First-magnitude stars are the brightest stars in the

night sky The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. Natural light sources in a night sky include moonlig ...

, with

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

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

Hipparchus Hipparchus (; , ; BC) was a Ancient Greek astronomy, 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 equinoxes. Hippar ...

, in the

1st century BC The 1st century Before Christ, BC, also known as the last century BC and the last century Common Era , BCE, started on the first day of 100 BC, 100 BC and ended on the last day of 1 BC, 1 BC. The Anno Domini, AD/BC notation does not ...

, 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 Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or microsc ...

. In the 19th century, this ancient scale of apparent magnitude was

logarithm In mathematics, the logarithm of a number is the exponent by which another fixed value, the base, must be raised to produce that number. For example, the logarithm of to base is , because is to the rd power: . More generally, if , the ...

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

such as

Sirius Sirius is the brightest star in the night sky. Its name is derived from the Greek word (Latin script: ), meaning 'glowing' or 'scorching'. The star is designated Canis Majoris, Latinized to Alpha Canis Majoris, and abbr ...

(-1.5),

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

(-4), the full Moon (-12.7), and the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

(-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, 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 (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.

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) is a dwarf galaxy and satellite galaxy of the Milky Way. At a distance of around , the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal Galaxy, Sagittarius Dwarf ...

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

, Carina Nebula, Hyades,

Pleiades The Pleiades (), also known as Seven Sisters and Messier 45 (M45), is an Asterism (astronomy), asterism of an open cluster, open star cluster containing young Stellar classification#Class B, B-type stars in the northwest of the constellation Tau ...

and the

Alpha Persei Cluster The Alpha Persei Cluster, also known as Melotte 20 or Collinder 39, is an open cluster of stars in the northern constellation of Perseus (constellation), Perseus. To the naked eye, the cluster consists of several blue-hued spectral type B stars. ...

(with Eta Carinae, Theta Tauri, Alcyone and Mirfak 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