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In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. The lower the color index, the more blue (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is.
The apparent magnitudes of stars in the system are often used to determine the color indices B−V and U−B, the difference between the B and V magnitudes and the U and B magnitudes respectively. [1] The system is defined using a set of color optical filters in combination with an RMA 1P21 photomultiplier tube. [2]
Effective temperature of a black body compared with the B−V and U−B color index of main sequence and supergiant stars in what is called a color-color diagram. [1] Stars emit less ultraviolet radiation than a black body with the same B−V index. Although stars are not perfect blackbodies, to first order the spectra of light emitted by stars ...
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. This type of diagram is what is often called an observational Hertzsprung–Russell diagram, or specifically a color–magnitude diagram (CMD), and it is often used by ...
Main-sequence stars vary in surface temperature from approximately 2,000 to 50,000 K, whereas more-evolved stars – in particular, newly-formed white dwarfs – can have surface temperatures above 100,000 K. [3] Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest.
U−B color index: −1.06 [2] B−V color index: −0.28 [2] Variable type: Suspected [4] Astrometry; ... "On the origin of the O and B-type stars with high ...
A-type star In the Harvard spectral classification system, a class of main-sequence star having spectra dominated by Balmer absorption lines of hydrogen. Stars of spectral class A are typically blue-white or white in color, measure between 1.4 and 2.1 times the mass of the Sun, and have surface temperatures of 7,600–10,000 kelvin.
This was done so the color index of these stars would be 0. [13] Although this system is often called "Vega normalized", Vega is slightly dimmer than the six-star average used to define magnitude 0.0, meaning Vega's magnitude is normalized to 0.03 by definition.