It is easy to find information about star categories O B A F G K and M, but years ago, in a children's primer (I was a child then) I came across an unelaborated reference to categories W, R, N and S. can you tell me more about the characteristics of these classes? (color and mass range and a lead more information would suffice).
The latter four letters that you have alluded to do not correspond to any star category. The main categories of stars are O,B,A,F,G,K and M. Later, astronomers detected brown dwarf stars, which have too less mass to have nuclear fusion (and so are not stars by conventional definition). These stars are classified into L and T dwarf stars. L dwarfs have temperatures in the range of 1300-2000 K while T dwarfs have temperatures less than 1300 K. Both L and T dwarfs have masses less than 0.08 times the mass of the Sun. So, the full spectral sequence is OBAFGKMLT.
In addition, there are a special class of stars called Wolf-Rayet stars (W-R stars), which have temperatures greater than 20,000 K (and may be as high as 90,000 K) and have anamalously strong and borad emission lines. These are thought to be very massive stars (> 10 times mass of the Sun) which are stripped of their outer hydrogen envelopes. W-R stars are further classified into WN, WC or WO stars depending on their composition.
This page updated on June 27, 2015