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.
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.
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.
Alpha Centauri System Data Table
|Bayer||alpha (α) Cen|
|RA (J2000)||14h 39m 36s|
|DEC (J2000)||-60d 50m 08s|
|Other designations||Rigil Kentaurus, Rigil Kent, Toliman, Bungula|
|Notable feature||Closest star system to the Solar system|
Alpha Centauri A Data Table
|Name||Alpha Centauri A|
|RA (J2000)||14h 39m 36.5s|
|DEC (J2000)||-60d 50m 02s|
|Surface Temp (K).||5,790|
|Age (years)||5.75 Billion|
|Other designations||HR 5459, HD 128620, HIP 71683|
Alpha Centauri B Data Table
|Name||Alpha Centauri B|
|RA (J2000)||14h 39m 35s|
|DEC (J2000)||-60d 50m 15s|
|Surface Temp (K).||5,260|
|Age (years)||5.75 Billion|
|Other designations||HR 5460, HD 128621, HIP 71681|
Proxima Centauri Data Table
|RA (J2000)||14h 29m 43s|
|DEC (J2000)||-62d 40m 46s|
|Surface Temp (K).||3,040|
|Age (years)||4.85 Billion|
|Other designation||HIP 70890|