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Stars can be classified according to their colour and a star’s colour is related to its surface temperature. The colours are on a scale from blue to red where the hottest stars appear blue. The colour changes from blue to white to yellow to orange, as temperature decreases, with the coolest stars appearing red.
These colours are categorised into the classes O, B, A, F, G, K and M as shown in the table below:
| Class | Surface temperature | Colour | Mass compared to the Sun | Luminosity compared to the Sun |
| O | 30,000K+ | Blue | 16 | 30,000+ |
| B | 10,000–30,000K | Blue-white | 2.1–16 | 25,000–30,000 |
| A | 7,500–10,000K | White | 1.4–2.1 | 5–25 |
| F | 6,000–7,500K | Yellow-white | 1.04–1.4 | 1.5–5 |
| G | 5,200–6,000K | Yellow | 0.8–1.04 | 0.6–1.5 |
| K | 3,700–5,200K | Orange | 0.45–0.8 | 0.08–0.6 |
| M | 2,400–3,700K | Red | 0.08–0.45 | Less than 0.08 |
From the table you can see that there is a relationship between the mass of the star and its surface temperature. The greater the star’s mass, the higher the surface temperature, and the smaller the mass of the star, the lower its surface temperature. There is also a direct relationship between the surface temperature and the luminosity of the star. Luminosity is a measure of the amount of light emitted by an object per second. The higher the surface temperature of the star, the higher the luminosity. This information can be plotted on a Hertzsprung-Russell diagram (HR diagram).
Hertzsprung-Russell diagrams
A Hertzsprung-Russell diagram (HR diagram) plots the luminosities of stars on the axis against their surface temperature on the axis, and allows us to see a trend between the two values.
A typical HR diagram is shown below:
When looking at the HR diagram we can see that there are three main regions.
The main sequence stars are shown in a diagonal band going from a high luminosity and high temperature to low luminosity and low temperatures. The hotter white dwarf stars are shown towards the bottom left of the diagram and the cooler red giants and super giants are shown at the top right. These three specific areas show stars in different stages of their life cycles and show how their surface temperature and luminosities change from one stage to another.
The HR diagram also shows how the temperature and colour of a star is related along the axis. When a star is hotter (the further to the left) then the colour is closer to blue but turns to red when the star is cooler.
Absolute magnitude
There are two ways in which the brightness of a star can be described. We can use the apparent magnitude, which is a measurement of the brightness of the star seen by someone on Earth.
However, this is not an accurate method as the brightness of a star seen from Earth can vary depending on how far away the star is from Earth and the amount of light it emits. Imagine you can see two stars in the sky. One may be very bright and one may be dull. However, this does not necessarily mean that the bright star has a higher luminosity. It may be that the dull one is much further away. Since some stars are very far away, their luminosity as it appears from Earth will be different to the true value.
The second way in which a star’s brightness can be measured is through the use of absolute magnitude. Absolute magnitude is the luminosity of a star as if it was measured at a point which is a standard distance from the Earth. The distance that absolute magnitude is measured from is equal for all stars, which means that the brightness will only depend on the luminosity of the star. This provides a much fairer and more accurate comparison of the brightness of different stars which only depends on the luminosity of the star. This means that the absolute magnitude of two stars can be compared to give a true representation of the energy each one gives off rather than just what we see from our position in the universe.
The standard distance used for absolute magnitude is 32.6 light years (3 x 1017m). This value can be used in calculations to measure a star’s luminosity. This calculation uses the inverse square law and assumes that if the distance between Earth and the star is doubled, the star will appear four times less bright.
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