Astronomical Photometry

a branch of practical astrophysics, which develops and studies methods of measuring the brightness of stars as well as the brightness of extended celestial objects. Astronomical photometry originated in the second century B.C, when Hipparchus, on the basis of naked-eye estimates of their brightness, assigned the visible stars to one of six classes of stellar magnitude. As exact measurements have shown, stellar magnitudes express the subjective perception of the brightness of stars, corresponding to the logarithm of the objective stimulation of the eye retina by the light from the star. The range of five stellar magnitudes has been found to correspond to a luminosity ratio of 100.

The introduction of photometers into practice in the middle of the 19th century made it possible to determine the brightness ratios of stars and thereby to calculate more exact values of their stellar magnitudes. The zero point in the range of stellar magnitudes was selected to match the ancient ones, so that the stellar magnitudes of our time would approximately coincide with those of Hipparchus. Extensive catalogs containing visual stellar magnitudes of all the stars visible to the unaided eye were compiled in the 19th and early 20th centuries.

Astronomical photometric work based on measurements of the effect of stellar light on photographic emulsions began in the 20th century. A system of photographic stellar magnitudes differing from the system of visual magnitudes as well as a system of photovisual magnitudes (photography on an isochromatic emulsion through a yellow filter) almost coinciding with the visual system were constructed. The zero point of the new systems was assigned so that the visual photographic magnitudes and the photovisual stellar magnitudes would coincide for white stars belonging to spectral class A0.

The precision of photometric measurements increases by almost a whole order of magnitude when a photocathode is used as a sensor of stellar radiation. The electrophotometric method has not yet led to the compilation of photometric catalogs of all the stars visible to the unaided eye. However, it made it possible to construct a series of photometric standards for many regions of the sky, predominantly those containing star clusters with stars to the 21st magnitude. The electrophotometric method has made it possible to extend the system of stellar magnitudes to such extended objects as nebulas and comets by comparing the luminous flux emanating from them and from the stars. The principal source of errors in astronomical photometry is the earth’s atmosphere with its changing transparency and turbulent motions causing fluctuations in the light of the stars.