magnitude

A measurement of the brightness of a star or other celestial object. On the magnitude scale, the lowest numbers refer to objects of greatest brightness. The magnitude system was initially a qualitative attempt to classify the apparent brightness of stars. The Greek astronomer Hipparchus (c. 120 BC) ranked stars on a magnitude scale from "first" for the brightest stars to "sixth" for those just detectable in a dark sky by the unaided eye. This qualitative description was standardized in the mid-nineteenth century. By this time it was understood that each arbitrary magnitude step corresponded roughly to a similar brightness ratio. (In other words, the magnitude scale is a logarithmic scale of brightness.) In 1856, N. R. Pogson proposed that a difference in magnitude of 5 should correspond to a brightness ratio of 100:1, a system that is now universally accepted. If two stars differ by one magnitude, their brightnesses differ by a factor equal to the fifth root of 100, i.e. 2.512. This number is known as Pogson's ratio. The zero point of the scale was set by assigning standard magnitudes to a small group of stars near the north celestial pole, called the North Polar Sequence. The brightness of stars as observed from the Earth, and hence their apparent magnitude, depends on both their intrinsic luminosity and their distance. Absolute magnitude is a measure of intrinsic luminosity on the magnitude scale, defined as the apparent magnitude an object would have at the arbitrary distance of ten parsecs. The magnitude of an object varies with the wavelength range of the radiation observed. Visual magnitude corresponds to the normal sensitivity of the human eye. Photographic magnitude usually refers to the response of a standard photographic emulsion, which is chiefly in the blue and violet part of the spectrum. However, different photographic materials and detectors may have very different colour responses, and quoted magnitudes should include information about the method of measurement. Bolometric magnitudes take account of all radiation, both visible and outside the visible range. Magnitudes measured over a defined wavelength range are often described as "colours". The accurate determination of such magnitudes is achieved by photometry.