If you look at the sky in the night you will observe that some stars are brighter than the others. Have you ever wondered why this is so? On looking through a powerful telescope, we not only observe differences in their brightness, but also in their colours. The brightness and colour of a star, in fact, depend on its temperature. The higher the temperature, the brighter is the star. There is a relationship between the colour and temperature of a star, which helps us to determine its brightness. Stars that appear to be red or orange have the lowest surface temperature. Stars whose colour is yellow or green have slightly higher temperature. ‘White’ stars are hotter than ‘yellow’ or ‘green’ stars. Stars that are blue in colour 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 ‘blue’ stars. The surface temperature of a typical yellow star is approximately 6,000°C. The stars that appear to be red are cooler and fainter than yellow stars. Their surface temperature is around 1,650°C or even less. These facts show that the brightness of stars is directly related to their surface temperature. Another factor that influences the brightness of the stars is the distance. The greater the distance of the star, the fainter the star appears. Thus, stars with very high surface temperature but at a great distance from us, looks less brighter than those whose surface temperature is less but are nearer to the Earth. It is the same in the case of street lamps. Distant street lamps look fainter than the closer ones.