Color Index

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 light. 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 the object is. Conversely, the larger the color index, the more red the object is. This is a consequence of the logarithmic magnitude scale, in which brighter objects have smaller magnitudes than dimmer ones. For comparison, the yellowish Sun has a B-V index of 0.656±0.005, while the bluish Rigel has B-V -0.03.Color indices of distant objects are usually affected by interstellar extinction i.e. they are redder than those of closer stars.