By using increasingly powerful telescopes, fitted with various instruments to analyse the quality of the light collected from objects such as stars and galaxies, and by making calculations based on their observations, astronomers down the centuries have been able to gather a considerable amount of information about a large number of stars and other objects.
One item of information that can be assessed is the “absolute magnitude” of a star. It is not surprising that stars that are relative close to Earth will appear brighter than those that are further away. However, it is only possible to compare the actual visual luminosity of stars if it is possible to calculate their distances and make the necessary adjustment. When this is done, astronomers can set a standard and declare the absolute, as opposed to the apparent, magnitude of every object in the night sky. The standard that is set is the magnitude that a star would have if it were at a distance of 10 parsecs (a parsec is equivalent to 3.26 light years).
Absolute magnitude is expressed as a factor of its apparent magnitude, either plus or minus. Thus the brightest objects, in absolute terms, record a figure of minus 10 and the dimmest are at plus 15. The Sun’s absolute magnitude is plus 4.8.
Astronomers are also interested in how hot stars are, and this is directly related to their colour. Thus a relatively cool star such as Betelgeuse produces light at long wavelengths, which make the star look red, and a hotter star, such as Rigel, has its intensity curve skewed towards short wavelengths, so it appears blue. By using a light-sensitive device such as a photomultiplier, and a standard set of colour filters, it is possible to arrive at an accurate assessment of a star’s colour.
By using a spectroscope it is possible to obtain a full spectrum of the light coming from a star, and the nature of the spectrum is also directly related to the surface temperature of a star. Astronomers use a spectral scale of star types which are given the letters O, B, A, F, G, K and M, with O stars being the hottest (surface temperatures above 35,000 degrees K) and M stars the coolest (at around 3,000 degrees K).
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. As shown above, 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. There 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.
Abell, G. (et al) Exploration of the Universe. 5th ed. Saunders, 1987
Kaufmann, W. Universe. 2nd ed. Freeman, 1987