Astrophysics
MYP Year 4 Physics — Astrophysics unit
Astrophysics is just ordinary physics — gravity, forces, motion, waves — stretched across distances so large you need AU and light-years instead of metres. Every question in this unit reduces to one of two moves: (1) explain what gravity is doing to an orbiting body, or (2) read a relationship out of real astronomical data. Examiners consistently reward students who name the force and state its direction, and who quote actual numbers from a table or graph instead of describing a vague trend.
Overview: How This Topic Fits Together
The two halves of astrophysics at this level
The unit splits into a 'nearby' half and a 'whole universe' half. The Solar System half is about gravity keeping bodies in orbit — planets around the Sun, moons around planets, satellites around Earth — and uses Kepler's laws to connect distance and orbital period. The Big Bang half zooms out to the origin and expansion of the entire universe, using redshift and the Cosmic Microwave Background as the evidence base.
- The Sun contains about 99.8% of the Solar System's total mass — that's why its gravity dominates and everything else orbits it, not the other way round.
- Astronomical Unit (AU) km, the average Earth–Sun distance — used because kilometres become unwieldy at planetary scales.
- Every orbit, from the Moon around Earth to a galaxy around a supermassive black hole, is caused by the same single force: gravity.
- The Big Bang isn't a separate topic bolted on — it's what you get when you ask 'if gravity pulls things together, why is everything flying apart?' The answer (expansion of space) is the whole second half of this unit.
The shape of the chapter
Command terms this unit tests hardest
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| State | give a specific, concise answer with no reasoning expected | AO1 | one correct fact scores full marks; adding explanation earns nothing extra |
| Identify | name or select the correct feature from given information | AO1 | just the correct name — over-explaining wastes time, not marks, but won't add credit |
| Describe | give a detailed account of a pattern or feature | AO2 | must reference actual data or a specific feature, not a generic trend |
| Explain | give reasons — a cause-and-effect chain | AO2 | needs mechanism, not just the fact — 'gravity holds it' with no direction/mechanism loses the mark |
| Construct | build a graph/diagram/table from given data | AO2 | axes, scale, labels and plotted points are each separately marked |
| Analyse | break down data to find a relationship | AO3 | must quote actual numbers from the table, not just say 'it increases' |
| Suggest | propose a plausible idea beyond taught content | AO3 | any scientifically sound, topic-relevant idea is accepted if justified |
| Discuss | give a balanced account covering more than one angle | AO3 | needs at least two distinct points — one well-explained point caps the mark |
Key point
Overview