Space Physics — CIE IGCSE Physics Revision Notes
What you'll learn
- How the rotation and orbit of the Earth produce days and years
- The structure of the Solar System, the order of the planets, and how it formed
- Why orbits are elliptical, and why a comet speeds up as it approaches the Sun
- How to calculate orbital speed from radius and period
- The Sun as a star, and the fusion process that powers it
- Astronomical distances, the light-year, and how to convert it
- The life cycle of a star, on both of its possible endings
- Redshift, the Big Bang, and how the Hubble constant gives an age for the Universe
Key terms and definitions
Rotation — the spinning of a body on its own axis.
Orbit — the path one body follows around another under gravitational attraction.
Orbital period (T) — the time taken to complete one full orbit.
Solar System — the Sun together with the planets, minor planets, moons and smaller bodies orbiting it.
Minor planet — a rocky body orbiting the Sun, smaller than a planet; most lie in the asteroid belt.
Accretion — the process by which a cloud of gas and dust collapses and clumps together to form a star and planets.
Nebula — a cloud of gas and dust in space, from which stars form.
Protostar — a collapsing clump of gas and dust that has not yet begun fusion.
Nuclear fusion — the joining of light nuclei to form heavier ones, releasing energy.
Main sequence star — a stable star in which the outward pressure from fusion balances inward gravitational attraction.
Red giant / red supergiant — the expanded, cooler stage a star enters when its core hydrogen is exhausted.
White dwarf — the hot, dense remnant left when a low-mass star sheds its outer layers.
Supernova — the explosive end of a massive star.
Neutron star — an extremely dense remnant left after some supernovae.
Black hole — a remnant so dense that light cannot escape it.
Galaxy — a large collection of stars, gas and dust held together by gravity.
Light-year — the distance light travels in one year, about 9.5 × 10¹⁵ m.
Redshift — the increase in observed wavelength of light from a source moving away from the observer.
Hubble constant (H₀) — the ratio of a galaxy's recession speed to its distance.
Cosmic microwave background radiation (CMBR) — faint microwave radiation from all directions, left over from the early Universe.
Core concepts
The Earth, Sun and Moon
The Earth rotates on its own axis once every 24 hours. The side facing the Sun experiences day, the side facing away experiences night. It is the rotation that produces day and night, not the orbit — a distinction questions test directly.
The Earth orbits the Sun once every 365¼ days. That quarter day is why a leap year is needed.
The Moon orbits the Earth in approximately one month. The Moon does not emit light; it is seen by reflected sunlight.
Orbital speed
For a body in a roughly circular orbit, the distance travelled in one orbit is the circumference 2πr, covered in the orbital period T. So:
v = 2πr ÷ T
For the Earth, r ≈ 1.5 × 10¹¹ m and T = 365¼ days = 3.16 × 10⁷ s.
v = (2 × π × 1.5 × 10¹¹) ÷ (3.16 × 10⁷) ≈ 3.0 × 10⁴ m/s
The most common error here is using the period in days while the radius is in metres. Convert T to seconds first, every time.
The Solar System
The Sun lies at the centre. The eight planets in order of increasing distance are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
The four inner planets are small and rocky; the four outer planets are large and gaseous. The asteroid belt, containing most minor planets, lies between Mars and Jupiter.
Two general patterns are worth knowing. Planets further from the Sun have longer orbital periods and slower orbital speeds. And the further a planet is from the Sun, the weaker the gravitational field strength it experiences, because gravitational field strength decreases with distance.
The Solar System formed by accretion: a cloud of gas and dust collapsed under gravitational attraction, most of the mass forming the Sun at the centre, with the remainder clumping together into planets and smaller bodies.
Why orbits work
Orbits are maintained by gravitational attraction, which provides the centripetal force — the force directed towards the centre of the orbit. Without it a body would travel in a straight line.
Orbits are elliptical rather than perfectly circular. For a strongly elliptical orbit such as a comet's, the Sun is not at the centre, so the distance varies considerably over one orbit.
This produces a result worth understanding rather than memorising. As a comet approaches the Sun, the gravitational attraction on it increases, so it speeds up; as it moves away, it slows down. A comet therefore travels fastest when closest to the Sun and slowest when furthest away.
The Sun as a star
The Sun is a star of average size. It emits most of its radiation in the visible and infrared parts of the spectrum.
Its energy comes from nuclear fusion in its core: hydrogen nuclei fuse to form helium, releasing energy. This is the source of essentially all the energy in the Solar System.
