Spectral class
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In astronomy one method of classifying stars is through the analysis of their absorption spectra, by this method stars are assigned a spectral class. Since the absorption spectra is dependent on the surface temperature of the star the spectral class gives an indications of the temperature of the star.
The current system of naming spectral class was adopted in 1910 and consists of a letter and a number from 0 to 9, for example the spectral class of the sun is G2. The letters used are in decreasing order of temperature
O B A F G K M
The number is used to sub-divide these with the lower the number the higher the temperature so a star classed as F4 would be hotter than a star classed as F9.
The O-type stars are commonly referred to as blue stars. They have weak He lines in their spectra due to their extremely high temperatures; most of the helium is completely ionized even at the surface of the star. The O-type is the hottest of all visible stars.
The B-type stars are closely related to the O-type, being blue-white stars. Slightly cooler, the helium at their surface is not as heavily ionized and gives strong lines in the spectra.
The A-type stars continue the cooling trend, and are considered white stars. They have strong hydrogen lines, as well as some ionized metals.
The F-type stars are cooler still, and visibly seem yellow-white in color. The hydrogen lines in these stars is noticeably weaker than in the A-type.
The G-type stars are stars similar to Earth's Sun. These are considered yellow stars. Their hydrogen lines are even weaker, and neutral metals begin to show up in the spectral lines.
The K-type stars are cooler than the Sun, and have an orange cast to their light. Thus, it is not surprising that they are considered the yellow-orange stars. Their hydrogen lines are extremely weak, while neutral metals predominate.
The M-type stars are the coolest of the mainline stars. These are the red stars, usually near the end of their lives, whether they be a red giant (M1) or a red dwarf (M9). The hydrogen has been entirely burned away, leaving only heavier elements to undergo fusion. Molecular metals, such as titanium oxide, can be found in the spectra of red stars, indicating their extremely low surface temperature.
There are other types of spectral classes that cover small, odd, or unusual stellar types. The above are the main-sequence stars. Some of the unusual stellar types include:
C-type stars are carbon stars, which have an unusually high percentage of carbon in their spectra.
D-type stars follow after many red stars; these are the white dwarfs left after the main sequence star has burned away.
L-type stars are more commonly known as brown dwarfs and lack the mass to undergo the usual hydrogen fusion process.
S-type stars are nearly identical to M-type stars, but contain zirconium oxide instead of titanium oxide.
T-type stars are also brown dwarfs, but are characterized by being even smaller and containing methane in their spectra.
W-type stars are considered to be very young, possibly in a short-lived first stage in their stellar evolution. They are characteristically very hot and bright, and usually show broad, bright lines in their spectra.
Y-type stars are theoretical brown dwarfs, smaller than even than the T-type. Jupiter is almost a Y-type star.
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