Classification
The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line. It was not known at the time that the major influence on the line strength was temperature; the hydrogen line strength reaches a peak at over 9000 K, and is weaker at both hotter and cooler temperatures. When the classifications were reordered by temperature, it more closely resembled the modern scheme.
There are different single-letter classifications of stars according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types have special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures: class O0 and O1 stars may not exist.
In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by the surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Most stars belong to the main sequence, which consists of ordinary hydrogen-burning stars. These fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type. Our Sun is a main sequence G2V yellow dwarf, being of intermediate temperature and ordinary size.
Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.
White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature index.
Class O Stars
Class O stars are very hot and luminous, being blue in colour. Naos (in the constellation Puppis) shines with a power close to a million times solar. These stars have prominent ionized and neutral helium lines and only weak hydrogen lines. Class O stars emit most of their radiation in ultra-violet.
Class B Stars
Class B stars are again very luminous, Rigel (in the great constellation Orion) is a prominent B class blue supergiant. 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 appear clustered together in the OB associations, which are associated with giant molecular clouds. The Orion OB association is an entire spiral arm of our Galaxy.
Class A Stars
Class A stars are amongst the more common naked eye stars. Deneb in Cygnus is another very powerful star. Sirius, that appears the brightest star as seen from Earth, is also an A class star, but not nearly as powerful. As with all class A stars, they are white. Many white dwarfs are also A. They have strong hydrogen lines and also ionized metals.
Class F Stars
Class F stars are still quite powerful and they are average-sized, such as Fomalhaut in Pisces Australis. Their spectra is characterized by the weaker hydrogen lines and ionized metals, their colour is white with a slight tinge of yellow.
Class G Stars
Class G stars are probably the most well known 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.
Class K Stars
Class K are orange 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 smaller. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals.
Class M Stars
Class M is the most common class by the number of stars. All red dwarfs, such Proxima Centauri, the closest star to our Solar Sysem, go in here, and they are plentiful. M is also host to most giants and some supergiants such as Antares in Scorpio and Betelgeuse in Orion, as well as Mira variable stars. These red giants are old stars. 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.
M stars may be dwarf stars or supergiant stars, and A stars can be white dwarfs or white giants as well. However, not all combinations are possible. For example, F and G stars must be average-sized stars. This can be understood through the Hertzsprung-Russell diagram, that is very important in astrophysics and relates temperature and spectral classification of stars with their luminosity and size. While these descriptions of stellar colors are traditional in astronomy, they really describe the light as we see them from Earth, after it has been scattered by the atmosphere. The Sun is not in fact a yellow star, but has the color temperature of a body of 5780 K, that is a white with no trace of yellow which is sometimes used as a definition for standard white.