Stability comes from a balance of two opposing effects. The outward pressure produced by fusion balances the inward gravitational attraction of the star's own mass. A star in this balance is a main sequence star, and the Sun will remain one for billions of years.
Stars, galaxies and distances
The Sun is one of many stars making up the Milky Way galaxy, and the Milky Way is one of very many galaxies in the Universe.
Astronomical distances are far too large for metres to be convenient, so the light-year is used: the distance light travels in one year.
1 light-year = speed of light × seconds in a year = (3.0 × 10⁸) × (3.16 × 10⁷) ≈ 9.5 × 10¹⁵ m
You should be able to reproduce that calculation rather than only quoting the value.
The life cycle of a star
All stars begin the same way. A nebula of gas and dust collapses under gravitational attraction, forming a protostar. When the core becomes hot and dense enough, fusion begins and the star becomes a stable main sequence star.
What happens next depends on mass.
For a star of similar mass to the Sun: core hydrogen is eventually exhausted, and the star expands and cools to become a red giant. It then sheds its outer layers, leaving the hot dense core behind as a white dwarf, which gradually cools.
For a star much more massive than the Sun: it expands to become a red supergiant, then explodes as a supernova. The remnant left behind is either a neutron star or, if massive enough, a black hole.
Two points are commonly dropped. The route taken is determined by the star's mass, so any answer must state which mass is being described. And supernovae matter beyond the star itself: they scatter the heavier elements formed in the star into space, and those elements go on to form new stars and planets.
Redshift and the expanding Universe
Light from distant galaxies is observed at longer wavelengths than expected — it is redshifted. This indicates the galaxies are moving away from us.
The crucial observation is that more distant galaxies show greater redshift, meaning they are receding faster. This is evidence that the Universe is expanding.
The Big Bang theory interprets this as expansion from an initial single point. Two independent lines of evidence support it: the redshift of distant galaxies, and the cosmic microwave background radiation, faint microwave radiation arriving from all directions which is understood as radiation left over from the early Universe, stretched to microwave wavelengths by the expansion since.
The Hubble constant
The Hubble constant relates a galaxy's recession speed to its distance:
H₀ = v ÷ d
where v is speed of recession and d is distance. Its units are therefore s⁻¹ when v is in m/s and d in m.
Because 1 ÷ H₀ has units of time, it gives an estimate of the age of the Universe — the time for which expansion has been proceeding.
Using H₀ ≈ 2.2 × 10⁻¹⁸ s⁻¹:
age ≈ 1 ÷ (2.2 × 10⁻¹⁸) ≈ 4.5 × 10¹⁷ s
Converting to years: (4.5 × 10¹⁷) ÷ (3.16 × 10⁷) ≈ 1.4 × 10¹⁰ years, about 14 billion years.
This estimate assumes the rate of expansion has been constant, which is why it is an approximation rather than a precise figure.
Worked examples
Example 1: Orbital speed
A planet orbits the Sun at a radius of 2.3 × 10¹¹ m with a period of 687 days. Calculate its orbital speed. (3 marks)
Convert the period to seconds: T = 687 × 24 × 3600 = 5.94 × 10⁷ s (1)
v = 2πr ÷ T = (2 × π × 2.3 × 10¹¹) ÷ (5.94 × 10⁷) (1)
v = 2.4 × 10⁴ m/s (1)
Note this is slower than the Earth's 3.0 × 10⁴ m/s, as expected for a planet further from the Sun.
Example 2: Converting a light-year
Show that one light-year is approximately 9.5 × 10¹⁵ m. (3 marks)
Seconds in one year = 365¼ × 24 × 3600 = 3.16 × 10⁷ s (1)
Distance = speed × time = (3.0 × 10⁸) × (3.16 × 10⁷) (1)
= 9.5 × 10¹⁵ m (1)
Example 3: Comet speed
Explain why a comet travels faster when it is close to the Sun. (3 marks)
A comet has a highly elliptical orbit, so its distance from the Sun varies greatly (1). As it approaches, the gravitational attraction on it becomes stronger because gravitational field strength increases as distance decreases (1). The greater force produces a greater acceleration towards the Sun, so the comet speeds up, reaching its maximum speed when closest (1).
