Precision measurement of the brightness, colour, and spectrum of stars and other celestial objects to obtain data on their structure, temperature, and composition. About 130 BC Hipparchus used a system that divided the stars into six magnitudes, from brightest to faintest. Beginning in the 17th century, use of the telescope led to the discovery of many fainter stars, and the scale was extended. The use of photographic and, since the 1940s, photoelectric equipment has vastly extended the sensitivity and wavelength range of astronomical photometry. The main (UBVRI) classification system uses wave bands in the ultraviolet, blue, visual, red, and infrared ranges. More elaborate systems can distinguish giant and dwarf stars, detect metals in stars, and determine surface gravity.
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