spectrum

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    The SPECTRAL CLASS of a star commonly found in astronomical documents is a designation describing the temperature in its chromosphere, analysed by splitting its light into a rainbow of colors and absorption lines. Each color or line indicates an ion of an element, a "fingerprint" of the chemical composition of the chromosphere.

     Most stars are classified as "O", "B", "A", "F", "G", "K" and "M", based on their temperature signature. "O" stars are the hottest (blue) and "M" class the coolest (red). Originally, the sequence used all letters from A to O in order, but when star temperatures became better understood, the classes were reordered to the sequence now used.

     To better define classifications, the Morgan-Keenan (MK) system appends a number from 0 to 9 to each class, indicating tenths of the range between two star classes. "A5" is five tenths between "A" and "F", and "G2" is two tenths of the range from "G" to "K".

     Another refinement is a luminosity class expressed by the Roman numbers "I", "II", "III", "IV" and "V". This feature is a rough measure of the size of the star, proportinal to its total luminosity output. Class "I" are called supergiants, class "III" giants and class "V" are dwarfs or main sequence stars.

     For example our Sun has the spectral type "G2V", which might be interpreted as a "yellow/white, two tenths towards orange, main sequence (dwarf) star".

Class "O" stars are very hot (>30,000 K)and luminous (>1,000,000 Suns). These stars display absorption and emission lines for He-II, ionized Si-IV, O-III, N-III, and C-III, and neutral helium. They are massive, have very hot cores, thus burn through their hydrogen fuel very quickly are the first stars to leave the main sequence. Less than 0.3% of stars are class "O".
Examples are Zeta Orionis, Zeta Puppis, Lambda Orionis, and Delta Orionis

Class "B" (10,000-30,000 K) stars are extremely luminous and blue, with neutral helium and moderate hydrogen spectral lines. Ionized metal lines include Mg-II and Si-II. Like class "O", "B" are very short lived and tend to cluster in OB associations, with giant molecular clouds. About 0.1% of main sequence stars are class "B".
Examples include Rigel, Spica, and the brighter stars of the Pleiades (M45).

Class "A" (7,500-10,000 K) stars are white or bluish-white with strong hydrogen lines and lines of ionized metals Fe-II, Mg-II, and Si-II. The presence of Ca-II lines is strengthening by A5. While representing only 0.6% of main sequence stars, they are among the more common naked eye stars.
Examples of "A" stars are Vega, Sirius, and Deneb.

Class "F" (6,000-7,500 K) stars have strengthening H and K lines of Ca-II, and metals Fe-I and Cr-I gain on ionized metal lines by late "F". Spectra are characterized by weaker hydrogen lines and ionized metals. Their color is white, about 3% of main sequence stars.
Examples are Arrakis, Canopus, and Procyon.

The most important Class "G" (5,200-6,000 K) star is our Sun, one of 8% of main sequence stars. Notable are the H and K lines of Ca-II. "G" displays weaker hydrogen lines than "F", but with the ionized and neutral metals, and a prominent spike in the G band of CH molecules.
Famous "G" stars are The Sun, Alpha Centauri A, Capella, and Tau Ceti.

Class "K" are orangish stars (3,700-5,200 K), slightly cooler than the Sun. "K" stars include both supergiants and main sequence stars with extremely weak hydrogen lines and mostly neutral metals Mn-I, Fe-I, and Si-I. About 12% of the main sequence stars are Class "K" stars.
Examples include Alpha Centauri B, Epsilon Eridani, Arcturus, and Aldebaran.

Class "M" (<3,700 K) is by far the most common class, with about 76% of all stars being "M" red dwarfs or "M" giants and supergiants, plus some hotter brown dwarfs that are above the L (infrared) spectrum. The spectrum of an "M" star shows lines of molecules and all neutral metals, but hydrogen lines are usually absent.
Betelgeuse is a "M" supergiant. "M" red dwarfs are Proxima Centauri, Barnard's star, and Gliese 581.

References used in researching information for this page
"Stellar Classification", From Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Stellar_classification

Australia Telescope Outreach and Education - Stellar Classes
http://outreach.atnf.csiro.au/education/senior/astrophysics/spectral_class.html

Spectral Class Characteristics, Data from J. C. Evans, George Mason University
http://hyperphysics.phy-astr.gsu.edu/hbase/starlog/staspe.html

"Spectra"
http://www.astro.uiuc.edu/~jkaler/sow/spectra.html

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Spectral Classes