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The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature. 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).
The spectral type is not a numerical quantity, but the sequence of spectral types is a monotonic series that reflects the stellar surface temperature. Modern observational versions of the chart replace spectral type by a color index (in diagrams made in the middle of the 20th Century, most often the B-V color) of the stars.
A G-type main-sequence star (spectral type: G-V), also often, and imprecisely, called a yellow dwarf, or G star, is a main-sequence star (luminosity class V) of spectral type G. Such a star has about 0.9 to 1.1 solar masses and an effective temperature between about 5,300 and 6,000 K (5,000 and 5,700 °C; 9,100 and 10,000 °F).
It is one of the largest white 'A' spectral type stars known. Deneb is a bluish-white star of spectral type A2Ia, classifying it as a blue supergiant star [31] with a surface temperature of 8,500 kelvin. Since 1943, its spectrum has served as one of the stable references by which other stars are classified. [5] Its mass is estimated at 19 M ☉.
Blue giant is not a strictly defined term and it is applied to a wide variety of different types of stars. They have in common a moderate increase in size and luminosity compared to main-sequence stars of the same mass or temperature, and are hot enough to be called blue, meaning spectral class O, B, and sometimes early A.
The MK spectral types of the two stars have been measured a number of times, and they are both consistently assigned a luminosity class of III indicating a giant star. [69] The composite spectrum appears to be dominated by the primary star due to its sharper absorption lines ; the lines from the secondary are broadened and blurred by its rapid ...
This was based on the relative strengths of certain spectral lines understood to be sensitive to the luminosity of a star. [47] In the Bright Star Catalogue 5th edition it is given the spectral class F0II, the luminosity class indicating a bright giant. [48] Balmer line profiles and oxygen line strengths indicate the size and luminosity of ...
O class main sequence stars are already highly luminous. The giant phase for such stars is a brief phase of slightly increased size and luminosity before developing a supergiant spectral luminosity class. Type O giants may be more than a hundred thousand times as luminous as the sun, brighter than many supergiants.