firstmagnitude

First magnitude is a historic and still-in-use term in astronomy referring to the brightest category in the apparent magnitude scale, which measures how bright an object appears from Earth. In this inverted scale, smaller numbers denote brighter objects, and a difference of five magnitudes corresponds to a brightness ratio of 100. The first magnitude encompasses the brightest naked-eye stars, typically those with apparent magnitudes around zero or brighter, and often extending up to about +1 under ordinary observing conditions. Objects with higher magnitude numbers (second, third, etc.) appear progressively fainter, with sixth magnitude marking the faint limit of naked-eye visibility.

The magnitude scale was formalized by Norman Pogson in 1856. He defined that a change of one magnitude corresponds to a brightness factor of approximately 2.512, so a difference of five magnitudes equals exactly 100 in brightness. This logarithmic relation remains a core aspect of how astronomers compare stellar brightness, now applied to a wide range of celestial objects beyond stars.

Notable first-magnitude stars include Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Betelgeuse, and Rigel. These stars are among the brightest visible from Earth under typical conditions, and they serve as common reference points in stellar catalogs and sky observations.

In modern practice, the term is largely historical, with preference given to the broader concepts of apparent magnitude and naked-eye visibility. The distinction between first magnitude and fainter classes remains a useful historical framework for describing the brightness hierarchy of stars as seen from Earth, as well as for understanding ancient and pre-modern astronomical observations.