First-magnitude stars are the brightest stars in the night sky, with

apparent magnitudes Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

lower (i.e. brighter) than +1.50.

Hipparchus Hipparchus (; el, Ἵππαρχος, ''Hipparkhos''; BC) was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the equ ...

, in the 1st century B.C., introduced the magnitude scale. He allocated 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 Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...

was logarithmically defined—so that a star of 1.00 mag is exactly 100 times brighter than a star of 6.00 magnitude. The scale also was extended to even brighter

celestial bodies An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...

Sirius Sirius is the brightest star in the night sky. Its name is derived from the Greek word , or , meaning 'glowing' or 'scorching'. The star is designated α Canis Majoris, Latinized to Alpha Canis Majoris, and abbreviated Alpha CMa ...

(-1.5 mag),

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...

(-4 mag),

full Moon The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic longitudes of the Sun and Moon differ by 180°). This mean ...

(-12.7 mag) and Sun (-26.7 mag).

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 ''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 Bellatrix is the third-brightest star in the constellation of Orion, positioned 5° west of the red supergiant Betelgeuse (Alpha Orionis). It has the Bayer designation γ Orionis, which is Latinized to Gamma Orionis. With a slightly va ...

, 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 Epsilon Canis Majoris is a binary star system and the second-brightest object in the constellation of Canis Major. Its name is a Bayer designation that is Latinised from ε Canis Majoris, and abbreviated Epsilon CMa or ε CMa. This is ...

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) ;

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 The Large Magellanic Cloud (LMC), or Nubecula Major, is a satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (≈160,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the ...

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked ey ...

Carina Nebula The Carina Nebula or Eta Carinae Nebula (catalogued as NGC 3372; also known as the Great Carina Nebula) is a large, complex area of bright and dark nebulosity in the constellation Carina, located in the Carina–Sagittarius Arm of the Milky Wa ...

, Hyades,

Pleiades The Pleiades (), also known as The Seven Sisters, Messier 45 and other names by different cultures, is an asterism and an open star cluster containing middle-aged, hot B-type stars in the north-west of the constellation Taurus. At a distance ...

and the Alpha Persei Cluster (with

Eta Carinae Eta Carinae (η Carinae, abbreviated to η Car), formerly known as Eta Argus, is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around distant in th ...

Aldebaran Aldebaran (Arabic: “The Follower”, "الدبران") is the brightest star in the zodiac constellation of Taurus. It has the Bayer designation α Tauri, which is Latinized to Alpha Tauri and abbreviated Alpha Tau or α Tau. Aldeb ...

, Alcyone and

Mirfak Alpha Persei ( Latinized from α Persei, abbreviated Alpha Per, α Per), formally named Mirfak (pronounced or ), is the brightest star in the northern constellation of Perseus, outshining the constellation's best-known sta ...

as the brightest stars of the latter four).

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

{{reflist Stellar astronomy Observational astronomy Photometry