Colours of the Stars
With the naked eye it is obvious that stars are not all the same colour. Rigel (Beta Orionis) is blue, Sirius (Alpha Canis Majoris) is white, our sun is yellowish, Aldebaran (Alpha Tauri) is orange and Betelgeuse (Alpha Orionis) is red. Stars have different colours because they have different surface temperatures. Blue stars are hotter than white stars that are hotter than yellow stars. These in turn are hotter than orange stars, which are hotter than red stars.
Each colour of star has its own type of spectrum and is classified using the letters O, B, A, F, G, K, M, which define its spectral class. Blue O-class stars have surface temperatures of over 25000 K. B-class stars have surface temperatures of 25000 to 11000 K and are bluish white, and so on down to red M-class stars which have surface temperatures below 3500 K. Each class is further divided into 10 numbered sub-classes, in which 0 is the hottest and 9 is the coolest. Our sun, with a surface temperature of around 5800 K, is a yellow G2 star.
Early in the 20th century, the Danish astronomer Ejnar Hertzsprung and the American Henry Russell plotted the positions of stars on a graph of absolute magnitude (a star's luminosity relative to others) against surface temperature. They found that stars group in some areas and are absent from other areas of the graph, known as the Hertzsprung-Russell (H-R) diagram.
Most stars fall into the "main sequence", a grouping which wends its way diagonally across the diagram, from dim red stars at the bottom right to bright blue stars at the top left. The Sun falls roughly in the middle of the main sequence. A star's position on the sequence depends on its mass, with the least massive stars at the bottom right of the diagram and the most massive at the top left, because more massive stars burn hotter. Below the main sequence is a line of dim white stars, white dwarfs; above is a group of variable stars and luminous red stars, known as giants. Across the top of the diagram are a line of extremely luminous stars, the supergiants.
The Hertzsprung-Russell diagram is a snapshot of the galaxy as we see it today; some stars on it are new and some old, but it can tell us a lot about how stars evolve. New stars appear just above the main sequence. As they settle to steady hydrogen fusion, they stabilize on the sequence, with less massive stars burning cool, red and dim and more massive ones burning hot, blue, and bright. The bluer a star on the sequence, the shorter its life.
Older stars move off the main sequence when their fusion reactions change, becoming luminous giants or supergiants. Stars in this stage are unstable and their output changes make them variables. The more massive a star is at "birth", the brighter it is at the giant stage. As a star reaches the end of its giant phase, it gets redder and moves to the right of the diagram. Supergiants eventually explode and disappear off the diagram altogether. Stars the size of our Sun end up as dim white dwarfs below the main sequence after they have ceased fusion in their cores.