Glossary term: Magnitude
Description: In astronomy, the magnitude is a measure of how bright a celestial object is. The magnitude system used in astronomy originated in antiquity as a ranking of stars from the brightest to the least bright. That is why a smaller (or more negative) magnitude value means that the object is brighter, and a larger number means a fainter object. Hence a star with magnitude -1 is brighter than a star with magnitude 0 which in turn is brighter than a star of magnitude 1.
Magnitude has a logarithmic scale, in which a magnitude difference of five corresponds to a factor of 100 difference in the amount of energy received: a star with magnitude 10 is a hundred times dimmer than a star with magnitude 5.
There are different types of magnitude: apparent magnitude measures the apparent brightness of an object, which depends both on the object's luminosity – how much light the object emits – and on its distance from Earth.
In contrast, the absolute magnitude is the value we would obtain if the object were at a standard distance of 10 parsecs (32.6 light years) from Earth. (For reflecting objects such as asteroids, there is a different definition.)
In practice, magnitude is specified for observations through a specific filter, corresponding to an object's brightness in a given wavelength range of light. Numerous "photometric systems" for specifying filters, and corresponding magnitudes, exist. In contrast, the bolometric magnitude is a direct measure of the luminosity of an object: the total electromagnetic energy emitted in unit time. Visual magnitudes correspond to the brightness as perceived by the human eye.
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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher
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