First-magnitude star

First-magnitude stars are the brightest stars in the night sky, with apparent magnitudes lower (i.e. brighter) than +1.50. 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).

Hipparchus ranked his stars in a very simple way. He listed the brightest stars as "of the first magnitude", which meant "the biggest." Stars less bright Hipparchus called "of the second magnitude", or second biggest. The faintest stars visible to the naked eye he called "of the sixth magnitude".

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

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.

Epsilon Canis Majoris has an apparent magnitude of almost exactly 1.5, so it may be considered a first magnitude sometimes due to minor variations.

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.

Of the 22 1st-magnitude stars, only 18 of them were visible in Hipparchos' Greece.

Beside stars there are also deep-sky objects that are first-magnitude objects, accumulatively brighter than +1.50, such as the Large Magellanic Cloud, Milky Way, Carina Nebula, Hyades, Pleiades and the Alpha Persei Cluster (with Eta Carinae, Theta Tauri, Alcyone and Mirfak as the brightest stars of the latter four).

• Absolute magnitude
• List of brightest stars

• Jeffrey Bennett et al., 2010: Astronomie. Die kosmische Perspektive (Ed. Harald Lesch), Chapter 15.1 (p. 735–737). Pearson Studium Verlag, München,ISBN 978-3-8273-7360-1
• H.Bernhard, D.Bennett, H.Rice, 1948: New Handbook of the Heavens, Chapter 5 (Stars of the Southern Sky). MaGraw-Hill, New York
• Patrick Moore, 1996: Brilliant Stars Cassell Publishers LimitedISBN 978-0-3043-4903-6
• James. B Kahler, "First Magnitude: A Book of the Bright Sky". World Scientific, 2013. 239 pages.ISBN 9814417424, 9789814417426