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Our nearest stellar neighbour (apart from the Sun) is a triple star system consisting of a red dwarf called Proxima Centauri, revolving around two sun-like stars - Alpha Centauri A and B.
Alpha Centauri A has a spectral type of G2, which means that its temperature and colour is the same as our Sun. The only difference is its luminosity: with a mass 1.09 times that of our Sun, Alpha Centauri A emits a steady 54 per cent more light than our Sun. In fact, of all the nearest stars within 12 light years of the Sun, Alpha Centauri A is the one that resembles the Sun the most.
Alpha Centauri B is an orange-yellow star of spectral type K1. This means it is cooler and smaller than the Sun. With a mass of only 0.88, Alpha Centauri B has a stable brightness of 44 per cent of the standard solar value.
The two stars orbit each other every 80 years and are separated by at least 11 astronomical units (ie. Sun-Saturn distance) to a maximum of 35 astronomical units (ie. Sun-Neptune) when they are at their farthest.
In 1986, scientists from Yale University, USA - Pierre Demarque, D.B. Guenther and William van Altena - used the most recent data on the luminosity and mass of the Alpha Centauri system to estimate the age of the stellar system. Calculations indicate that both Alpha Centauri A and B are about 5 billion years old - more than enough time for intelligence to arise on a habitable planet.
Although no planetary system has been detected so far, a mathematical study of this stellar system suggest stable planetary orbits should exist around both stars up to a distance of at least 2 astronomical units. Since the Sun's fourth planet, Mars, is only 1.5 astronomical units, an Earth-like planet may be lurking within the Alpha Centauri system, but again no direct evidence exists to support this.
The only evidence we do have suggestive of unseen planets is from a careful spectroscopic analysis of the chemical composition of stars: For life and a technology to arise on a planet, there must be elements heavier than those that constitute a typical star. For instance, we need silicon and oxygen for the formation of rocks; carbon, nitrogen and oxygen for the development of life; and iron, titanium, uranium, and other metals for the development of a technology.
In our Sun, there is a residue of about two per cent of such heavy elements. As for Alpha Centauri A and B, the residues of heavy elements are in much greater quantities than in our Sun! (Croswell 1991, pp. 28-37).
Presently, the closest star to our Sun is a very faint red dwarf called Proxima Centauri (spectral type M5). This particular star is of the 'flare-up' variety, meaning that it emits every now and then great bursts of energy. It orbits the other two larger luminous spheres at a distance of some 13,000 astronomical units (430 times Sun-Neptune distance) and takes millions of years to complete one orbit.
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