The sun's closest stellar neighbors are three stars in the Alpha Centauri system. The two main stars are Alpha Centauri A and Alpha Centauri B, which form a binary pair. The third star, which may or may not be part of the system, is Proxima Centauri. It is about 4.22 light-years from Earth and is the closest star other than the sun.
Alpha Centauri A and B orbit a common center of gravity every 80 years. The average distance between them is about 11 astronomical units (AU) — about the same distance as the sun is to Uranus. These stars are an average of 4.3 light-years from Earth. It would take about four years, three months and 18 days to travel there at the speed of light, which is thought to be theoretically impossible (or is it?).
Proxima Centauri is about one-fifth of a light-year or 13,000 AUs from the two other stars, a distance that makes some astronomers question whether it should be considered part of the same system. It may be passing through the system and will leave the vicinity in several million years, or it may be gravitationally bound to the binary pair. If it's bound, it has an orbital period around the other two of about 500,000 years.
To the naked eye, the two main stars shine as one, making them the third brightest "star" in our night sky. The two separate stars can be seen through a small telescope; one of the finest binary stars that can be observed. Proxima Centauri is too faint to see unaided, and through a telescope it appears about four diameters of the full moon away from the other two.
By itself, Alpha Centauri A is the fourth brightest star in the sky; just a bit dimmer, by 0.02 of a magnitude, than Arcturus. It is a yellow star of the same type (G2) as the sun, and it is about 25 percent larger. Alpha Centauri B is an orange K2-type star, slightly smaller than the sun. Proxima Centauri is a red dwarf about seven times smaller than the sun, or one-and-a-half times bigger than Jupiter. All three stars are a bit older — 4.85 billion years old — than the sun, which is about 4.6 billion years old. [Infographic: Alpha Centauri Stars & Planet Explained: Our Nearest Neighbors]
The system is in the Southern sky and is not visible to observers above the latitude of 29 degrees north — a line that passes near Houston, Texas, and Orlando, Florida. In the Southern Hemisphere, it's easy to find because the cross-piece of the Southern Cross (from Delta to Beta Crucis) points the way. Its right ascension is 14h 39m 41s and its declination is minus 60 degrees 50 minutes 7 seconds.
Scientists say the Alpha Centauri system, particularly Alpha Centauri B, has the ingredients for an Earth-like planet. Much of its matter — even more so than the sun — is made up of elements heavier than hydrogen and helium, so there would be plenty of heavier material to make planets from. And because it is a double- or triple-star system, the processes that form large gas giant planets would be suppressed. It would be more likely for the system to produce terrestrial exoplanets.
Astronomers announced in October 2012 that they had detected an Earth-size planet orbiting Alpha Centauri B. The newfound world, known as Alpha Centauri Bb, is about as massive as Earth, but its surface may be covered with molten rock, researchers said. The existence of the planet suggests that undiscovered worlds may lurk farther away from its star — perhaps in the habitable zone, that just-right range of distances where liquid water can exist. [Infographic: The Nearest Stars to Earth]
— Tim Sharp, SPACE.com Reference Editor