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Here's a famous astronomical mnemonic for remembering the descending order of classification of stars. The actual classification is a spectral one, but some also call it the "temperature sequence" or "temperature type" as the spectrum of each star is linked to its surface temperature:

In the spectral class system, our own Sun is a class G star.

Stars vary greatly in colour, the contrast between blue-white Sirius (the Dog Star) and red Betelgeuse (in Orion) being very marked for instance, and a star's colour is usually a measure of its surface temperature. However stars are not classed by colour or temperature but by spectrum - they are arranged into spectral classes that reflect the steady change in the strengths of their representative spectral absorbtion lines.

The long-established basic phrase "Oh, Be A Fine Girl - Kiss Me!" (representing the 7 principal types) has helped several generations of astronomers to learn the stellar classifications. Ironically the mnemonic, still used today, refers to an ordered system developed by a woman - Annie Jump Cannon (1863-1941). She greatly simplified all earlier classification systems by applying an earlier "arbitrary" division of stars into spectral classes O, B, A, F, G, K, M etc. [ Her "eye" for stellar spectra was phenomenal, and her Draper catalogs (which ultimately listed nearly 400,000 stars) were highly valued as the work of a single observer.

Her classification letters have led students to create many alternative mnemonics for fun, including:

Nearly all star information is derived from studying the light they emit. Through their spectra we obtain information about their compositions, temperatures, motions and ages. Most information comes from studying the absorption lines (which appear as dark strips) in a star's spectrum. Absorption lines represent specific wavelengths in a spectrum which have been eliminated.

Patterns of absorption lines were first observed in the spectrum of our sun in the 1800's by Joseph von Fraunhofer. Later Secci and Pickering noted that stellar spectra could be divided into about 22 groups according to similarities in their spectral lines, and classified the groups alphabetically by their strength of hydrogen lines (A being strongest and Q being weakest). Annie Jump Cannon then refined the groupings into the order O, B, A, F, G, K and M used ever since. Each of these classes are nowadays divided into ten subclasses. Thus the spectral types are: O0,O1,O2,... to O7,O8,O9, then B0,B1,B2... etc. The Sun is classified as a G2 star.

In the 1930's and 1940's it was realized that the factor most responsible for determining the spectral properties of a star is its surface temperature. The surface temperature determines what ions are present. For example, type O stars which have a temperature of around 40,000°K will have a large amount of ionized helium. Radiation of wavelengths x and y is absorbed by ionized helium. Therefore in O type stars dark absorption lines characteristically exist at these wavelengths. M stars are cool stars, having temperatures of around 3000°K. with different absorption lines created by the absorption by molecules in stellar atmospheres. These lines are absent in hotter stars since molecules are broken apart at high temperatures.

A further refinement of the classification was introduced in the 1940's and 1950s when it was determined that stars of the same temperatures but having different sizes will show subtle differences in their spectrum and thus a different luminosity. This refinement introduced a Roman numeral to the end of the spectral type to indicate the luminosity class. The numeral I indicates a supergiant star; II indicates a bright giant; III indicates a giant; IV indicates a subgiant star; and V indicates a main sequence star. Our Sun, a main sequence star, is a G2V.

A separate mnemonic exists for recalling the ten brightest stars in the Earth sky.