First-magnitude stars are the brightest stars in the night sky, with apparent magnitudes lower (i.e. brighter) than +1.50.[1][2] Hipparchus, in the 1st century BC, introduced the magnitude scale. He allocated the first magnitude to the 20 brightest stars and the sixth magnitude to the faintest stars visible to the naked eye.
In the 19th century, this ancient scale of apparent magnitude was logarithmically defined, so that a star of magnitude 1.00 is exactly 100 times as bright as one of 6.00. The scale was also extended to even brighter celestial bodies such as Sirius (-1.5), Venus (-4), the full Moon (-12.7), and the Sun (-26.7).