First magnitude star

First magnitude stars are the brightest stars in the night sky. Hipparchos, in the 1st century B.C., introduced the magnitude scale. He allocated 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 1,00 mag is exactly 100 times brighter than a star of 6,00 magnitude. The scale also was extended to even brighter celestial bodies like Sirius (-1,5 mag), Venus (-4 mag) or Sun (-26,7 mag).

During a series of lectures given in 1736 at the University of Oxford, its then Professor of Astronomy explained

The fixed Stars appear to be of different Bignesses, not because they really are so, but because they are not all equally distant from us. Those that are nearest will excel in Lustre and Bigness; the more remote Stars will give a fainter Light, and appear smaller to the Eye. Hence arise the Distribution of Stars, according to their Order and Dignity, into Classes; the first Class containing those which are nearest to us, are called Stars of the first Magnitude; those that are next to them, are Stars of the second Magnitude ... and so forth, 'till we come to the Stars of the sixth Magnitude, which comprehend the smallest Stars that can be discerned with the bare Eye. For all the other Stars, which are only seen by the Help of a Telescope [...]

And even among those Stars which are reckoned of the brightest Class, there appears a Variety of Magnitude; for Sirius or Arcturus are each of them brighter than Aldebaran [...] And there are some Stars of such an intermedial Order, that the Astronomers have differed in classing of them; some putting the same Stars in one Class, others in another. For Example: The little Dog was by Tycho placed among the Stars of the second Magnitude, which Ptolemy reckoned among the Stars of the first Class [...]

In the modern scale, the 20 brightest stars of Hipparchos have magnitudes between -1,5 (Sirius) and +1,6 (Bellatrix, γ Orionis). The table below shows 22 stars brighter than +1,5 mag, but 5 of them the Greek astronomers probably didn't know for their far southern position.

Twelve of the 22 brightest stars are on the actual Northern sky, ten on Southern sky. But on the seasonal evening sky, they are unevenly distributed: In Europe and USA 12-13 stars are visible in winter, but only 6-7 in summer. Nine of the brightest winter stars are part of the Winter Hexagon or surrounded by it.

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Wikipedia