How to Read a Hertzsprung Russell Diagram

A Hertzsprung-Russell diagram (sometimes abbreviated to “HR diagram”) is essentially a description of the different types of stars.

Devised around one hundred years ago by scientists Ejnar Hertzsprung and Henry Norris Russell, it plots the spectral class of numbers of stars along the x-axis and their absolute magnitude along the y-axis.

What do we mean by “spectral class” and “absolute magnitude”?

Spectral class is really a description of what a star looks like – its colour. There are several classes: O, B, A, F, G, K and M.  O stars are blue, B blueish white, A white, F yellowish-white, G yellow, K orange and M red.  There is considerable variation within these groups. It is also worth pointing out these groupings are based on old observations, and so do not always match more accurate modern observations.  There have also been new groups created according to modern discoveries: for example, to include stars that have only recently been identified, such as brown dwarfs – not technically stars because they are too small to sustain hydrogen burning at their cores.

The colour of a star tells us lots about it, primarily its heat. Blue stars are the hottest and red the coolest. The Sun is a G-type star, a larger than average yellowish-white star. However, most stars are M-type stars, because most stars are red dwarfs – smaller than the Sun and with surface temperatures of around 3000C.

Absolute magnitude refers to a star’s actual brightness and therefore the amount of energy it is radiating. The Sun’s absolute magnitude is 4.83, but the supergiant star Rigel has an absolute magnitude of -6.7 – much, much brighter. Some Hertzsprung-Russell diagrams may use luminosity rather than this, but it is a very similar measure. Luminosity describes the total amount of energy radiated by a star, and is a comparative measure. The Sun’s luminosity is defined as 1, so other stars are measured as either more or less luminous than the Sun.

So what can a Hertzsprung-Russell diagram tell us about stars?

It can tell us about the stage of existence stars are at. Stars are not evenly distributed throughout the diagram, but fall into distinct stripes, or branches.

Most HR diagrams will plot a sample of stars – often a thousand or more, to show the average distribution of stars across it. Smaller samples will not give as strong an indication of the lines and branches on the diagram.

There is a long branch, full of stars, that runs from upper left corner to lower right corner of each HR diagram containing what are known as “main sequence” stars – stars like the Sun or most red dwarf stars, which are burning hydrogen at their cores. Confusingly, these stars, which are in the early or middle phases of their lives, are generally known as “dwarf” stars – even if, like the Sun, they are larger than around 85% of known stars. 

As you read along this branch, you move towards the darker and cooler stars (and generally smaller). Towards the top left-hand corner are the stars that are blue and very bright – there are very few of these on the main sequence. Towards the lower right-hand corner are the red dwarf stars: these are smaller than the Sun and cooler, but there are far more of them – the majority of all stars. At the very bottom right hand corner, not on the main sequence because they do not burn hydrogen in fusion reactions, are the brown dwarfs. Most stars on any sample will be somewhere along the main sequence branch.

Above the main sequence on the diagram are stars that have finished burning hydrogen and have started burning helium, or perhaps carbon and oxygen. They have therefore left the main sequence branch of the diagram and are in the upper half of the diagram. These are the “giants” and “supergiants”.  Supergiant stars are the largest stars (by mass) in the universe, between around 10 and 70 times the mass of the Sun.  When a star this massive finishes its core hydrogen burning, it begins to expand to its supergiant state. An example of a supergiant star is Betelgeuse, which if placed in the Sun’s position would extend out past Jupiter.

Stars at the top left side of the diagram are blue supergiants: very bright, blue stars and stars at the top right side are red supergiants, cooler, reddish stars. Despite seeming very different, both these types of star are nearing the end of their lives, and are huge, inflated versions of their earlier main-sequence selves. A single star can wander across this line of the Hertzsprung-Russell diagram before dying in a supernova explosion.

Between the main sequence branch and the supergiant line at the top are the giants: smaller or medium mass stars which are nearing the end of their lives. The Sun will become one of these in around five billion years’ time.  It will have moved from the centre of the HR diagram to the upper right hand side, below the supergiants.

Before the Sun reaches the giant part of the diagram it will probably pass through the “subgiant” phase, just higher than the main sequence line but below the giant line. Subgiants are on their way to giant or supergiant phases.

The Hertzsprung-Russell diagram therefore shows how stars of different masses come to the end of their lives. On many HR diagrams you will see the stars given a size-based class, from I to V. I class stars are supergiants, II and III giants, IV subgiants and V main sequence stars.

There is also a branch at the lower left-hand side of the diagram, whose stars have low magnitudes and luminosities and are also white in colour. These are dead stars: the white dwarfs, which are the very dense, collapsed cores of stars like the Sun or smaller, which are not big enough to undergo supernova explosions at the end of their lives.

A Hertzsprung-Russell diagram will therefore not tell you a star’s position in the sky, or about its planetary system. It does not, technically, reveal a star’s age. What it does is to tell what stage of “life” the star is at. If it is on the main sequence, it is young or middle-aged and it is burning hydrogen at its core. If it is higher, it is a star that is beginning its journey to its end; if it is at the lower left hand corner, it is a star that has already finished its journey.

HR diagrams are useful tools in describing the ages of stars, which also meant that they were important in the development of astrophysics: because they set out different stages in a star’s life, they prompted scientists to study what might be causing these changes.