Alpha Centauri

α CentauriAlphaα Centauri ARigil KentaurusACentauriαα Cenα Centauri BAlpha Centauri A
Alpha Centauri (Latinized from α Centauri, abbreviated Alpha Cen or α Cen) is the closest star system to the Solar System at 4.37 ly from the Sun.wikipedia
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Proxima Centauri

Alpha Proximaits host starProxima
It is a triple star system, consisting of three stars: α Centauri A (officially Rigil Kentaurus ), α Centauri B (officially Toliman ), and α Centauri C (officially Proxima Centauri ).
Proxima Centauri forms a third component of the Alpha Centauri system, currently with a separation of about 12950 AU and an orbital period of 550,000 years.

Centaurus

Cena picture of himselfCentaur
Alpha Centauri A and B are Sun-like stars (Class G and K), and together they form the binary star Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus. To the naked eye, Alpha Centauri AB appears to be a single star, the brightest in the southern constellation of Centaurus.
Notable stars include Alpha Centauri, the nearest star system to the Solar System, its neighbour in the sky Beta Centauri, and V766 Centauri, one of the largest stars yet discovered.

List of brightest stars

brightest starsbrightest starone of the brightest stars
Alpha Centauri A and B are Sun-like stars (Class G and K), and together they form the binary star Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus. When considered among the individual brightest stars in the sky (excluding the Sun), it is the fourth brightest at an apparent magnitude of −0.01, being slightly fainter than Arcturus at an apparent magnitude of −0.04.
Stellar brightness is traditionally based on the apparent visual magnitude as perceived by the human eye, from the brightest stars of 1st magnitude to the faintest at 6th magnitude. Since the invention of the optical telescope and the documenting of binary stars and multiple star systems, stellar brightness could be expressed as either individual (separate) or total (combined) magnitude. The table is ordered by combined magnitude of all naked eye components appearing as if it they were single stars. Such multiple star systems are indicated by parentheses showing the individual magnitudes of component stars bright enough to make a detectable contribution. For example, the double star Alpha Centauri has the total or combined magnitude of −0.27, while its two component stars have magnitudes of +0.01 and +1.33.

K-type main-sequence star

orange dwarfKK-type star
Alpha Centauri A and B are Sun-like stars (Class G and K), and together they form the binary star Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus.
Better known examples include Alpha Centauri B (K1 V) and Epsilon Indi.

G-type main-sequence star

yellow dwarfGG-type
Alpha Centauri A and B are Sun-like stars (Class G and K), and together they form the binary star Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus.
Besides the Sun, other well-known examples of G-type main-sequence stars include Alpha Centauri A, Tau Ceti, and 51 Pegasi.

Canopus

α Carinaea first magnitude starCanopean
Alpha Centauri A and B are Sun-like stars (Class G and K), and together they form the binary star Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus.
English explorer Robert Hues brought it to the attention of European observers in his 1592 work Tractatus de Globis, along with Achernar and α Centauri, noting: "Now, therefore, there are but three Stars of the first magnitude that I could perceive in all those parts which are never seene here in England. The first of these is that bright Star in the sterne of Argo which they call Canobus. The second is in the end of Eridanus. The third is in the right foote of the Centaure."

Binary star

spectroscopic binaryeclipsing binarybinary
Alpha Centauri A and B are Sun-like stars (Class G and K), and together they form the binary star Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus.
Orbital periods can be less than an hour (for AM CVn stars), or a few days (components of Beta Lyrae), but also hundreds of thousands of years (Proxima Centauri around Alpha Centauri AB).

Sirius

SothisDog StarSirius B
Alpha Centauri A and B are Sun-like stars (Class G and K), and together they form the binary star Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus.
This proximity is the main reason for its brightness, as with other near stars such as α Centauri and in contrast to distant, highly luminous supergiants such as Canopus, Rigel or Betelgeuse.

Arcturus

ArcturiansArcturianArcturan
When considered among the individual brightest stars in the sky (excluding the Sun), it is the fourth brightest at an apparent magnitude of −0.01, being slightly fainter than Arcturus at an apparent magnitude of −0.04.
With an apparent visual magnitude of −0.05, Arcturus is the brightest star in the northern celestial hemisphere and the fourth-brightest star in the night sky, after Sirius (−1.46 apparent magnitude), Canopus (−0.72) and α Centauri (combined magnitude of −0.27).

Apparent magnitude

apparent visual magnitudemagnitudevisual magnitude
Alpha Centauri A and B are Sun-like stars (Class G and K), and together they form the binary star Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus. When considered among the individual brightest stars in the sky (excluding the Sun), it is the fourth brightest at an apparent magnitude of −0.01, being slightly fainter than Arcturus at an apparent magnitude of −0.04.

Absolute magnitude

Hbolometric magnitudeabsolute magnitude (H)
Alpha Centauri A and B have absolute magnitudes of +4.38 and +5.71, respectively.
Alpha Centauri A has a parallax of 0.742″ and an apparent magnitude

Crux

Southern CrossSouthern Cross constellationconstellation
some 4.5° west, points to the constellation Crux—the Southern Cross.
Alternatively, if a line is constructed perpendicularly between Alpha Centauri and Beta Centauri, the point where the above-mentioned line and this line intersect marks the Southern Celestial Pole.

First magnitude star

brightestfirst magnitudefirst magnitude stars
To the naked eye, Alpha Centauri AB appears to be a single star, the brightest in the southern constellation of Centaurus.

Beta Centauri

Hadarβ Cenβ Centauri
It forms the outer star of The Pointers or The Southern Pointers, so called because the line through Beta Centauri (Hadar/Agena),
A line made from the other pointer, Alpha Centauri, through Beta Centauri leads to within a few degrees of Gacrux, the star at the north end of the cross.

