12/16/2023 0 Comments Star chart 3.0The Johnson–Cousins filter system, named after the originators of the system (see references). This system of filters is sometimes called The passbands most optical astronomers use are the UBVRI filters, where the U, B, and V filters are as mentioned above, the R filter passes red light, and the I filter passes infrared light. A good approximation can be obtained by considering stars as black bodies, using Ballesteros' formula (also implemented in the PyAstronomy package for Python): T = 4600 K ( 1 0.92 B - V + 1.6 ( − ) + 1 1.5 B - V + 0.62 ). In principle, the temperature of a star can be calculated directly from the B−V index, and there are several formulae to make this connection. The difference in magnitudes found with these filters is called the U−B or B−V color index respectively. The set of passbands or filters is called a photometric system. 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). Traditionally, the color index uses Vega as a zero point. For comparison, the yellowish Sun has a B−V index of 0.656 ± 0.005, whereas the bluish Rigel has a B−V of −0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B−V = −0.03). This is a consequence of the logarithmic magnitude scale, in which brighter objects have smaller (more negative) magnitudes than dimmer ones. Conversely, the larger the color index, the more red (or cooler) the object is. The lower the color index, the more blue (or hotter) the object is. 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. For the term in geology, see Color index (geology). For the colorant reference database, see Colour Index International.
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