When we look up into the night sky the stars look much the same. Some stars appear brighter than others but they all look white. If the stars are looked at through a pair of binoculars some appear to be different colours. Many stars look quite orange in colour and some have a blue / white tint. The colour of a star depends on its surface temperature and indirectly on there size.
Stars vary in colour ranging from orange through true white to blue. Colour is caused by the temperature of the surface of the star. Our Sun is classified as a yellow dwarf and is very much an average mid range star with a surface temperature of about 6000ºC.
Blue or even blue/green stars are very hot up to 50,000ºC and are normally giant young stars which are very active or even ‘hyper-active’. They are all young because they don’t live very long. They live very active short lives and die young usually in a massive explosion called a Super Nova.
White to blue-white stars are bigger, more powerful and hotter than our Sun with surface temperatures up to 12,000ºC and generally live longer than blue stars but not as long as our Sun will.
Yellow stars are normally from twice the mass (mass is the stuff they are made of) down to about one tenth of the mass of our sun. These stars are in their mid life and are normal average stars.
Orange or Red stars have used up most of their Hydrogen fuel and are approaching the end of their lives. Because the fuel (hydrogen burning - fusion) is running out, the waste product Helium has built up in the centre of the star. Helium can only burn in large stars so the nuclear furnace has developed into a shell above the Helium central core. As there is not so much of the star left above the nuclear furnace shell there is less gravitational force to hold in the outer regions. The nuclear reaction will actually become more fierce and the outer part of the star gets pushed out by the increased radiation and blows up like a balloon. The star develops into a huge Red Giant with a diameter large enough to engulf all the planets in our solar system out to Mars, if it was at the position of our Sun. With the heat spread over this huge surface area the surface will become quite cool for a star, between 2500 ºC and 4000 ºC.
Another type of red star is the Red Dwarf. This is a very small star perhaps as small as one hundredth of the mass of our Sun. Red Dwarfs are small and dim and are therefore very hard to see even in large telescopes, only the closest to us are visible.
GREEN / WHITE Type W & O 36000+oC
Giant very hot active stars
BLUE Type B 28600+ oC
Very hot Helium stars
WHITE Type A 10700+ oC
Large hot stars
YELLOW / WHITE Type F 7500+ oC
Stars larger & brighter than Sun
YELLOW Type G 6000+ oC
ORANGE Type K 4800+ oC
ORANGE / RED Type M 3400+ oC
Old dying stars
RED Type N & S 2500+ oC
Cool Carbon stars
There are other types stars besides the normal ones which are just hotter than other ordinary stars. Some vary in brightness, some stars blow off great clouds of gas and dust, some burn Helium or Carbon instead of Hydrogen.
When medium sized stars like our Sun are very young they tend to be very active and hot. Giant stars burn their fuel very fast and are also very hot both the types of star shine white or blue. Old stars become bloated into giants so the heat they produce and spread over a large surface area so they appear cooler and shine with a red colour much like when an electric fire element is cooling down. Very small stars don’t produce so much heat so they appear red and cooler.