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The science of the measurement of light. The earliest estimates of star brightnesses were made with the naked eye, which formed a basis for the magnitude scale used in astronomy. Now, professional astronomers use photometers to compare objects under study with standard stars. Broad-band filters are used for the faintest objects to achieve sufficient signal, while brighter stars can be measured with intermediate-band or narrow-band filters. The filters are carefully chosen to reveal characteristics of the spectrum such as absorption lines in stars and galaxies, and emission lines and bands in nebulae. Magnitudes from different filters are usually compared to obtain a colour index. Satellites have extended astronomical photometry into the ultraviolet and infrared.

Subjects: Astronomy and Astrophysics.

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