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A slash (/) means that a star is either one class or the other. A dash (-) means that the star is in between the two classes. For example, a star classified as A3-4III/IV would be in between spectral types A3 and A4, while being either a giant star or a subgiant.
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 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 ...
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 term subgiant was first used in 1930 for class G and early K stars with absolute magnitudes between +2.5 and +4. These were noted as being part of a continuum of stars between obvious main-sequence stars such as the Sun and obvious giant stars such as Aldebaran, although less numerous than either the main sequence or the giant stars.
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 ...