However, because main-sequence stars of spectral class M have such low luminosities, none are bright enough to be visible to see with the unaided eye. About 76% of the main-sequence stars in the Solar neighborhood are class M stars. There is a suggestion that K Spectrum stars may potentially increase the chances of life developing on orbiting planets that are within the habitable zone.Ĭlass M stars are by far the most common. By late K, molecular bands of titanium oxide become present. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals (Mn I, Fe I, Si I). There are also giant K-type stars, which range from hypergiants like RW Cephei, to giants and supergiants, such as Arcturus, whereas orange dwarfs, like Alpha Centauri B, are main-sequence stars. They make up about 12%, nearly one in eight, of the main-sequence stars in the solar neighborhood. Ionized calcium, neutral and ionized metalsĬlass K stars are orangish stars that are slightly cooler than the Sun. While they do this, they do not stay for long in the yellow supergiant G classification as this is an extremely unstable place for a supergiant to be. Supergiant stars often swing between O or B (blue) and K or M (red). G is host to the "Yellow Evolutionary Void". There is a prominent spike in the G band of CH molecules. They have even weaker hydrogen lines than F, but along with the ionized metals, they have neutral metals. Most notable are the H and K lines of Ca II, which are most prominent at G2. They make up about 7.5%, nearly one in thirteen, of the main-sequence stars in the solar neighborhood. Hydrogen and ionized metals, calcium and ironĬlass G stars are probably the best known, if only for the reason that the Sun is of this class. About 1 in 33 (3.03%) of the main-sequence stars in the solar neighborhood are class F stars. Their spectra are characterized by the weaker hydrogen lines and ionized metals. Neutral metals (Fe I, Cr I) beginning to gain on ionized metal lines by late F. About 1 in 160 (0.625%) of the main-sequence stars in the solar neighborhood are class A stars.Ĭlass F stars have strengthening H and K lines of Ca II. The presence of Ca II lines is notably strengthening by this point. They have strong hydrogen lines, at a maximum by A0, and also lines of ionized metals (Fe II, Mg II, Si II) at a maximum at A5. Thus, due to the low probability of kinematic interaction during their lifetime, they do not, and are unable to, stray far from the area in which they were formed.Ĭlass A stars are among the more common naked eye stars, and are white or bluish-white. ![]() As O and B stars are so powerful, they only live for a relatively short time. Ionized metal lines include Mg II and Si II. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Mintaka, Kalidar, Zeta Puppis, Eta CarinaeĬlass B stars are very luminous and blue. Class O stars frequently have complicated surroundings which make measurement of their spectra difficult Some of the most massive stars lie within this spectral class. About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the solar neighborhood are class O stars. These are the rarest of all main-sequence stars. This indicates that is a Spectral Class G star of considerable temperature, and that it is a Type V (main sequence) star.įrom Memory Alpha's Article: Stellar classification Contents Spectral Class OĬlass O stars are very hot and extremely luminous, with most of their radiated output in the ultraviolet range. Luminosity class 0 or Ia+ stars for hypergiants, class I stars for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class VI for sub-dwarfs, and class VII for white dwarfs.įor example, Earth's Sun is classified as type G2V. This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. In the MK system, a luminosity class is added to the spectral class using Roman numerals. A8, A9, F0, F1 form a sequence from hotter to cooler). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. Most stars are currently classified under the Morgan–Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type). In astronomy, stellar classification is the classification of stars based on their spectral characteristics.
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