apparent magnitude

astronomy

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June 18, 2026

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In astronomy, apparent magnitude is a measure of how bright a celestial object appears from Earth. The brighter an object, the lower its magnitude number. This system, refined by Norman Robert Pogson in 1850, uses a scale where one magnitude difference equates to a brightness ratio of approximately 2.512. A difference of five magnitudes represents a 100:1 brightness ratio. Negative magnitudes are used for objects brighter than the traditional scale's brightest stars.12

For example, the Sun has an apparent magnitude of -26.7, the full Moon is around -11, and Sirius, a bright star, is -1.5. The faintest objects detectable by the Hubble Space Telescope have an apparent magnitude of about 30. This measurement is a key aspect of photometry, the science of measuring celestial brightness to understand objects' properties like temperature and distance.13

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star
Measuring starlight intensity
…expressed by means of their magnitudes, a usage inherited from classical times. A star of the first magnitude is about 2.5 times as bright as one of the second magnitude, which in turn is some 2.5 times as bright as one of the third magnitude, and so on. A star…
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…expressed by means of their magnitudes, a usage inherited from classical times. A star of the first magnitude is about 2.5 times as bright as one of the second magnitude, which in turn is some 2.5 times as bright as one of the third magnitude, and so on. A star…
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photometry
The earliest observations of the apparent brightness of the stars were made by Greek astronomers. The system used by Hipparchus about 130 bc divided the stars into classes called magnitudes; the brightest were described as being of first magnitude, the next class were second magnitude, and so on in equal…
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The earliest observations of the apparent brightness of the stars were made by Greek astronomers. The system used by Hipparchus about 130 bc divided the stars into classes called magnitudes; the brightest were described as being of first magnitude, the next class were second magnitude, and so on in equal…
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astronomical map
The constellations and other sky divisions
…stars into six brightness, or magnitude, classes. He listed 15 bright stars of the first magnitude but comparatively few of the faint, much more numerous but barely visible sixth magnitude at the other limit of his list. Al-Ṣūfī, a 10th-century Islamic astronomer, carried out the principal revision made to these…
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…stars into six brightness, or magnitude, classes. He listed 15 bright stars of the first magnitude but comparatively few of the faint, much more numerous but barely visible sixth magnitude at the other limit of his list. Al-Ṣūfī, a 10th-century Islamic astronomer, carried out the principal revision made to these…
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