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).

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".[3]

Naked-eye magnitude system

During a series of lectures given in 1736 at the University of Oxford, its then Professor of Astronomy explainedː[4]

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 [...]

Distribution on 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 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

(18 of them visible in Hipparchos' Greece)
  V Mag.
(m) 		Bayer designation Proper name Distance (ly) Spectral class SIMBAD
1 0.001−1.46[5] α CMa Sirius 0008.6 A1 V Sirius A
2 0.003−0.74[6] α Car Canopus 0310 F0 Ia Canopus
3 0.004−0.27 α Cen AB (α1,2 Cen) Rigil Kent, Toliman 0004.4 G2 V/K1 V Alpha Centauri
4 0.005−0.05 var[7] α Boo Arcturus 0037 K1.5 III Arcturus
5 0.03 α Lyr Vega 0025 A0 V Vega
6 0.08 α Aur Capella 0042 K0 III, G1 III Capella
7 0.12 β Ori Rigel 0860 B8 Iab Rigel
8 0.34 α CMi Procyon 0011 F5 IV-V Procyon
9 0.42 var α Ori Betelgeuse 0640 M2 Iab Betelgeuse
10 0.50 α Eri Achernar 0140 B3 Vpe Achernar
11 0.60 β Cen Agena, Hadar 0350 B1 III Hadar (Agena)
12 0.77 α Aql Altair 0017 A7 V Altair
13 0.77 α Cru Acrux 0320 B1 V Acrux A
14 0.85 var α Tau Aldebaran 0065 K5 III Aldebaran
15 1.04 α Vir Spica 0260 B1 III-IV, B2 V Spica
16 1.09 var α Sco Antares 0600 M1.5 Iab-b Antares
17 1.15 β Gem Pollux 0034 K0 IIIb Pollux
18 1.16 α PsA Fomalhaut 0025 A3 V Fomalhaut
19 1.25 α Cyg Deneb 2,600 A2 Ia Deneb
20 1.30 β Cru Mimosa, Becrux 0350 B0.5 IV Mimosa
21 1.39 α Leo Regulus 0077 B8 IV Regulus
22 1.50 ε CMa Adhara 0430 B2 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, Aldebaran, Alcyone and Mirfak as the brightest stars of the latter four).

See also

Literature

References

  1. ^ "First Magnitude Stars". stargazing.net. Retrieved 2017-08-31.
  2. ^ Learning the First-Magnitude Stars. Campbell, Frederick. Journal: Popular Astronomy, 1917. Vol. 25, p.245
  3. ^ Alan MacRobert (1 August 2006). "The Stellar Magnitude System". skyandtelescope.com. Sky & Telescope. Retrieved 31 August 2017.
  4. ^ Keill, John (1739). An Introduction to the True Astronomy, 3rd ed. London: Henry Lintot. p. 47.
  5. ^ Hoffleit, D.; Warren, Jr., W. H. (1991). "Entry for HR 2491". Bright Star Catalogue, 5th Revised Ed. (Preliminary Version). CDS.((cite web)): CS1 maint: multiple names: authors list (link) ID V/50.
  6. ^ Ducati, J. R. (2002). "Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D. Vizier catalog entry
  7. ^ Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.
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