Astrophysics Help for Physics Exams (IB, AP, A-level)

The astrophysics parts of school physics are full of big numbers and new ideas. With the physics understood and a clear method, they become some of the most reliable marks on the paper.

Who it's for

Students taking IB Physics, A-level Physics, AP Physics or a national curriculum, who want to understand stars, light and orbits properly — and set out answers that earn full marks.

What we cover

Gravitation and orbits
Wien's law and the Stefan–Boltzmann law
Stellar properties: luminosity, temperature, radius
The Hertzsprung–Russell diagram
Cosmology basics: redshift and Hubble's law
Exam technique and mark schemes

Where astrophysics appears in your course

Course	Where it is	What we cover
IB Physics (2025 syllabus, SL & HL)	Topic E.5, Fusion and stars, plus gravitational fields. There are no options any more — this is core content.	HR diagram, stellar parallax, stellar radii from Wien and Stefan–Boltzmann, hydrostatic equilibrium, stellar evolution, orbits
AQA A-level Physics	Option A: Astrophysics — one of the optional topics, if your school takes it	Telescopes, magnitudes, classifying stars, the HR diagram, cosmology and Hubble's law, detecting exoplanets
OCR A-level Physics A	Module 5.5, Astrophysics and cosmology — compulsory for everyone	Stars and their fate, the HR diagram, spectra and energy levels, Wien and Stefan–Boltzmann, astronomical distances, cosmology
AP Physics 1 & 2	No astrophysics unit, but the physics behind it is examined	Gravitation, circular motion and orbits (AP 1); photons, energy levels and spectra (AP 2)
Other courses	National curricula and other boards	Send me your syllabus and we follow it

How lessons work

We follow your syllabus. Each live lesson covers one idea in depth, then works through exam-style questions on a shared whiteboard. After each lesson you get a notes summary and practice questions, and we review your answers against mark-scheme logic.

Worked example: a star's radius from its luminosity and temperature

Problem. A main-sequence star has a luminosity of $L = 25\,L_\odot$ and a surface temperature of $T = 9900$ K. Find its radius. ( $L_\odot = 3.828\times10^{26}$ W, $R_\odot = 6.957\times10^{8}$ m, $\sigma = 5.67\times10^{-8}$ W m⁻² K⁻⁴.)

Method 1 — Stefan–Boltzmann directly. Treat the star as a black body:

L = 4\pi R^2 \sigma T^4 \quad\Rightarrow\quad R = \sqrt{\frac{L}{4\pi\sigma T^4}}.

R = \sqrt{\frac{25 \times 3.828\times10^{26}}{4\pi \times 5.67\times10^{-8} \times 9900^4}} \approx \sqrt{\frac{9.57\times10^{27}}{6.84\times10^{9}}} \approx 1.18\times10^{9}\ \text{m}.

So $R \approx 1.7\,R_\odot$ .

Method 2 — ratios (faster in an exam). Divide by the same equation for the Sun ( $T_\odot = 5772$ K):

\frac{R}{R_\odot} = \sqrt{\frac{L}{L_\odot}}\left(\frac{T_\odot}{T}\right)^2 = \sqrt{25}\times\left(\frac{5772}{9900}\right)^2 \approx 5 \times 0.340 \approx 1.70.

Exam tip. The ratio method cancels $4\pi\sigma$ and avoids huge powers of ten. Show the ratio equation explicitly — it usually carries a method mark.

Start with a trial lesson

A 30-minute trial costs €10. We set goals and you see how I teach. After that, lessons are €30 for 60 minutes, or less with a pack.

Book a €10 trial lesson See all prices

Questions

Which exam boards do you cover?

IB Physics (the 2025 syllabus, where stars are part of topic E.5, Fusion and stars), A-level Physics (AQA's astrophysics option and OCR's astrophysics and cosmology module), AP Physics 1 and 2 (gravitation, orbits and atomic spectra), and national curricula. Tell me your board and syllabus and we follow it.

Do you use past papers?

Yes. Past-paper questions show how marks are awarded. We practise setting out answers so every step earns credit.

Can you help shortly before an exam?

Yes. A few focused lessons on your weakest topics and on exam technique can make a real difference, even close to the exam.

I find the HR diagram confusing. Can you help?

Yes. The HR diagram makes sense once you see that luminosity depends on both temperature and radius. The blackbody and HR simulation on this site lets you move a star around and watch why.