Spectral Classification

The classification of stars according to their spectra; each major spectral classification is given a letter, with additional numbers providing further subdivisions.

The light produced by every single star is unique. The temperature of the star, its luminosity, and its chemical composition, will all have effects on the light that it produces, and these effects can be decoded by examining the star's spectrum. We see the Sun's spectrum (albeit in a very coarse form) whenever we look at a rainbow, or use a prism to split sunlight into its constituent colours. Starlight, too, is composed of these seven colours, but the relative intensities of light along the spectrum will vary considerably from star to star.

This fact, as well as providing a wealth of information about stars themselves, provides a basis for a systematic stellar classification. At its most basic, this system consists of sorting stars according to their surface temperature. Stars are divided into a number of basic types according to this temperature (and other corresponding factors), and each type is given an identifying letter. Within each type, stars can be further categorised by grading them from 0 to 9 (so for example, an M2 star is hotter than an M7).

The peculiar ordering of the identifying letters is a matter of historical accident, and the letters themselves have no particular significance. Most of the common types have associated colours, which are themselves a consequence of the temperature of the star (hotter stars are bluer, while cooler stars tend to be redder). In decreasing order of surface temperature, the main spectral types are shown below.

W W is a very rare type of intensely hot star, with surface temperatures up to 50,000 K. There is only one example in the sky that is visible to the naked eye, in the Suhail al Muhlif system in the constellation Vela.
O O-type stars are also relatively uncommon, but far more numerous than those of type W. These are bright blue stars which also have very high surface temperatures, in the range 25,000 K to 50,000 K. Examples are Alnitak (O9.5), Naos (O5), Hatysa (O9) and Heka (O8).
B The B type is the first of the really populous classes. Stars of this type are blue in colour and burn hotly, with surface temperatures lying between 11,000 K and 25,000 K. Prominent examples of blue B-type stars are Rigel (B8), Achernar (B3), Agena (B1) and Spica (also B1).
A A-type stars are those whose surface temperatures lie in the approximate range 7,500 K to 11,000 K. They are white in colour, and some of the brightest and most famous stars in the sky belong to this classification, including Sirius (A0), Vega (A0), Altair (A7) and Deneb (A2).
F F-type stars lie between the A-type white stars and G-type 'true' yellow stars, and have a distinctly yellowish light. Their surfaces have a temperature between 6,000 K and 7,500 K. Sometimes called Calcium Stars, examples of this type include Canopus (F0), Procyon (F5), Algenib in Perseus (F5) and Wezen (F8).
G The cooler a star, the more complex its chemistry tends to be. G-type stars, with temperatures ranging between 5,000 K and 6,000 K, have spectra that betray the existence of 'metals' (in this context, 'metal' refers to any element heavier than helium). Examples of yellow G-type stars are Alpha Centauri (G2), Capella (G5), Kraz (G5) and Mufrid (G0). The Earth's Sun is a G2 star, and also belongs to this type.
K K-type stars are occasionally referred to as Arcturian Stars, after the brightest of their number. Their surface temperatures are between 3,500 K and 5,000 K, low enough for simple molecules to form. K-type stars are orange in colour, and among the brightest in the sky are Arcturus (K2), Aldebaran (K5), Pollux (K0) and Atria (K2).
M The coolest of the common star types, red stars are classified as M-type. They have very cool surface temperatures below 3,500 K, allowing more complex molecules to form. Among the brightest red stars in the sky are Betelgeuse (M2), Antares (M1), Gacrux (M4) and Mirach (M0). The Sun's nearest neighbour in space, Proxima Centauri, is also a red star, classified as M5.

Two other rare classes are the C- and S-type stars. These are cool stars that overlap the K- and M-type classifications in terms of temperature, but are placed in separate categories due to unusual chemistry within the star. Very few of these stars are visible without optical aid, though the C-type U Hydrae, and the S-type Chi Cygni are unusual exceptions.

A star's full spectral classification often also includes a 'luminosity class', a Roman numeral from I to VII indicating the star's luminosity, which correlates with its mass. The luminosity class is simply appended to the spectral class. So, for example, the Sun's full spectral classification, including its luminosity class, is G2V. The seven luminosity classes are listed below.

I Supergiants: extremely massive and luminous stars, usually nearing the end of their lifespan. They are subclassified as Ia or Ib, with Ia representing the most luminous stars of all. Examples include Rigel (B8Ia), Betelgeuse (M2Ib) and Antares (M1Ib).
II Bright Giants: a relatively uncommon group of giant stars that are particularly luminous, and can be a thousand times more so than the Sun, or more. Examples include Adara (B2II), Sargas (F1II) and Kraz (G5II).
III Normal Giants: the giant stars in this category are typically a hundred times more luminous than Earth's Sun, and considerably more massive. Examples of this populous group include Arcturus (K2III), Agena (B1III) and Aldebaran (K5III).
IV Subgiants: though still far more massive and luminous than the Sun, subgiants fall short of the true giants. Examples include Acrux (B0.5IV), Shaula (B1.5IV) and Miaplacidus (A2IV).
V Dwarfs: a very numerous class of main sequence stars, whose mass and luminosity is generally comparable with that of the Sun. Examples include Sirius (A0V), Alpha Centauri (G2V) and Vega (A0V).
VI & VII These classes designate subdwarfs and white dwarfs, respectively. They are not now in common use, but are included here for completeness.

In addition to the luminosity class, spectral classifications sometimes also carry commentary additions, usually lower case letters added before or after the main type. For example, the full spectral classification for Achernar is B3Vp, with 'p' indicating that it has a peculiar spectrum, while Castor in Gemini is classified as A2Vm, with 'm' demonstrating that the spectrum contains strong metal lines, and so on. These prefixes and suffixes lie rather outside the range of this brief article.

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