Stars between Zero and 4.3 l.y.

Number of stars: 4




NOTE: The mass of the Sun compared to other stars is represented here by the size of the sphere. Colour is approximate only.

A typical yellow dwarf (spectral type G2) of diameter 1,392,530km. The Sun has a family of nine planets, of which the third planet, in increasing distance from the star, supports life.


A : 1.09
B : 0.89
C : 0.11

1.54 : A
0.43 : B
0.00006 : C

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 where we live, Alpha Centauri A is the one that resembles the Sun the most.

Alpha Centauri B is an orange-yellow star of spectral type K1, making it 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 79.91 years and are separated by at least 11 astronomical units (i.e. Sun-Saturn distance) to a maximum of 35 astronomical units (i.e. 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 is likely to be lurking within the Alpha Centauri system (perhaps even two Earth-like planets if we included both stars as capable of supporting life), but again no direct evidence exists to support this. Of course, you might be wondering why we can't detect planets by now given our current technology of orbiting telescopes, radio astronomy and all the rest. Understandable considering the triple star system is the closest to our Sun. The biggest problem astronomers have in detecting planets in our neighbouring star system, despite how close it is to our Sun, is because of the brightness of the stars, and the way both stars already wobble side-to-side quite significantly as they move together through space thanks to the gravitational pull of both stars on each other, thereby making it extremely hard to detect the tiniest of wobbles in the stars' motions from much smaller bodies we call planets. Only a spacecraft venturing out to this star system will answer the question once and for all (unless life on a habitable planet within this system decides to venture out to visit us first, which is more likely given the age of these stars).

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. In the case of Alpha Centauri A and B, the residues of heavy elements are in much greater quantities than in our Sun! (1).

Presently, the closest star to our Sun is a very faint red dwarf called Proxima Centauri (spectral type M5). It 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.

On 24 August 2016, observations made of Proxima Centauri with a telescope in Chile has revealed a hidden companion roughly the size of the Earth. Yes, there is a planet orbiting this red star. And its estimated distance from the star suggests liquid water can exist. The only complicating factor in this liquid water debate is the distance from from star: the planet is rather close to its solar source. Thus the risks of unexpected temperature variations as the star flares up at various times can whip up the wind speed of the alien atmosphere with great ease and ferocity, and heat up the surface to high levels. If water can still remain on the surface, the chances of primitive life is good so long as they are well-protected deep beneath the water and/or inside caves. But if the temperatures exceed a critical level, the surface is likely to be barren and dry. And, like Mercury in our solar system, the alien world is locked in its revolutions such that one side of the planet always faces the Sun. On the other side, it will be perpetually dark and potentially cold.

If we want to find highly advanced alien life, we would be better off making the extra effort to visit Alpha Centauri A and B. Here, the likelihood of finding planets around these stars (not detectable yet because of the brightness of the stars, the way the stars move around each other, and quite possibly even the size of the planets in question) are extremely high.