IGCSE · 26 September 2026 · 6 min read
IGCSE space physics questions: orbits, light-years and redshift
Space questions combine familiar mechanics and waves with unfamiliar scales. The arithmetic often becomes manageable once you separate radius from altitude, years from seconds and distance from time. Written questions need equally clear distinctions between what is observed and the explanation it supports.
Cambridge calls the topic Space Physics; Edexcel includes Astrophysics. Their detailed scope is not identical. Use these original examples to practise shared reasoning, then check your specification for the equations, star stages and cosmology detail required by your course.
Start with the model and the scale
If a question describes a circular orbit at constant speed, one revolution covers the circumference 2πr. The orbital radius is measured from the centre of the body being orbited, not from its surface. If you are given altitude, add the body's radius before using the circumference formula.
Convert the orbital period into seconds when calculating speed in metres per second. Keep powers of ten visible and estimate whether the answer should be a few metres per second or thousands. A consistent model and unit system are more valuable than memorising the appearance of one satellite calculation.
Worked example: orbital speed and changing the period
An idealised satellite travels in a circular orbit of radius 7.0 × 10⁶ m with period 6000 s. Its path length for one orbit is 2π × 7.0 × 10⁶ ≈ 4.40 × 10⁷ m. Average speed is that distance divided by 6000, giving about 7.3 × 10³ m/s, or 7.3 km/s.
As a mathematical comparison, if an object covered the same circular path in twice the time, its speed would halve. Do not infer that a freely orbiting satellite can adopt any chosen speed at the same radius: real orbital conditions must also satisfy gravitational dynamics. Distinguish a relationship calculation from a claim about which orbit is physically possible.
Explain why constant speed can still mean acceleration
In circular motion the velocity direction changes continuously even if the speed remains constant. That means there is acceleration towards the centre, requiring a resultant inward force. For a satellite, gravity supplies that force. Saying gravity is absent because the satellite stays above Earth contradicts the mechanism maintaining its curved path.
If the inward force were suddenly absent in the simplified model, the object would move along the instantaneous tangent rather than continue turning. Use a diagram with a tangential velocity arrow and an inward force arrow. They show different quantities and should not be drawn in the same direction merely because both belong to the satellite.
Worked example: light-years and travel time
Use 1 light-year = 9.5 × 10¹⁵ m for this practice question. A star at 4.0 light-years is approximately 4.0 × 9.5 × 10¹⁵ = 3.8 × 10¹⁶ m away. Light reaching us from it has travelled for about four years. Light-year describes the distance covered in a year, not a separate unit of time.
For another check, a distance of 2.85 × 10¹⁶ m corresponds to 2.85 × 10¹⁶ / (9.5 × 10¹⁵) = 3.0 light-years. If a question supplies a different rounded conversion or asks you to derive it from speed and seconds per year, use its stated values consistently. Avoid mixing kilometres and metres in the same division.
Worked explanation: use a spectral line as evidence
An identifiable spectral line measured at 500 nm in a laboratory is observed at 510 nm in light from a distant galaxy. The observed wavelength is longer by 10 nm, so the line is redshifted. The term refers to a shift towards longer wavelengths; it does not mean that every galaxy must look visibly red to your eye.
At the expected IGCSE level, the systematic redshift pattern in distant galaxies supports an expanding universe. State the observation before the inference. Do not claim that one shifted line alone supplies every detail of the universe's history. Cosmology questions may also ask about microwave background evidence, with the required explanation determined by your syllabus.
Build star-life-cycle explanations around mass and processes
For stellar evolution, identify whether the question concerns a lower-mass or a much more massive star before selecting an ending. A Sun-like pathway leads through a red giant stage towards a white dwarf remnant; a sufficiently massive star can undergo a supernova and leave a neutron star or black hole. Do not send every star through every possible final stage.
Connect stage names to physical changes rather than listing labels alone. Gravity gathers material; high temperatures allow fusion; changing fuel conditions alter the star's structure. In revision, draw two branches and explain why the question belongs on one. Then return to your specification for the exact terminology and depth instead of memorising a longer internet diagram than your course requires.
Questions, explained
Choose a question for a direct answer, then explore the explanation and supporting resources. Each answer has its own link to save or share.
Is a light-year a distance or a time?
A light-year is the distance light travels in one year in vacuum. It is approximately 9.5 × 10¹⁵ m using a common IGCSE rounding. A star four light-years away is seen using light that has travelled for about four years.
Do I use orbital radius or altitude in 2πr/T?
Use the radius of the circular path measured from the centre of the body being orbited. If the question gives height above the surface, add the body's radius first. Convert the period to seconds when the required speed is in m/s.
Why is an orbiting satellite accelerating at constant speed?
Velocity includes direction. A satellite following a circular path continuously changes direction, so its velocity changes even if speed does not. The required inward acceleration is produced by a resultant force towards the centre, supplied by gravity in the orbital model.
Does redshift mean a galaxy looks red?
Redshift means identifiable spectral features are observed at longer wavelengths than their reference values. It describes a wavelength comparison, not simply the visible colour of the whole galaxy. Explain the measured shift before relating it to motion or expansion in the question's model.
Do all stars become black holes?
No. A star's evolution and remnant depend strongly on its mass and subsequent development. A Sun-like star follows a different pathway from a sufficiently massive star that may leave a neutron star or black hole after a supernova. Use the mass information in the question.