Know your calculator!
When answering examination questions, it is important that your excellent knowledge of physics isn’t undermined by your calculator behaving in an unexpected way. You MUST be able to trust the answer given. But the situation is potentially complicated, not only because calculators don’t all behave the same way but also because they may not even be behaving the way you…
Read moreAt what rate is the Universe expanding?
When Edwin Hubble first used data to determine whether “nebulae” were part of our own galaxy, he found they were in fact moving away from the Milky Way at a rate proportional to their observed distance. The most distant of these objects (now known to be galaxies) were receding at the greatest speed. This in turn gave birth to the…
Read moreRay Diagram for a Telescope in Normal Adjustment
There is a summary of the key characteristics of astronomical telescopes in a previous post, at https://physbang.com/2026/03/15/a-level-guide-to-astronomical-telescopes/. This article builds on that general overview by explaining a specific skill; how to draw a ray diagram for a refracting telescope in normal adjustment. First, what is “normal adjustment”? It is when the objective and eyepiece lenses are arranged so that the…
Read moreRed-Shift and the Age of the Universe
Stars can be characterised by their absorption lines, which reveal their temperature and composition. There is more detail about this in two recent posts about the HR diagram and stellar classification. A similar approach can also be used to characterise entire galaxies but in this case the aim is to determine how quickly they are receding (moving away) from the…
Read moreThe formation, life and death of stars
All stars are born through the same basic mechanism but their evolution depends on their size. In particular, all stars go through a hydrogen-fuel stage but the time spent fusing hydrogen is determined by the star’s initial mass. The final fate of a star is also predicted by its mass; some stars will simply fade away whereas others depart in…
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