Stellar Classes - Introduction

The image to the right shows a comparison of stars from each class that are on the main sequence.  Click to enlarge. 
Type Description
0 Hyper-giant
I Luminous Supergiants
II Luminous giants
III Giants
IV Sub-Giants
V Main Sequence/Dwarf Stars
VI Sub-Dwarf
VII White Dwarf
Astronomy & Cosmology

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Stars - Stellar Classes

Stars are divided up into a series of classes (types O, B, A, F, G, K, M.  Remember as "Oh Be A Fine Girl, Kiss Me), as shown in the table on the right.  It is known as the Morgan-Keenan system.  This classification is based on their spectral characteristics; essentially indicating the temperature of the chromosphere.  There are further subdivisions within each class, indicated by a number from 0 to 9 so that a G2 star, like our sun for example, is two-tenths of the way between G0 and K0.  There are various extensions to this, some of which are shown in the lower part of the table, below the main classes; Variables, Carbon Stars etc.  The following pages are structured as the table from Type "O" down to Carbon Stars, with more information about stars of each type as well as some examples. 
WILLIAM & DEBORAH HILLYARD
Type
(Click for More Info)
General Color Surface Temperature
Type O Blue Over 25,000K
Type B Blue/
White
Between 11,000K
& 25,000K
Type A White Between 7,500K
& 11,000K
Type F Yellow/ White Between 6,000K
& 7,500K
Type G Yellow Between 5,000K
& 6,000K
Type K Orange Between 3,500K
& 5,000K
Type M Red Between 2,000K
& 3,500K
Variable Various Various
Wolf-Rayet Blue 25,000K to
over 50,000K
Types L,
T & Y
Brown/ Magenta Below 2,000K
Carbon Red Between 2,800K
& 5,100K

Luminosities:
An additional element is the luminosity which gives a reasonable indication of a star's size.  This is shown by the numerals 0 through VII as shown in the second table to the near right.  The Sun is a luminosity V star, so its full classification is  type G2V.  In fact, the luminosity scale is further subdivided, and you can find the complete luminosity scale here
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