IB Physics: E 2.6 – Hertzsprung-Russell Diagram The Hertzsprung – Russell is a diagram which models therelation between the temperature and luminosity of a star. ● On the x-axis, we have Temperature (K) ; ○ Please note, on the x-axis, we can also have spectral class. ● On the y-axis, we have Luminosity (Watts) ○ Please note, on the y axis, we can also have absolute magnitude Here is an example of a Hertzsprung Russell diagram:
Some key points you should take from this graph: ● The line of stars that lies diagonally are called main sequence stars ● White dwarf stars are what its name suggests – it’s small ○ If you remember Stefan-Boltzmann Law, L = σAT 4 , it suggests that Luminosity is proportional to bothsurface area and 4th power of area. So as you cansee, L is really small, whilst ‘T’ is reasonably large –this suggests that surface area ‘A’ is really small. ○ White dwarfs lie in the bottom left to center, where’sRed dwarfs lie in the bottom right to center – again,you might be tested on this in the exam. ● Red Giants again are what its name suggests – its large. In this graph, it is simply labelled as ‘Giant’, although theIB might ask for ‘Red Giant’ ○ Again, use the law above – as the Luminosity is high,the surface area should also be ‘high’ ● Super Giant: Basically a Giant on steroids – even higher luminosity, so on average even larger surface temperature Another thing you need to appreciate is that the scales on thegraph above are NOT linear! The IB loves to ask something likethis. Questions that can be asked: I’ve done a lot of research and problems about theHertzsprung-Russell diagram, and whilst the information aboveis important, being fully equipped to solving problems aboutthe Hertzsprung Russell diagram is even more important. So,some stuff that comes up generally everytime HR Diagramsappear
● Why star A is larger than star B? You can find this solutionin my description of the Stefan-Boltzmann Law seenabove – generally, this question comes up 9/10 times. ● Labeling the diagram ● Writing alternative labels for the axes E.g. Spectral class isgiven, what is an alternative for the x-axis?