Star Apparent Magnitude Calculator

Apparent magnitude: m = −2.5·log₁₀(F/F_ref)

Apparent magnitude m tells you how bright something in the sky looks from Earth. The scale is logarithmic and runs backwards; it goes back to Hipparchus (~150 BC) and was put on a firm footing by Pogson (1856): m = −2.5·log₁₀(F/F_ref), where F is the observed flux and F_ref is the reference flux (Vega ≈ 0). A gap of Δm = 5 corresponds to a 100× brightness ratio, which makes Δm = 1 roughly 2.512× brighter. A handful of reference points: Sun −26.7, full Moon −12.7, Venus max −4.9, Sirius −1.46, naked-eye limit +6, Hubble limit ~+31. The catch is that smaller (and negative) numbers mean brighter objects. It feels backwards, but the ordering is a holdover from history.

Applications

Amateur astronomers lean on it when planning a night out, asking whether a given object will be visible from their site, and planetarium software like Stellarium, SkySafari and KStars uses it under the hood. In astrophysics it feeds photometry of variable stars, exoplanet transits and supernovae light curves. It also shows up in light pollution work, where the Bortle scale is tied to the naked-eye limiting magnitude, and in satellite tracking, since Starlink trains often sit around m ≈ +3 to +5 right after launch.

FAQ

Why is the scale inverted? Hipparchus called the brightest stars "first magnitude" and the faintest he could still see "sixth." When Pogson made the scale logarithmic, he kept that original ordering, so brighter still means a lower number.

Does apparent magnitude depend on color filter? It does. The V (visual), B (blue) and R (red) bands each yield their own m, while the "bolometric" magnitude rolls all wavelengths together.

How does it relate to absolute magnitude? Through the distance modulus, m − M = 5·log₁₀(d/10 pc). Move the same star around and its m changes while M stays put.