus to check coverage. This prevents spending time on an advanced calculation while accidentally overlooking a required basic observation or explanation.
How did the Solar System form?
The Solar System formed as gravity gathered a rotating cloud of gas and dust into a disc, with most material accumulating in the young Sun. Collisions and gravitational accretion built smaller bodies and planets from the remaining material. Temperature differences across the disc influenced what could accumulate.
Accretion means material collecting together rather than planets being thrown out fully formed. The central concentration became hot and dense enough for the Sun to develop, while the surrounding disc contained material that could collide, stick and grow. Cooler outer regions allowed different substances to contribute than the hotter inner regions, helping explain broad contrasts between rocky inner planets and larger outer planets. This is a formation model on Solar System scales, not an explanation of how the Universe itself began. Keep those two histories distinct.
How do redshift and the cosmic microwave background support the Big Bang?
Large-scale redshift patterns support an expanding Universe, while the cosmic microwave background supports an earlier hot, dense state. Together they provide different observational evidence for the Big Bang model. Expansion stretches wavelengths; the background is ancient radiation observed after that stretching and cooling history.
Using two pieces of evidence is stronger than repeating the same observation twice in different words. Galaxy spectra provide wavelength shifts and a broad relationship between distance and recession speed. The microwave background is detected in many directions and concerns radiation from the Universe’s early history rather than a collection of ordinary nearby radio sources. The Big Bang model describes expansion from an early hot, dense condition; it should not be drawn as an ordinary explosion from one centre into pre-existing empty space. Local galaxy motions can complicate individual redshift observations.
Why is a main-sequence star stable?
A main-sequence star is approximately stable because an outward pressure gradient balances gravity’s inward effect. Fusion supplies energy that helps maintain the hot interior and pressure. Stability does not mean gravity disappears, particles stop moving, or the star remains unchanged forever.
Gravity tends to compress the star, while the pressure is greater deeper inside than near the surface. The resulting pressure force provides support. Fusion reactions release energy, replacing energy the star radiates away and helping maintain the conditions that support this balance. These are connected but distinct statements: fusion is an energy source, and a pressure gradient supplies the outward supporting effect. When the available fuel and internal conditions change, the structure evolves. Avoid describing stable stars as simple solid balls or saying that every individual particle has zero resultant motion.
Should I use altitude or distance from Earth’s centre for orbital speed?
The radius in v = 2πr/T is measured from the centre of the body being orbited. If only altitude above its surface is given, add the body’s radius. Convert the orbital period to seconds before calculating speed in metres per second.
For a circular orbit, the travelled distance in one complete period is the circumference, 2πr. Altitude is only the distance from the surface to the satellite, so using it as r makes that circumference too small. A sketch with the centre, surface and satellite on one radial line makes the addition clear. The formula assumes a circular orbit with constant speed; an elliptical orbit has changing distance and speed, so one surface height does not supply the radius of an equivalent circle without further assumptions.
Convert altitude to orbital radius
Earth’s radius is 6.4 × 10⁶ m. A satellite orbits 0.8 × 10⁶ m above the surface with a 6000 s period. Find its speed.
- r = 6.4 × 10⁶ + 0.8 × 10⁶ = 7.2 × 10⁶ m.
- v = 2π × 7.2 × 10⁶/6000.
v ≈ 7.54 × 10³ m/s.
Why does one divided by the Hubble constant estimate an age?
The estimate t ≈ 1/H₀ gives a time in seconds when H₀ is expressed in inverse seconds. It is an approximate timescale based on a simplified expansion history, not an exact measurement of cosmic age. Convert seconds to years only after calculating the reciprocal.
The relationship v = H₀d gives H₀ units of speed divided by distance. If both are in SI units, metres cancel and leave s⁻¹, so its reciprocal is a time. Astronomy often uses other distance units, which must be converted before treating a numerical Hubble constant as inverse seconds. The physical estimate effectively traces the expansion backwards under simplifying assumptions. The actual expansion rate has evolved, so the reciprocal is useful for an IGCSE model calculation without being a complete account of cosmological history.
Calculate a Hubble timescale
Use H₀ = 2.5 × 10⁻¹⁸ s⁻¹ and 1 year = 3.16 × 10⁷ s to estimate the timescale in years.
- t ≈ 1/H₀ = 4.0 × 10¹⁷ s.
- Divide by seconds per year: (4.0 × 10¹⁷)/(3.16 × 10⁷).
t ≈ 1.27 × 10¹⁰ years, an approximate model timescale.
Your learning checklist
Use these goals to check your understanding. Requirements vary by specification, tier and exam year; the official references below define your full course.
- 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.
Sources & how to cite these notes
Original explanations, illustrations and practice by Dr Desouky Physics Academy. Official specifications guide the course scope; questions and suggested marks on this page are our own revision material.
- Cambridge 0625 syllabus for 2026–2028 — section 6: Space physics
- Cambridge IGCSE Physics 0625 — official qualification and syllabus updates
Reference this page
Dr Desouky Physics Academy. IGCSE Physics: Space physics. Updated 2026-09-25. https://www.drdesouky.com/revision/igcse-physics/space-physics
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