If you look at the sky in the night you will observe that some stars are brighter than others. On viewing through powerful telescopes, we not only observe a difference in their brightness, but also in their colours.
The brightness and colour of the stars, in fact, depend on their temperature. The higher the temperature, the brighter is the star.
The relationship between colour and temperature helps in determining the brightness of a star. The Stars whose colour appears to be red or orange are colder than those whose colour is yellow or green. ‘White’ stars are hotter than ‘yellow’ or ‘green’ stars, and that the ‘blue’ stars have the highest surface temperature.
The surface temperature of ‘blue’ stars is around 27,750° C or even more. The Sun is a ‘yellow’ star and as such its temperature is much less than that of the ‘blue’ stars. Its surface temperature is nearly 6,000° C. The stars which appear to be red are colder and less bright. Their surface temperature is around 1,650° C or even less. All these facts make it clear that the brightness of stars is related to their surface temperature. Since brightness is reduced by distance, stars with very high surface temperature but at a great distance from us, look less bright than those whose surface temperature is less but are nearer to the Earth.