Thomas Henderson (astronomer)

Thomas HendersonHenderson, ThomasProfessor Thomas Henderson
The large proper motion of Alpha Centauri AB was discovered by Manuel John Johnson, observing from Saint Helena, who informed Thomas Henderson at the Royal Observatory, Cape of Good Hope of it. The parallax of Alpha Centauri was subsequently determined by Henderson from many exacting positional observations of the AB system between April 1832 and May 1833.
Thomas Henderson FRSE FRS FRAS (28 December 1798 – 23 November 1844) was a Scottish astronomer and mathematician noted for being the first person to measure the distance to Alpha Centauri, the major component of the nearest stellar system to Earth, the first to determine the parallax of a fixed star, and for being the first Astronomer Royal for Scotland.

Star system

multiple star systemmultiple systemstriple star
It is a triple star system, consisting of three stars: α Centauri A (officially Rigil Kentaurus ), α Centauri B (officially Toliman ), and α Centauri C (officially Proxima Centauri ). Alpha Centauri (Latinized from α Centauri, abbreviated Alpha Cen or α Cen) is the closest star system to the Solar System at 4.37 ly from the Sun.
Alpha Centauri is a triple star composed of a main binary yellow dwarf pair (Alpha Centauri A and Alpha Centauri B), and an outlying red dwarf, Proxima Centauri. Both A and B form a physical binary star, designated as Alpha Centauri AB, α Cen AB, or RHD 1 AB, where the AB denotes this is a binary system. The moderately eccentric orbit of the binary can make the components be as close as 11 AU or as far away as 36 AU. Proxima is much further away (~15,000 AU) from α Cen AB than they are to each other. Although this distance is still comparatively small to interstellar distances, it is still debatable whether Proxima, whose orbital period would be more than 500,000 years, is gravitationally bound to α Cen AB.

Robert T. A. Innes

Robert InnesRobert Thorburn Ayton InnesDr Robert T A Innes
Alpha Centauri C was discovered in 1915 by Robert T. A. Innes, who suggested that it be named Proxima Centaurus, later amended to Proxima Centauri.
In 1915, he found a faint star fairly close to and sharing the same large proper motion with Alpha Centauri, which until then was believed to be the closest star system to the Sun.

Alpha Crucis

Acruxα Crucisα Cru
Alpha Centauri was only the second binary star to be discovered, preceded by Alpha Crucis.
It is the southernmost first-magnitude star, 2.3 degrees more southerly than Alpha Centauri.

Solar System

outer Solar Systeminner Solar SystemSol system
Alpha Centauri (Latinized from α Centauri, abbreviated Alpha Cen or α Cen) is the closest star system to the Solar System at 4.37 ly from the Sun.
The closest is the triple star system Alpha Centauri, which is about 4.4 light-years away.

Angular diameter

apparent diameterangular sizeapparent size
As seen from Earth, Proxima Centauri is 2.2° southwest from Alpha Centauri AB, about four times the angular diameter of the Moon.
The angular diameter of the Sun is also about 250,000 times that of Alpha Centauri A (it has about the same diameter and the distance is 250,000 times as much; the Sun is 4×10 10 times as bright, corresponding to an angular diameter ratio of 200,000, so Alpha Centauri A is a little brighter per unit solid angle).

Royal Observatory, Cape of Good Hope

Royal ObservatoryCape ObservatoryRoyal Observatory at the Cape of Good Hope
The large proper motion of Alpha Centauri AB was discovered by Manuel John Johnson, observing from Saint Helena, who informed Thomas Henderson at the Royal Observatory, Cape of Good Hope of it. The parallax of Alpha Centauri was subsequently determined by Henderson from many exacting positional observations of the AB system between April 1832 and May 1833.
The second HM Astronomer, Thomas Henderson, aided by his assistant, Lieutenant William Meadows, made the first observations that led to a believable stellar parallax, namely of Alpha Centauri.

Manuel John Johnson

Manuel Johnson
The large proper motion of Alpha Centauri AB was discovered by Manuel John Johnson, observing from Saint Helena, who informed Thomas Henderson at the Royal Observatory, Cape of Good Hope of it. The parallax of Alpha Centauri was subsequently determined by Henderson from many exacting positional observations of the AB system between April 1832 and May 1833.
While comparing his results with those of Nicolas Louis de Lacaille he noted the high proper motion of Alpha Centauri and communicated these to Thomas Henderson at the Royal Observatory, Cape of Good Hope.

Stellar classification

spectral typeK-typeG-type
Alpha Centauri C, or Proxima Centauri, is a small and faint red dwarf (Class M).
There are also giant K-type stars, which range from hypergiants like RW Cephei, to giants and supergiants, such as Arcturus, whereas orange dwarfs, like Alpha Centauri B, are main-sequence stars.

Proper motion

proper motionsproper-motionhigh proper motion star
(Presently the ecliptic latitude is 43.5° South, but it has decreased by a fraction of a degree since Ptolemy's time due to proper motion.) In Ptolemy's time, Alpha Centauri was visible from Alexandria, Egypt, at 31° N, but, due to precession, its declination is now –60° 51′ South, and it can no longer be seen at that latitude.
After the Sun and the Alpha Centauri system, it is the nearest known star to Earth.

List of nearest stars and brown dwarfs

passing starsnearest starsclosest stars
Alpha Centauri (Latinized from α Centauri, abbreviated Alpha Cen or α Cen) is the closest star system to the Solar System at 4.37 ly from the Sun.