Colour tells you surface temperature
A star is classified by its colour, and its colour is set by its surface temperature (how hot its surface is). The order from hottest to coolest is blue → white → yellow → orange → red: blue stars are the hottest and red stars the coolest, with our yellow Sun in the middle. The key idea for marks: the bluer the star, the hotter its surface; the redder, the cooler. This feels backwards next to everyday "red-hot/white-hot" language, so learn the direction deliberately.
Life cycle of a Sun-mass star
For a star of similar mass to the Sun the stages, in order, are: nebula (a cloud of dust and gas, mostly hydrogen, gathered by gravity); protostar (the collapsing gas heats up, but fusion has not started); main sequence star (stable, fusing hydrogen into helium); red giant (the core hydrogen runs low, so the star swells and its surface cools, looking red); and white dwarf (the outer layers drift off, leaving the small, hot, dense core). The high-mass route is out of scope.
Main-sequence stability, and what mass decides
A main sequence star is stable because the outward radiation/gas pressure from hydrogen fusion balances the inward pull of gravity, so it keeps roughly the same size for billions of years until its core hydrogen runs low. The property that determines a star's whole evolutionary path is its mass — not its size or colour. For 4SD0 you only describe a star of similar mass to the Sun; the high-mass route ending in a supernova is out of scope.
Drawn from real examiner reports.
Compare by temperature and size, not colour
When comparing a red giant and a white dwarf, writing only "a red giant is red and a white dwarf is white" scores nothing — naming a colour just restates the name. Compare the physical properties: a white dwarf is hotter, a red giant cooler; a red giant is much larger, a white dwarf very small (and far denser). Colour only matters because it tells you the temperature.
June 2024 Paper 1P Q4(c); June 2024 Paper 1PR Q4(b): candidates stated only the colours; credit needed comparisons of surface temperature and size.
Mass, not size, decides the fate
Asked what determines how a star will evolve, the answer is its mass. Wrong answers earning no credit: "size", "how big it is", "brightness", "colour", "temperature". Do not confuse size (how large the star looks) with mass (how much matter it contains) — a red giant is enormous in size but its mass is still only about that of the Sun.
June 2024 Paper 1P Q4(b): the most common wrong answer for the property that determines the path was size instead of mass.
No fusion in a protostar
As a nebula collapses, the gravitational energy store decreases and the thermal store increases. But nuclear fusion has not started in a protostar, so the nuclear store stays the same — fusion only begins once the core is hot and dense enough, making it a main sequence star. Do not mix up chemical and nuclear reactions: stars release energy by nuclear fusion, not by burning.
June 2024 Paper 1P Q4(a): candidates confused chemical and nuclear stores; only the most able knew no fusion occurs in a protostar.
A Sun-mass star ends as a white dwarf
For 4SD0 you describe only the low-mass route: main sequence → red giant → white dwarf. Do not give the high-mass ending (red supergiant → supernova → neutron star or black hole) for a Sun-mass star — candidates were marked down for it. A star like the Sun finishes as a white dwarf, its final stable stage.
June 2024 Paper 1PR Q4(a): giving the high-mass route was penalised; keep to the low-mass life cycle to red giant and white dwarf.
Blue is hot, red is cool
The colour-temperature direction is counter-intuitive: blue stars are the hottest and red stars are the coolest, with yellow (like the Sun) in between. Everyday "red-hot then white-hot" thinking tempts students to rank red as hottest — here it is the coolest. Whenever you name a colour, immediately state the matching temperature to avoid slipping into the wrong direction.
Answer the question colour → temperature asks
When a question applies colour to temperature (for example on a graph or phase diagram), answer the question actually asked — turn the colour into a surface temperature and use it. Some candidates drifted onto a different, well-rehearsed answer (such as a liquid becoming a gas) and scored nothing. Apply blue = hotter, red = cooler to the context in front of you.
November 2024 Paper 1P Q1(d): only the ablest applied colour→temperature to the context; many answered a different question.
Always convert colour to temperature
Whenever you mention a colour, immediately turn it into a temperature: blue = hottest, red = coolest. Never leave a colour statement on its own — colour is only useful because it tells you the surface temperature.
Learn the Sun-mass sequence in order
Memorise the stages in order: nebula → protostar → main sequence → red giant → white dwarf. Do not end a Sun-mass star at a supernova — that is the out-of-scope high-mass route.
"Compare" needs comparative words
For a "compare" question use comparative words — hotter/cooler, larger/smaller, denser/less dense. Restating a name ("it is red") is not a comparison. Use the mark tariff: a 4-mark compare wants about four distinct differences, so plan that many before writing.
Formation energy transfers
For "energy as a star forms" questions: the gravitational store goes down and the thermal store goes up as the cloud is compressed and heats. No fusion happens yet, so the nuclear store is unchanged until the main sequence begins.
This topic is entirely describe and compare — there are no calculations in this part of the 4SD0 course. Marks are won by classifying stars by colour → temperature, knowing the two life-cycle sequences in the right order, and making genuine comparisons rather than just naming colours.
There is no equation to memorise for this topic. Instead, learn these fixed relationships:
Key definitions:
Which colour of star has the HIGHEST surface temperature? A) red B) yellow C) blue
Astronomers classify stars by their colour.
(a) State what a star's colour tells you about the star.
(b) Two stars are observed: one is blue and one is red. State which star has the higher surface temperature, and explain your answer.
