[[Image:Morgan-Keenan_spectral_classification.png|left|The Morgan-Keenan spectral classification]]

'''Class O'''

Class O stars are very hot and very luminous, being bluish in colour; in fact, most of their output is in the ultraviolet range. These are the rarest of all main sequence stars, constituting as few as 1 in 32,000. O-stars shine with a power over a million times our Sun's output. These stars have prominent ionized and neutral helium lines and only weak hydrogen lines. Because they are so huge, Class O stars burn through their hydrogen fuel very quickly, and are the first stars to leave the main sequence.

'''Class B''''

Class B stars are extremely luminous and blue. Their spectra have neutral helium and moderate hydrogen lines. As O and B stars are so powerful, they live for a very short time. They do not stray far from the area in which they were formed as they don't have the time. They therefore tend to cluster together in what we call OB1 associations, which are associated with giant molecular clouds. The Orion OB1 association is an entire spiral arm of our Galaxy (brighter stars make the spiral arms look brighter, there aren't more stars there) and contains all of the constellation of Orion. They constitute about 0.13% of main sequence stars -- rare, but much more common than those of class O.

'''Class A'''

Class A stars are amongst the more common naked eye stars. As with all class A stars, they are white or bluish-white. They have strong hydrogen lines and also ionized metals. They comprise perhaps 0.63% of all main sequence stars.

'''Class F'''

Class F stars are still quite powerful but they tend to be main sequence stars. Their spectra is characterized by the weaker hydrogen lines and ionized metals, their colour is white with a slight tinge of yellow. These represent 3.1% of all main sequence stars.

'''Class G'''

Class G stars are probably the best known, if only for the reason that our Sun is of this class. They have even weaker hydrogen lines than F, but along with the ionized metals, they have neutral metals. G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be. These are about 8% of all main sequence stars.

'''Class K'''

Class K are orangish stars which are slightly cooler than our Sun. Some K stars are giants and supergiants, such as Arcturus while others like Alpha Centauri B are main sequence stars. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals. These make up some 13% of main sequence stars.

'''Class M'''

Class M is by far the most common class if we go by the number of stars. All the red dwarfs go in here and they are plentiful; over 78% of stars are red dwarfs, such as Proxima Centauri. M is also host to most giants and some supergiants such as Antares and Betelgeuse, as well as Mira variables. The spectrum of an M star shows lines belonging to molecules and neutral metals but hydrogen is usually absent. Titanium oxide can be strong in M stars. The red color is deceptive; it is because of the dimness of the star. When an equally hot object, a halogen lamp (3000 K) which is white hot is put at a few kilometers distance, it appears like a red star.