The Alpha Centauri system appears to the naked eye as a single star of magnitude -0.27. It's the third brightest in the night sky although considerably fainter than Canopus (mag. -0.72) but obviously brighter than its nearest rival Arcturus (mag. -0.04). Yet Alpha Centauri is not a single star; it's a triple system that consists of two bright components and a feeble red dwarf star. For most of their orbit the main stars can be easily split with small telescopes but the red dwarf requires a small to medium size telescope just to be spotted. The star system is also the nearest to our Solar System.

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

Unusually for a bright star Alpha Centauri doesn't have a commonly agreed proper name. Arab astronomers knew it as Rigil Kentaurus "the foot of the Centaur". A shorter form "Rigel Kent" is commonly used today especially in air navigation. The Chinese referred to the star as Nan Mun "the Southern Gate". Another name used is Toliman, which means in Arabic "the ostriches". To astronomers it remains simply as Alpha Centauri.

What makes Alpha Centauri particularly important - from our perspective - is that it's the closest of all bright stars. It's only 4.3 light-years distant and hence twice as close as Sirius. It was also the first star to have its distance measured. Thomas Henderson a Scottish astronomer and mathematician was 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, slightly less than the modern agreed 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 due somewhat 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 labeled A, B and C although Alpha Centauri C is better known as Proxima Centauri. Alpha Centauri A is the primary and brightest component, a yellow type G2 star similar to the Sun. It's slightly larger than our star and about 60% more luminous. Alpha Centauri B is an orange type K1 star that's slightly smaller than the Sun and about half as luminous. The pair orbit around a common centre with a period of 79.9 years. It was Father Richaud while comet hunting in India who discovered the double nature of Alpha Centauri in 1689.

The third member of the group is Proxima Centauri. This star is a red dwarf that's much fainter than it's two neighbours. With an apparent magnitude of +11.1 it requires a small to medium size telescope to be seen. The star is much smaller than the Sun with a diameter of about 200,000 kilometres (120,000 miles) and if at the centre of the Solar System, Proxima would only produce 45 times the light of the full Moon! It's perhaps no surprise then that it was not until 1915 when Robert Innes discovered it at the Union Observatory in South Africa. Although it has a very low average luminosity, Proxima is a flare star that undergoes random dramatic increases in brightness due to magnetic activity. Indeed most stars, including the Sun, show flare activity but an outburst on a powerful star usually goes unnoticed whereas on a feeble dwarf it's much more evident. The name Proxima is Latin for "next to" or "nearest to" and the reason is because at 4.22 light-years the star is closer to us than both Alpha Centauri A or B. With a separation of 0.21 light-years (15,000 AU) from the main stars there has been much debate if Proxima is a true member of the system or just simply a star passing close by. Modern measurements suggest that Proxima is indeed a true member with a large 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 29N. It can't be seen at all from Europe and most of North America. From Los Angeles it never manages to climb above the horizon but 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 southern parts of Australia, all of New Zealand and much of the Southern Hemisphere, Alpha Centauri is circumpolar and therefore never sets. The star is best visible during the months of April, May and June.

Through a telescope Alpha Centauri is a glorious double with John Herschel described it as the most imposing of all double stars. The main stars are currently separated by 4 arc seconds (2015) which is close to the minimum possible. In November 2037, the separation will shrink to 1.7 arc seconds before increasing to a wide 22 arc seconds by 2062. Even now the two stars are easily split in small telescopes. An refractor of 80mm (3.1 inch) aperture at about 100 magnification will do the job. The brighter A star is at magnitude 0.0 and appears yellowish with the fainter B orange star shining at mag +1.3. Proxima Centauri is not even in the same field of view; positioned 2.2 degrees to the southwest.

Finder Chart for Alpha Centauri (credit:- freestarcharts)

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

Alpha Centauri System Data Table

NameAlpha Centauri
Bayeralpha (α) Cen
RA (J2000)14h 39m 36s
DEC (J2000)-60d 50m 08s
Distance (ly)4.3
Apparent Mag.-0.27
Absolute Mag.4.20
Luminosity (Sol)1.78
Other designationsRigil Kentaurus, Rigil Kent, Toliman, Bungula
Notable featuresClosest 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 (ly)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 (million years)5,750
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 (ly)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 (million years)5,750
Other designationsHR 5460, HD 128621, HIP 71681

Proxima Centauri Data Table

NameProxima Centauri
RA (J2000)14h 29m 43s
DEC (J2000)-62d 40m 46s
Distance (ly)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 (million years)4,850
Other designationsHIP 70890

Sky Highlights - April 2017

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