Alpha Centauri is the third brightest star in the night sky. To the naked eye it shines at apparent magnitude -0.27, which is fainter than Canopus (mag. -0.72) but brighter than Arcturus (mag. -0.04). However, Alpha Centauri is not a single star; it's a triple consisting of two bright components and a feeble red dwarf. For most of their orbit, the main stars are easily split with small telescopes. This is also the nearest star system to the Solar System.

Alpha Centauri (credit:- ESO/Digitized Sky Survey 2/Davide De Martin)

Unusual for a bright star, Alpha Centauri does not have a commonly agreed proper name. Arab astronomers called it "Rigil Kentaurus", meaning "the foot of the Centaur". A shorter form of this name, "Rigel Kent", is commonly used today. The Chinese referred to it as Nan Mun or "the Southern Gate". Another name is Toliman, which in Arabic means "the ostriches". To astronomers, the star remains simply as Alpha Centauri.

What makes Alpha Centauri particularly important - from our perspective - is it's the closest star system to us. At 4.3 light-years, it's twice as near as the brightest star in the night sky, Sirius. Alpha Centauri was also the first star to have its distance measured. Thomas Henderson, a Scottish astronomer and mathematician, was the director of the Royal Observatory at the Cape of Good Hope in South Africa (between April 1832 and May 1833). Using the method of parallax he was able to calculate a distance of about 4 light-years. This is slightly less than the agreed modern value, but nevertheless remarkably accurate. However, Henderson disliked the area intensely and after only 13 months returned to his native Scotland. In no hurry to publish his results, and somewhat due to a lack of confidence in his measurements, Henderson finally released his findings on January 9, 1839. In the meantime, Friedrich Wilhelm Bessel at the Königsberg Observatory in Germany measured the distance of 61 Cygni and published his result in December 1838. This was a few weeks before Henderson and therefore he won the "race" to determine the distance to the stars.

The three stars of the Alpha Centauri System are labelled A, B and C; although Alpha Centauri C is better known as Proxima Centauri. Alpha Centauri A is the primary and brightest component. It's a yellow type G2 star similar to the Sun, but slightly larger and about 60% more luminous. Alpha Centauri B is an orange type K1 star, slightly smaller than the Sun and about half as luminous. The pair orbits each other around a common centre of gravity every 79.9 years. The double nature of Alpha Centauri was discovered by Father Richaud in 1689, while comet hunting in India.

The third group member is the red dwarf, Proxima Centauri. With an apparent magnitude of +11.1, it requires a small to medium size scope to be seen. The star has a diameter of about 200,000 kilometres (120,000 miles) and therefore is much smaller than the Sun. If placed at the centre of the Solar System, Proxima would shine only 45 times brighter than the full Moon when seen from Earth. It's perhaps not surprising that Robert Innes at the Union Observatory in South Africa didn't discover it until 1915. Proxima is a flare star and therefore undergoes random dramatic increases in brightness, due to magnetic activity. Indeed most stars, including the Sun, show flare activity but outbursts on more powerful stars often go unnoticed, whereas on feeble dwarf stars they are more evident. The name, Proxima, is Latin for "next to" or "nearest to" because at 4.22 light-years distant, this star is closer to us than both Alpha Centauri A and B. Since Proxima is separated by 0.21 light-years (15,000 AU) from the main pair, there has been much debate whether it's a true system member or simply a star passing close by. Modern measurements suggest that Proxima is indeed a true member with an orbital period of about 500,000 years.

The apparent orbit of Alpha Centauri (credit:- freestarcharts)

Alpha Centauri is located at a declination of almost -61 degrees and therefore only visible for latitudes south of +29 degrees. It can't be seen at all from Europe and most of North America. For example, from Los Angeles the star never manages to climb above the horizon. However, it can be glimpsed low down from southern Florida, southern Texas and Hawaii. It's also visible from the Spanish Canary Islands and practically all of India. On the other hand in New Zealand, southern parts of Australia and many other Southern Hemisphere locations, the star is circumpolar and therefore never sets. It's best seen during the months of April, May and June.

John Herschel described this as the most imposing of all double stars. The main stars are currently separated by about 4 arc seconds (2017), which is close to the minimum possible. In November 2037, the separation will shrink to 1.7 arc seconds before increasing up to 22 arc seconds by 2062. Even now the stars can easily be split through small telescopes. A refractor of 80mm (3.1 inch) aperture, at about 100 magnification, will easily do the job. The brighter A star shines at magnitude 0.0 and appears yellowish, with the fainter B star, orange in colour (mag +1.3). Proxima Centauri is not even in the same field of view as the main pair and can be found 2.2 degrees to the southwest.

Finder Chart for Alpha Centauri (credit:- freestarcharts)

Finder Chart for Alpha Centauri - pdf format (credit:- freestarcharts)

Alpha Centauri System Data Table

NameAlpha Centauri
Bayeralpha (α) Cen
ConstellationCentaurus
RA (J2000)14h 39m 36s
DEC (J2000)-60d 50m 08s
Distance (light-years)4.3
Apparent Mag.-0.27
Absolute Mag.+4.20
Luminosity (Sol)1.78
Other designationsRigil Kentaurus, Rigil Kent, Toliman, Bungula
Notable featureClosest star system to the Solar system

Alpha Centauri A Data Table

NameAlpha Centauri A
RA (J2000)14h 39m 36.5s
DEC (J2000)-60d 50m 02s
Distance (light-years)4.35
Apparent Mag.+0.01
Absolute Mag.+4.38
Spectral TypeG2V
Radius (Sol)1.227
Surface Temp (K).5,790
Luminosity (Sol)1.60
Age (years)5.75 Billion
Other designationsHR 5459, HD 128620, HIP 71683

Alpha Centauri B Data Table

NameAlpha Centauri B
RA (J2000)14h 39m 35s
DEC (J2000)-60d 50m 15s
Distance (light-years)4.35
Apparent Mag.+1.33
Absolute Mag.+5.71
Spectral TypeK1V
Radius (Sol)0.865
Surface Temp (K).5,260
Luminosity (Sol)0.45
Age (years)5.75 Billion
Other designationsHR 5460, HD 128621, HIP 71681

Proxima Centauri Data Table

NameProxima Centauri
RA (J2000)14h 29m 43s
DEC (J2000)-62d 40m 46s
Distance (light-years)4.22
Apparent Mag.+11.13
Absolute Mag.+15.60
Spectral TypeM6V
Radius (Sol)0.141
Surface Temp (K).3,040
Luminosity (Sol)0.00005
Age (years)4.85 Billion
Other designationHIP 70890

Sky Highlights - September 2017

Opposition
Neptune reaches opposition on September 5th

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for September

Northern Hemisphere
Evening
West:- Jupiter (mag. -1.7)
Southwest:- Saturn (mag. +0.5)
East:- Neptune (mag. +7.8)
Midnight
South:- Neptune
East:- Uranus (mag. +5.7)
Morning
West:- Neptune
South:- Uranus
East:- Venus (mag. -3.9), Mars (mag. +1.8) (from second week), Mercury (mag. +0.5 to -1.3) (from second week)

Southern Hemisphere
Evening
West:- Jupiter
Northwest:- Saturn
East:- Neptune
Midnight
West:- Saturn
North:- Neptune
Northeast:- Uranus
Morning
West:- Neptune
Northwest:- Uranus
Northeast:- Venus
East:- Mars (end of month)

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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