The Sun: a medium-sized star
The Sun is a star -- the closest star to Earth. It is medium-sized, made mostly of hydrogen and helium, and radiates most of its energy in the infrared, visible and ultraviolet regions. It holds most of the mass of the Solar System, so its gravitational attraction keeps planets, comets and asteroids in orbit. (Extended) It is powered by fusion of hydrogen nuclei into helium, and its field strength -- so orbital speed -- falls with distance.
Light-year: a distance, not a time
A light-year (ly) is the distance light travels in one year in a vacuum -- a unit of distance, never of time. With c = 3.0 × 10^8 m/s and one year ≈ 3.15 × 10^7 s, 1 ly = c × t ≈ 9.5 × 10^15 m. On the Solar-System scale, rearrange speed = distance / time to t = d / c: light from the Sun, 1.5 × 10^11 m away, takes 500 s (about 8.3 minutes). (Extended) Orbital speed is v = 2πr / T, where 2πr is the circumference and T the period.
Star life cycle depends on mass
Every star begins as a protostar: gas and dust pulled together by gravity until fusion starts, giving a main-sequence star. What follows depends on mass. A Sun-mass star becomes a red giant, then a white dwarf, shedding a planetary nebula. A much more massive one becomes a red supergiant, explodes as a supernova and leaves a neutron star -- or a black hole if huge. (Extended) That supernova spreads a nebula from which stars form.
Drawn from real examiner reports.
(Extended) Fusion, not burning or fission
The Sun releases energy by nuclear fusion -- hydrogen nuclei join to form helium. It is not combustion: nothing is burning in oxygen. It is not fission, which splits large nuclei and is the reactor process. And it is not radiation: radiation is how the energy leaves the Sun, not how it is produced.
Flagged Jun 2022 P23 Q31; Nov 2022 P41 Q6c
(Extended) Orbital speed uses 2πr, not πr²
In one period T an orbiting object travels once around the orbit -- a distance equal to the circumference 2πr -- so v = 2πr / T. Candidates who substitute the area πr², or write πr or 2r, use the wrong distance and lose every mark that depends on it. Learn the formula with the 2 in front.
Flagged Jun 2022 P41 Q12b
(Extended) Further from the Sun means slower
The Sun's gravitational field strength decreases with distance, so a more distant planet feels a weaker pull and orbits at a smaller speed, taking far longer to complete one orbit. The instinct that distant planets must be racing round faster is wrong -- the close-in planets are the fast ones.
Flagged Jun 2022 P42 Q12aiii
A light-year is a distance
A light-year measures how far light travels in one year (about 9.5 × 10^15 m), not how long anything takes. Writing that a signal "took three light-years" gives a distance where a duration was asked, and scores nothing. If the question asks for a time, the answer must carry a time unit -- seconds, minutes or years.
Sun-mass stars never go supernova
Keep the two routes apart. A star about the Sun's mass ends: red giant → white dwarf, shedding a planetary nebula. A star much more massive ends: red supergiant → supernova → neutron star, or a black hole if very massive. Sending a Sun-like star to a supernova, neutron star or black hole is a guaranteed mark loss.
Planetary nebula ≠ supernova
Both scatter gas and dust, but they belong to different routes. A planetary nebula is the shell gently thrown off by a Sun-mass star as it becomes a white dwarf. A supernova is the violent explosion of a much more massive star. Answering just "a nebula" for both throws away the distinguishing mark.
Starting the life cycle at "red giant"
A described or labelled life cycle must start at the protostar formed from a cloud of gas and dust, then the main-sequence star, before any giant stage. Arrows on a flow diagram must run in life-cycle order; a reversed or truncated chain loses the sequencing marks even when every stage name is correct.
Show t = d / c and give the unit
Write d and c in standard form, divide the numbers, subtract the powers of ten, then end with the unit. A bare number scores no unit mark. Example: d = 1.5 × 10^11 m with c = 3.0 × 10^8 m/s gives t = 500 s.
Match the command word
State wants the term only ("the Sun"). Describe wants the ordered chain -- protostar → main sequence → red giant → white dwarf. Explain wants the causal link. Answering the wrong command word loses marks on otherwise correct physics.
Orbit answers need mass AND gravity
For "explain why the planets orbit the Sun", two marks need two ideas: the Sun holds most of the mass of the Solar System, and that mass produces the gravitational attraction pulling each planet into a curved path. Saying only "gravity" earns one mark at best.
Scale and Big-Bang facts to learn
A galaxy holds billions of stars; the Milky Way is about 100 000 light-years across, one of billions of galaxies. (Extended) Big-Bang evidence: the Universe began from a single dense, hot point, is still expanding, and is about 13.8 billion years old.
Cambridge 0654 spec reference: Section P6 "Space physics", sub-topic P6.2 (Core + Extended). This leaf covers the Sun as the closest star, light-years and light-travel time, why objects orbit the Sun, the Sun's composition and radiation, star formation and life cycles by mass, galaxies and the Milky Way, and (Extended) orbital speed , how the Sun's gravity and orbital speed vary with distance, hydrogen fusion, supernova nebulae, and Big-Bang evidence.
Scope note: Hubble's law / the Hubble constant and quantitative red-shift are NOT in this leaf (the 0654 Big-Bang requirement is qualitative evidence only). Mass-defect/binding-energy are out of scope -- P5/P6 are qualitative in 2025-27.
| Quantity | Relationship | Notes |
|---|---|---|
| Light-travel time (Core) | ; answer in seconds | |
| Light-year (Core) | a distance, not a time | |
| Orbital speed (Extended) | circumference (not area ); in m/s |
Symbols: = distance (m); = speed of light in a vacuum (m/s); = orbital radius (m); = orbital period (s); = orbital speed (m/s). (IR = infrared, UV = ultraviolet, ly = light-year.)
(Core) Define a light-year.
(Core) The Sun is a distance of from Earth. The speed of light is .
Calculate the time taken for light from the Sun to reach Earth. Show your working. (2 marks)
