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. To measure the index, one observes the
magnitude of an object successively through two different filters, such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light (see also:
UBV system). The set of passbands or filters is called a
photometric system. The difference in magnitudes found with these filters is called the U-B or
B-V color index, respectively. The smaller 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. This is a consequence of the logarithmic magnitude scale, in which brighter objects have smaller (more negative) magnitudes than dimmer ones. For comparison, the yellowish
Sun has a B-V index of 0.656±0.005, while the blueish
Rigel has B-V -0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B-V=-0.03).
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The difference in brightness of a star, as measured at two different wavelengths. The brightness is measured using photometry through different coloured filters. These filters isolate specific wavelength ranges which can be either very narrow or a lot wider. Careful choice of filters and measurements can yield a range of information about a celestial object.
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