Stars, Galaxies and the Universe
Space physics is mostly sequences and definitions — the life cycle of a star, the structure of the Universe, and the evidence for the Big Bang. The marks are for order and precise wording.
1. Structure of the Universe
Planet → Solar System → Galaxy → Universe
- A galaxy is a huge collection of billions of stars, held together by gravity
- Our galaxy is the Milky Way, and the Sun is one star within it
- The Universe contains billions of galaxies
2. The light year
A light year is the DISTANCE light travels in one year — about 9.5 × 10¹⁵ m.
A light year measures DISTANCE, not time. This was a recorded error, and it is examined directly. The word “year” is in the name, but the quantity is a distance.
Where the number comes from: speed of light (3 × 10⁸ m/s) × seconds in a year (≈ 3.15 × 10⁷ s) ≈ 9.5 × 10¹⁵ m.
Take care with the power of ten. Writing 9.5 × 10⁻⁸ instead of 9.5 × 10¹⁵ was a recorded error — check that huge distances have large positive powers.
Know the speed of light: 3 × 10⁸ m/s. Forgetting it was recorded.
3. Nuclear fusion in stars
Stars release energy by nuclear FUSION — hydrogen nuclei join to form helium, releasing enormous energy.
Stars run on FUSION, not fission. Identifying the Sun’s process as fission was a recorded error. Fusion joins small nuclei; fission splits large ones.
The most abundant elements in the Sun are HYDROGEN and helium — not nitrogen and oxygen, a recorded guess.
Fusion needs extremely high temperature and pressure to overcome the repulsion between positively charged nuclei — conditions found only in stellar cores.
A star is stable while the outward pressure from fusion exactly balances the inward pull of gravity.
4. The life cycle of a star
Nebula → Protostar → Main sequence star → …
Get the order right — this is the most examined part of the topic.
- Nebula — a cloud of dust and gas
- Gravity pulls the material together, heating it — it becomes a protostar
- When hot enough, fusion begins and it becomes a stable main sequence star (like our Sun), where fusion pressure balances gravity
The nebula comes FIRST, then the protostar. Reversing them was a specific recorded error. The cloud collapses into the protostar.
Then the path depends on mass:
A star about the size of the Sun:
main sequence → red giant → white dwarf (→ black dwarf)
A star much more massive than the Sun:
main sequence → red supergiant → supernova → neutron star, or a black hole if massive enough
Only massive stars go supernova. A Sun-sized star ends quietly as a white dwarf. Getting the two paths crossed was recorded.
What drives the change: when the hydrogen in the core runs out, fusion slows, gravity wins, the core contracts and the outer layers expand and cool — turning the star red.
A supernova scatters heavier elements into space, which become the raw material for new stars and planets. This is where the elements heavier than helium come from.
5. Star colour and temperature
Blue stars are the HOTTEST; red stars are the COOLEST.
Hotter objects emit radiation at higher frequencies (shorter wavelengths), and blue light has a higher frequency than red.
Uncertainty over which colour was hotter was recorded. Link it to the electromagnetic spectrum: blue = higher frequency = hotter.
6. Redshift and the expanding Universe
Redshift is the observed increase in wavelength (and decrease in frequency) of light from a galaxy moving away from us — the light is shifted towards the red end of the spectrum.
Blueshift is the opposite: wavelength decreases for an object moving towards us.
Moving away → wavelength INCREASES → shifted to RED. Confusing redshift with blueshift, and misunderstanding which way the wavelength changes, were both recorded — this is the single most examined idea in space physics.
The evidence and what it means:
- Light from distant galaxies is redshifted
- The further away a galaxy is, the greater its redshift, so the faster it is moving away
- Therefore all galaxies are moving apart — the Universe is expanding
The full three-step chain earns the marks: redshift → galaxies moving away → the more distant ones move faster → the Universe is expanding. Tutors noted explicitly that marks are awarded for describing this properly, so give the whole chain rather than the conclusion alone.
7. The Big Bang theory
The Universe began from an extremely hot, dense point and has been expanding ever since.
The two main pieces of evidence:
- Redshift of distant galaxies — everything is moving apart, so it was once together
- Cosmic microwave background radiation (CMBR) — microwave radiation from all directions, the cooled remnant of the radiation from the early Universe
CMBR is in the MICROWAVE region — it has stretched to longer wavelengths as the Universe expanded and cooled.
Working backwards from the expansion rate gives an age for the Universe of roughly 14 billion years.
8. Mistakes that cost marks
Saying a light year measures time.
Getting the power of ten wrong for a light year.
Saying stars are powered by fission.
Naming the wrong elements in the Sun.
Putting the protostar before the nebula.
Giving a supernova ending to a Sun-sized star.
Saying red stars are hottest.
Confusing redshift with blueshift.
Saying wavelength decreases in redshift.
Giving only the conclusion (“the Universe is expanding”) without the redshift reasoning.
Forgetting the CMBR as evidence.
Frequently asked questions
What is a light year? The distance light travels in one year — about 9.5 × 10¹⁵ m.
What process powers a star? Nuclear fusion — hydrogen nuclei joining to form helium.
What keeps a star stable? The outward pressure from fusion balances the inward pull of gravity.
What is the order of the life cycle of a star? Nebula → protostar → main sequence star, then red giant → white dwarf (small stars), or red supergiant → supernova → neutron star or black hole (massive stars).
Which stars become supernovae? Only stars much more massive than the Sun.
Which colour star is hottest? Blue. Red stars are the coolest.
What is redshift? An increase in wavelength of light from a galaxy moving away from us.
What does redshift tell us? That galaxies are moving apart — the Universe is expanding.
What is the evidence for the Big Bang? Redshift of distant galaxies and the cosmic microwave background radiation.
What is the CMBR? Microwave radiation from all directions — the cooled remnant of the early Universe.
Quick revision checklist
- I know the structure: planet → solar system → galaxy → Universe
- I know a light year is a distance, ≈ 9.5 × 10¹⁵ m
- I know the speed of light
- I know stars run on fusion of hydrogen into helium
- I know fusion needs high temperature and pressure
- I can explain what keeps a star stable
- I know the life cycle in order, starting from the nebula
- I know the two different endings and which stars take each
- I know blue stars are hottest
- I can define redshift and say what happens to the wavelength
- I can give the full redshift → expansion argument
- I know both pieces of Big Bang evidence
- I know CMBR is microwave radiation
These notes cover stars, galaxies and the Universe in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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