Spectral Classification

spectral classification System of classifying the spectra of stars (see SPECTRUM). In the mid-19th century, when astronomers began to observe the brighter stars with SPECTROSCOPES, they discovered a rich variety of spectra that required some system of classification. Of several original schemes, that by Father Angelo SECCHI was the most widely adopted. His system contained five types that were based on both the stars' colours and the details within their spectra: Type I contained blue-white stars with strong hydrogen ABSORPTION LINES (for example, Sirius and Vega); Type II referred to yellow and orange stars with numerous metallic spectrum lines (the Sun, Capella, Arcturus); Type III contained orange-red stars with metallic lines and bands, now known to be caused by titanium oxide (TiO), that shaded to the blue (Betelgeuse, Antares); Type IV included deep-red stars that had dark carbon bands shaded to the red; Type V was reserved for stars with bright EMISSION LINES.

As spectroscopes improved and photography became available, better spectroscopic detail required a more comprehensive description. A system devised between 1890 and 1901 by Harvard's E.C. PICKERING and his assistants (Williamina FLEMING, Antonia MAURY and Annie J. CANNON) originally used letters from A to Q based primarily on the strengths of the hydrogen lines such that A-D belonged to Secchi's Type I, E-L to Type II, M to Type III, carbon-line N to Type IV, and O-Q to Type V.

With improved observations, several letters were dropped as unnecessary, while the work of Maury and Cannon showed better continuity of all the lines if B were to come before A and O before B. The final result was the classic spectral sequence OBAFGKM. Cannon then added for each class simple numerical subtypes from 0 to 9, such that B0 to B9 is followed by A0 to A9 and so on (the modern system beginning at O3). Cannon's classification of about 225,000 stars was published between 1918 and 1924 in the henry draper catalogue. (A later extension increased the count to 359,082.)

Based in part on the presence of emission lines, M was originally subdivided into Ma to Md and O from Oa to Oe, but these were eventually made numeric as well ('e' and 'f' are now descriptive terms for emission lines). R STARS, which are the warmer versions of class N CARBON STARS, were added in 1908. SSTARS, which are intermediate carbon stars with bands of zirconium oxide, were added in 1924.

The system was quickly seen to correlate with colour that progressed from blue to red, and thus with temperature, class O to M ranging from 50,000 to 2000 K. The development of physical theory demonstrated that the spectral sequence O to M represents an ionization and excitation sequence, not one of chemical composition. R, N and S, however, are different, containing various enhancements of carbon. Though some carbon stars are DWARF STARS (with their own classification schemes), all the R, N and S stars are GIANT STARS. Temperatures in the classes R and N (now combined into class C) characterize the track on the Hertzsprung-Russell diagram of ordinary stars evolving from mid-G to late M, whereas class S temperatures track only class M.

Improved infrared technology in the last decade of the 20th century began to turn up stars that were not classifiable on the original scheme. In 1999 class L (see L STARS) was added to account for deep-red (really infrared) stars in which the strong TiO bands of class M ( see MSTARS) were replaced by powerful absorptions of hydrides and the alkali metals, the temperatures falling from 2000 to 1500 K. TSTARS, at the end, near 1000 K, are defined by methane bands. Class L is a mixture of RED DWARFS and BROWN DWARFS, while T is reserved for low-mass brown dwarfs.

Various comments are commonly added to the basic spectral types to describe details. Dwarfs and giants were originally prefixed by 'd' and 'g' (the Sun, for example, is a dG2 star, Arcturus is gK1). A separate classification by Maury used a, b and c to describe line widths. Sharp-lined stars, class c, were later seen to be SUPERGIANTS. The descriptive is still in use: Lc stars, for example, are irregular supergiant variables. Also, 'e', 'm', 'p', 'wk', respectively, stand for 'emission', 'metallic', 'peculiar' and 'weak-line' (Sirius is an A1m star).

The HARVARD SYSTEM was not adequate to deal with the differences between dwarfs, giants and supergiants, that is, stars of different luminosities at a given temperature. In 1943 William MORGAN, Philip Childs Keenan (1908-2000) and Edith Marie Kellman (1911- ) redefined the spectral types and introduced a two-dimensional classification system. The MORGAN-KEENAN CLASSIFICATION (MK or MKK) scheme retained the decimally subdivided OBAFGKM temperature sequence. To each of the classes is added a Roman numeral that describes the luminosity of the star, I to V standing for supergiant to dwarf, the Sun (a G2 dwarf) is thus classified as G2 V. Keenan later added Arabic '0' to define super-supergiants (hypergiants). The MK system gives precise details of which ratios of spectrum line strengths are to be used to determine the spectral types, and it provides photographic examples of bright stars that act as classification standards. Summaries of both Harvard and MK classes are given in the accompanying tables.

For accurate work, finer subdivisions are necessary, such as Iab (intermediate between Ia and Ib) and IIa or lib (respectively on the bright or faint side of class II). The decimal divisions can also be more finely divided, B0.5 falling between B0 and B1. Intermediate luminosity classes are expressed by hyphenated Roman numerals, 09 IV-V falling between subgiant and giant.

The MK system is applicable only to stars of normal (that is, solar) chemical composition. Roman numeral VI for metal-poor SUBDWARFS is therefore inconsistent. WHITE DWARFS were originally classified on a pseudo-Harvard scheme, with D preceding the type: DA white dwarfs have strong hydrogen lines; DB and DO strong helium lines; and DC only a continuum. These types bear more relation to odd chemical compositions than they do to temperature. DA now refers to white dwarfs with hydrogen envelopes, DB or non-DA to the others (those with helium-rich atmospheres). A more complex scheme is now available. For stars that contain spectral abnormalities, an MK type may be assigned together with an indication of the strength of the peculiarity; for example K0III-CN3 shows that the star has anomalously strong bands of CN, and K2II-Ba5 indicates that the K2 giant is an extreme BARIUM STAR. Carbon content is also added to the spectral class, for example C2,4, where the latter number indicates band strength.

To be properly classified on the MK system, the spectrum of a star must be photographed in the blue-violet with a SPECTROGRAPH of prescribed type and with an appropriate classification dispersion. Higher dispersions can reveal lines that are not seen under low-dispersion conditions and can lead to mis-classification. It is common, however, to calibrate a different system of absorption lines in a different part of the spectrum (red, ultraviolet) against the MK standards. Such calibration is also necessary for classifications achieved with modern CCD (charge-coupled device) detectors, whose data are rendered graphically rather than photographically. A variety of quantitative and computer-aided schemes are also available. For consistency, however, all need to be calibrated against the original MK standards.

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Yellow-colored two tenths towards orange primary sequence starA neighborhood.

In every class, there are 10 neighborhoods:
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Hi there You left out and also carbon stars, Class And OBAFGKMN. All stars tend to be classified by range. Sorry, I do not really know the criteria regarding G2 to G5. Do that hyperlink: http://www.answers.com/main/ntquery?s=spectral%2

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