First-magnitude star

First-magnitude stars are the brightest stars in the night sky, with apparent magnitudes lower 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, Venus, the full Moon, and the Sun.

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, its then Professor of Astronomy explainedː

Distribution on the Sky

In the modern scale, the 20 brightest stars of Hipparchos have magnitudes between -1.5 and +1.6. 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.

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	Spectral class	SIMBAD
1	0.001−1.46	α CMa	Sirius	0008.6	A1 V
2	0.003−0.74	α Car	Canopus	0310	A9 II
3	0.004−0.27	α Cen AB	Alpha Centauri	0004.4	G2 V + K1 V
4	0.005−0.05 var	α Boo	Arcturus	0037	K1.5 III
5	0.03	α Lyr	Vega	0025	A0 V
6	0.08	α Aur	Capella	0042	G8 III + G0 III
7	0.12	β Ori	Rigel	0860	B8 Iab
8	0.34	α CMi	Procyon	0011	F5 IV-V
9	0.42 var	α Ori	Betelgeuse	0640	M2 Iab
10	0.50	α Eri	Achernar	0140	B3 Vpe
11	0.60	β Cen	Hadar	0350	B1 III
12	0.77	α Aql	Altair	0017	A7 V
13	0.77	α Cru	Acrux	0320	B1 V
14	0.85 var	α Tau	Aldebaran	0065	K5 III
15	1.04	α Vir	Spica	0260	B1 III-IV + B2 V
16	1.09 var	α Sco	Antares	0600	M1.5 Iab-b
17	1.15	β Gem	Pollux	0034	K0 IIIb
18	1.16	α PsA	Fomalhaut	0025	A3 V
19	1.25	α Cyg	Deneb	2,600	A2 Ia
20	1.30	β Cru	Mimosa	0350	B0.5 IV
21	1.39	α Leo	Regulus	0077	B8 IV
22	1.50	ε CMa	Adhara	0430	B2 II

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.

Literature

Jeffrey Bennett et al., 2010: Astronomie. Die kosmische Perspektive, Chapter 15.1. Pearson Studium Verlag, München,
H.Bernhard, D.Bennett, H.Rice, 1948: New Handbook of the Heavens, Chapter 5. 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