First-magnitude star

Last updated

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.

Contents

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

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

  V Mag. Bayer designation Proper nameDistance (ly) Spectral class SIMBAD
1−1.46 [5] α CMa Sirius 8.6A1 V Sirius A
2−0.74 [6] α Car Canopus 310A9 II Canopus
3−0.27α Cen AB (α1,2 Cen) Alpha Centauri 4.4G2 V + K1 V Alpha Centauri
4−0.05 var [7] α Boo Arcturus 37K1.5 III Arcturus
50.03α Lyr Vega 25A0 V Vega
60.08α Aur Capella 42G8 III + G0 III Capella
70.12β Ori Rigel 860B8 Iab Rigel
80.34α CMi Procyon 11F5 IV-V Procyon
90.42 varα Ori Betelgeuse 640M2 Iab Betelgeuse
100.50α Eri Achernar 140B3 Vpe Achernar
110.60β Cen Hadar 350B1 III Hadar
120.77α Aql Altair 17A7 V Altair
130.77α Cru Acrux 320B1 V Acrux A
140.85 varα Tau Aldebaran 65K5 III Aldebaran
151.04α Vir Spica 260B1 III-IV + B2 V Spica
161.09 varα Sco Antares 600M1.5 Iab-b Antares
171.15β Gem Pollux 34K0 IIIb Pollux
181.16α PsA Fomalhaut 25A3 V Fomalhaut
191.25α Cyg Deneb 2,600A2 Ia Deneb
201.30β Cru Mimosa 350B0.5 IV Mimosa
211.39α Leo Regulus 77B8 IV Regulus
22ε CMa Adhara 430B2 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).

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, W. H. Jr. (1991). "Entry for HR 2491". Bright Star Catalogue (5th revised (Preliminary Version) ed.). CDS. 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.