Example 4: Age of the Universe
The Hubble constant is 2.2 × 10⁻¹⁸ s⁻¹. Estimate the age of the Universe in years. (3 marks)
age ≈ 1 ÷ H₀ = 1 ÷ (2.2 × 10⁻¹⁸) = 4.5 × 10¹⁷ s (1)
Convert to years: (4.5 × 10¹⁷) ÷ (3.16 × 10⁷) (1)
≈ 1.4 × 10¹⁰ years (1)
An answer left in seconds will not gain the final mark if the question asks for years — check the unit demanded.
Example 5: Life cycle
Describe what will happen to the Sun after it leaves the main sequence. (4 marks)
When the hydrogen in its core is exhausted, the Sun will expand and cool to become a red giant (2). It will then shed its outer layers, leaving the hot dense core behind (1), which will remain as a white dwarf and gradually cool (1).
Because the Sun is a low-mass star it will not become a supernova — including that in an answer loses marks.
Common mistakes and how to avoid them
Saying the Earth's orbit causes day and night. Rotation causes day and night; the orbit gives the year.
Leaving the period in days in v = 2πr ÷ T. Convert to seconds whenever the radius is in metres.
Describing the Sun as being at the centre of an elliptical orbit. For an ellipse the Sun is not at the centre, which is exactly why the orbital distance varies.
Saying the Moon produces its own light. It is seen by reflected sunlight.
Giving the wrong ending for a star's mass. Sun-like mass → red giant → white dwarf. Much greater mass → red supergiant → supernova → neutron star or black hole. State the mass your answer assumes.
Confusing redshift with the Universe expanding into something. Redshift shows galaxies are receding and more distant ones recede faster. That is the evidence; expansion is the interpretation.
Forgetting the second piece of Big Bang evidence. Redshift alone is worth fewer marks than redshift plus the cosmic microwave background radiation.
Quoting the age of the Universe without the assumption. The 1 ÷ H₀ estimate assumes a constant rate of expansion.
Muddling minor planets and moons. Minor planets orbit the Sun; moons orbit planets.
Exam technique for Space Physics
Write the equation, substitute, then evaluate. Marks are awarded for the correct equation and for correct substitution even when the arithmetic goes wrong. An unsupported final answer that is wrong scores nothing.
Use standard form and keep the powers of ten visible. These quantities are large, and losing a power of ten is the commonest arithmetic error in this topic. Write 3.0 × 10⁸ rather than 300000000.
Check the unit asked for. Seconds or years; metres or light-years. Conversions are frequently the final mark.
Give three significant figures unless told otherwise, and keep full accuracy in the calculator until the last step rather than rounding partway.
For descriptive questions, use the sequence. The life cycle of a star is a chain: nebula → protostar → main sequence → and then the mass-dependent branch. Marks are awarded per correct stage in order.
Explain with a cause, not a restatement. "The comet speeds up because it is closer" restates the observation. "Gravitational field strength increases as distance decreases, so the force and therefore the acceleration are greater" explains it.
Learn the planet order. It appears in ordering and multiple-choice questions and is free marks.
Quick revision summary
- Earth rotates once per 24 hours (day and night) and orbits the Sun once per 365¼ days (the year)
- The Moon orbits the Earth in about one month and shines by reflected sunlight
- Orbital speed v = 2πr ÷ T — always convert T to seconds
- Earth's orbital speed ≈ 3.0 × 10⁴ m/s
- Planet order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
- Inner planets rocky, outer planets gaseous, asteroid belt between Mars and Jupiter
- Further from the Sun means longer period, slower speed and weaker gravitational field
- The Solar System formed by accretion of a collapsing cloud of gas and dust
- Gravitational attraction provides the centripetal force keeping bodies in orbit
- Orbits are elliptical; a comet is fastest closest to the Sun and slowest furthest away
- The Sun is an average star fusing hydrogen into helium; fusion pressure balances gravity
- 1 light-year = 3.0 × 10⁸ × 3.16 × 10⁷ ≈ 9.5 × 10¹⁵ m
- Life cycle: nebula → protostar → main sequence → red giant → white dwarf (low mass)
- Or → red supergiant → supernova → neutron star or black hole (high mass)
- Supernovae scatter heavy elements that form later stars and planets
- Greater redshift at greater distance shows the Universe is expanding
- Big Bang evidence: redshift of distant galaxies and the cosmic microwave background radiation
- H₀ = v ÷ d; age ≈ 1 ÷ H₀ ≈ 1.4 × 10¹⁰ years, assuming constant expansion