Alpha Centauri: Nearest Star System to the Sun
A photo of Alpha Centauri.
Credit: MSX/IPAC/NASA

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. They are an average of 4.3 light-years from Earth. The third star 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. 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.

Proxima Centauri 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.

Astronomers announced in August 2016 that they had detected an Earth-size planet orbiting Proxima Centauri. The newfound world, known as Proxima b, is about 1.3 times more massive than Earth, which suggests that the exoplanet is a rocky world, researchers said.

The planet is also in the star's habitable zone, that just-right range of distances where liquid water can exist. Proxima b lies just 4.7 million miles (7.5 million kilometers) from its host star and completes one orbit every 11.2 Earth-days. As a result, it's likely that the exoplanet is tidally locked, meaning it always shows the same face to its host star, just as the moon shows only one face (the near side) to Earth.

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. [Infographic: The Nearest Stars to Earth]

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.

Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz.
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Star Quiz: Test Your Stellar Smarts
Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz.
Open Star Cluster Messier 50
0 of questions complete