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The spectral classes O through M, as well as other more specialized classes discussed later, are subdivided by Arabic numerals (0–9), where 0 denotes the hottest stars of a given class. For example, A0 denotes the hottest stars in class A and A9 denotes the coolest ones.
A good example is Plaskett's star, a close binary consisting of two O type giants both over 50 M ☉, temperatures over 30,000 K, and more than 100,000 times the luminosity of the Sun (L ☉). Astronomers still differ over whether to classify at least one of the stars as a supergiant, based on subtle differences in the spectral lines.
The new class S was simply left as either S or Se depending on the existence of emission lines. It was considered that the Se stars were all LPVs and the S stars were non-variable, [6] but exceptions have since been found. For example, π 1 Gruis is now known to be a semiregular variable. [7]
Below there are lists the nearest stars separated by spectral type. The scope of the list is still restricted to the main sequence spectral types: M, K, F, G, A, B and O. It may be later expanded to other types, such as S, D or C. The Alpha Centauri star system is the closest star system to the Sun.
The MK classification assigned each star a spectral type—based on the Harvard classification—and a luminosity class. The Harvard classification had been developed by assigning a different letter to each star based on the strength of the hydrogen spectral line before the relationship between spectra and temperature was known.
υ Orionis is a main sequence star of spectral type O9.7, although it has sometimes been given the spectral type B0V; Plaskett's star, a massive binary consisting of two O-class stars in orbit around each other and also one of the most massive binaries known.
The revised Yerkes Atlas system [7] listed a dense grid of A-type dwarf spectral standard stars, but not all of these have survived to this day as standards. The "anchor points" and "dagger standards" of the MK spectral classification system among the A-type main-sequence dwarf stars, i.e. those standard stars that have remained unchanged over years and can be considered to define the system ...
The nearest example is the Taurus-Auriga T association (Tau-Aur T association), located at a distance of 140 parsecs from the Sun. [11] Other examples of T associations include the R Corona Australis T association, the Lupus T association, the Chamaeleon T association and the Velorum T association. T associations are often found in the vicinity ...