Apparent magnitude

Apparent magnitude is a logarithmic measure of a celestial object's brightness as observed from Earth. It depends on the object's emitted light, distance, and intervening absorption.

Pogson's ratio: A brightness ratio of about 2.512 between objects differing by one magnitude. It sets the flux change represented by each one-step difference on the scale.
Absolute magnitude: The magnitude an object would have if placed at a standard distance of ten parsecs. Unlike apparent magnitude, it removes distance as a source of brightness difference.
Variable star: A star whose apparent brightness changes over time. Repeated magnitude measurements reveal its brightness cycle and variability.
Hipparchus: An ancient Greek astronomer associated with an early system for ranking stars by brightness. His tradition of six brightness classes underlies the magnitude scale.
Astronomical magnitude: A logarithmic system for expressing the brightness of celestial objects. Apparent magnitude is the observed-brightness form of this broader system.
Luminosity: The total energy a celestial object emits per unit time. It describes intrinsic output, not the brightness received at Earth.
Cepheid variable: A pulsating star whose period of brightness variation is related to its luminosity. Its apparent magnitude and pulsation period help determine distances.
Ptolemy: An ancient Greek astronomer whose Almagest preserved a catalog of stars with brightness classes. The surviving catalog helped transmit the early magnitude classifications.
Flux (astronomy): The energy from an astronomical source received per unit area and time. Observed flux is converted into apparent magnitude through a logarithmic relation.
Distance modulus: The difference between apparent and absolute magnitude, related to an object's distance. It expresses how distance changes the observed magnitude when extinction is accounted for.

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