First-magnitude stars are the brightest stars in the night sky, with apparent magnitudes lower (i.e. brighter) than +1.50.Learning the First-Magnitude Stars. Campbell, Frederick. Journal: Popular Astronomy, 1917. Vol. 25, p.245 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 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".
Naked-eye magnitude system During a series of lectures given in 1736 at the University of Oxford, John Keill, its then Professor of Astronomy explainedː
Distribution in the sky 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 magnitude, 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.
12 of the 22 brightest stars are in the actual Northern sky, with the other 10 in the Southern sky. However, in the seasonal evening sky, they are unevenly distributed: In Europe and the United States, 12–13 stars are visible in winter, but only 6–7 in summer. 9 of the brightest winter stars are part of the Winter Hexagon or surrounded by it.
Table of the 22 first-magnitude stars Of the 22 1st-magnitude stars, only 18 of them were visible in Hipparchos' Greece. V Mag. Bayer designation Proper name Distance (ly) Spectral class SIMBAD 1 0.001−1.46 ID V/50. α CMa Sirius 0008.6 A1 V Sirius A 2 0.003−0.74 Vizier catalog entry α Car Canopus 0310 A9 II Canopus 3 0.004−0.27α Cen AB (α1,2 Cen) Alpha Centauri 0004.4G2 V + K1 V Alpha Centauri 4 0.005−0.05 var α Boo Arcturus 0037K1.5 III Arcturus 5 0.03 α Lyr Vega 0025A0 V Vega 6 0.08 α Aur Capella 0042G8 III + G0 III Capella 7 0.12 β Ori Rigel 0860B8 Iab Rigel 8 0.34 α CMi Procyon 0011F5 IV-V Procyon 9 0.42 var α Ori Betelgeuse 0640 M2 Iab Betelgeuse 10 0.50 α Eri Achernar 0140B3 Vpe Achernar 11 0.60 β Cen Hadar 0350B1 III Hadar 12 0.77 α Aql Altair 0017A7 V Altair 13 0.77 α Cru Acrux 0320B1 V Acrux A 14 0.85 var α Tau Aldebaran 0065K5 III Aldebaran 15 1.04 α Vir Spica 0260B1 III-IV + B2 V Spica 16 1.09 var α Sco Antares 0600M1.5 Iab-b Antares 17 1.15 β Gem Pollux 0034K0 IIIb Pollux 18 1.16 α PsA Fomalhaut 0025A3 V Fomalhaut 19 1.25 α Cyg Deneb 2,600A2 Ia Deneb 20 1.30 β Cru Mimosa 0350B0.5 IV Mimosa 21 1.39 α Leo Regulus 0077B8 IV Regulus 22 1.50ε CMa Adhara 0430B2 II Adhara
First-magnitude deep-sky objects 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, respectively).
See also Absolute magnitude List of brightest stars
Literature Jeffrey Bennett et al., 2010: Astronomie. Die kosmische Perspektive (Ed. Harald Lesch), Chapter 15.1 (p. 735–737). Pearson Studium Verlag, München, 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 Limited James. B Kahler, "First Magnitude: A Book of the Bright Sky". World Scientific, 2013. 239 pages. , 9789814417426
References