A-Level Physics: Astrophysics – Telescopes and Stars

Astrophysics is the optional topic most often chosen by A level Physics students, and for some specifications it is part of the core content (for example, OCR A includes astrophysics and cosmology in its compulsory modules, while AQA offers it as one of the Paper 3 options). This material covers the first half of that content: how telescopes collect and resolve light, and what starlight tells us about the stars themselves. Its companion material in this category covers cosmology: Doppler shift, Hubble's law, the Big Bang and exoplanets.

The quiz (12 questions) starts with instruments. You calculate the angular magnification and length of a refracting telescope in normal adjustment, pick out the genuine advantages of large reflecting telescopes over refractors (and reject some tempting false claims about spherical aberration and secondary mirrors), apply the Rayleigh criterion to a large radio dish, and compare charge-coupled devices with the human eye in terms of quantum efficiency, exposure time and stored images. The second half is about stars: the logarithmic magnitude scale (how much brighter a star three magnitudes brighter appears), distance from apparent and absolute magnitude, the parsec, surface temperature from Wien's law, the radius of a star from its peak wavelength and luminosity using Stefan's law, why Balmer absorption lines are strongest in class A stars, the order of the spectral classes, the properties of main-sequence stars and the life cycle of a Sun-like star.

The flashcards (12 cards) give the magnification formula, the Rayleigh criterion, Wien's and Stefan's laws, the distance modulus, chromatic aberration, quantum efficiency, the parsec, the spectral sequence, the main sequence, the Cassegrain arrangement and integration time.

The written work (8 questions) asks you to explain the refracting telescope and derive its magnification, compare refractors and reflectors including both types of aberration, decide whether a telescope can resolve two stars and explain radio interferometry, compare CCDs with the eye, calculate a distance from magnitudes and relate it to the inverse-square law, find the temperature and radius of a red supergiant, explain the spectral classes and Balmer lines, and describe the Hertzsprung–Russell diagram and the evolution of a Sun-like star.

The content is based on the astrophysics content of A level Physics specifications in England (for example the AQA Astrophysics option, sections 3.9.1 and 3.9.2, and OCR A module 5.5). Formulae and constants are given in the questions where needed; specifications differ in some details, so check your own board's data sheet.

• Calculate the magnification and length of a refracting telescope in normal adjustment
• Compare reflecting and refracting telescopes, including chromatic and spherical aberration
• Apply the Rayleigh criterion to optical and radio telescopes and compare CCDs with the eye
• Use the magnitude scale and m − M = 5 log(d/10) to find brightness ratios and distances
• Use Wien's and Stefan's laws, spectral classes and the Hertzsprung–Russell diagram to describe stars and their evolution

Practice material written by Zestly, based on the astrophysics content of A level Physics specifications in England (for example the AQA 7408 Astrophysics option, sections 3.9.1 telescopes and 3.9.2 classification of stars, and OCR A module 5.5).