The Hertzsprung-Russell diagram

In 1911 the Danish astronomer Ejnar Hertzsprung (1873-1967), who was mainly interested in studying star clusters, decided to plot the luminosities of the stars in a cluster against their colours. In 1913 the American astronomer Henry Norris Russell did something very similar when studying “local” stars in the neighbourhood of the Sun, although he used spectral classes rather than colours. Both colour and spectral class relate directly to surface temperature, so the two astronomers were really doing the same thing, although quite independently of each other. The type of diagram in question is therefore known by the names of two astronomers who never actually worked together but are forever linked in the “Hertzsprung-Russell diagram” (or “H-R diagram” as it is often referred to as).

The x axis of an H-R diagram is of the spectral classes from O to M (left to right), which is also of temperatures from high to low. The y axis is of absolute visual magnitude, brightest at the top, and dimmest at the bottom. A hot, bright star will therefore be plotted towards the top left of the diagram and a cool, dim star towards the bottom right.

An H-R diagram can be plotted for any population of stars that is desired. Hertzsprung was interested in plotting all the members of a star cluster, but it can also be used to plot stars seen in a particular area of the night sky, for example, or the named stars that are visible with the naked eye.

What becomes apparent when plotting virtually any group of stars is that some parts of the diagram are far better populated than others. It is noticeable that a band of stars appears in a progression from top left to bottom right, with other parts having far fewer members. This band has become known as the “main sequence”, and the Sun features at just about the mid-point of the sequence, being averagely bright and hot when compared with most of the stars that are easily visible from Earth. The H-R diagram can therefore be read as a means of comparing the Sun with any other star on the diagram.

Two particular classes of star lie outside the main sequence and are of interest in their own right. Towards the top right of the diagram are a number of highly luminous but relatively cool stars. They gain their high absolute magnitude from their large surface area, as they constitute the red giants and supergiants. Red giants are anything between 10 and 100 times larger than the Sun, but much cooler. Supergiants are much rarer than giants, but include colossal stars such as Betelgeuse in the constellation Orion which, were it in the same place as the Sun, would occupy all the space out to the orbit of Mars and beyond.

To the left of the main sequence is a group of stars known as white dwarves. These are hotter than most main sequence stars but not as bright. They are therefore not visible to the naked eye.

One important lesson to be learned from an H-R diagram is that stars are not distributed uniformly across it but there are whole areas where stars are absent (or extremely rare). This is because of the second very useful feature of these diagrams, which is that they demonstrate the evolution of stars; the position of a star on an H-R diagram can tell the user a lot about how old or young it is and the likely fate that will eventually befall it.

For example, a supergiant like Betelgeuse is quite likely to explode as a supernova, and a white dwarf is the remnant of a star that has burned nearly all its fuel but never had sufficient mass to become a supernova.

A main sequence star is “safe” for the immediate future, but, when its hydrogen fuel is exhausted it is likely to move out of the main sequence into the red giant area as it becomes cooler but not necessarily less luminous. The next stage is for it to throw off its outer layers to leave a dense and extremely hot central core, so it will move across the H-R diagram into the hotter and less bright zone, thus becoming a white dwarf.

The H-R diagram is a very useful tool when particular areas of the sky are being surveyed, because comparisons can then be made with the “standard” H-R diagram that includes the Sun. For example, H-R diagrams of globular clusters reveal them to be composed of older stars, many of them in the red giant stage, and with the upper end of the main sequence missing due to the lack of very hot stars.

© John Welford