Up Studying Stars Star Layers Magnitude The Dippers Closest Stars Star Colors Star Life Temperature HOME    Stars are classed in different letter groups. This type of classifying was developed at the Harvard University and is known as the Harvard Classification. This system uses letters of the alphabet to group the stars by their color and temperature. The system was developed during the 1890’s through the 1900’s time period.  When the system first started out, the groups were lettered A through Q.  Over the years, however, some groups were dropped, some were joined together, and some were reordered.  

    The final classification that we use today includes only seven of the original seventeen spectral classes (or types). They are O, B, A, F, G, K, and M.  The 'O' class is the hottest, containing the very hottest of the blue stars. The 'B' class is the next hottest, containing hot blue stars.  The 'A' class has the blue to blue-white stars in it.  The 'F' class has all of the white stars in it.  The 'G' class has the yellow stars in it, such as the Sun.  The 'K' class has orange to red stars in it. 'M' is the last class, and it has the coolest red stars in it.