Science & history · Star guide
Alpha Centauri, the nearest star system
Alpha Centauri is the nearest star system to the Sun: three stars a little over four light-years away. To the naked eye the two bright ones merge into a single point, the third-brightest in the night sky.
How to spot it: The brighter of the two Pointers beside the Southern Cross. Source (opens in a new tab)
Three stars
NASA describes (opens in a new tab) a system of three. Alpha Centauri A and B orbit each other every 80 years. The third, Proxima Centauri, is a small red dwarf that takes more than half a million years to circle the pair. Proxima, 4.25 light-years away, is the closest of the three to us, and it has at least one planet, Proxima b, about 1.27 times Earth's mass on an 11-day orbit.
One point of light
The two bright stars are too close together to split with the eye. ESA's Hipparcos table (opens in a new tab) ranks their combined light third among all stars, at 4.4 light-years. The Bright Star Catalogue lists them separately, and the IAU (opens in a new tab) has named them: A is Rigil Kentaurus, B is Toliman, and C is Proxima Centauri.
How to find it
With Hadar, beta Centauri, it forms the Pointers (opens in a new tab): two bright stars side by side that lead the eye to the Southern Cross. Alpha is the brighter and the one farther from the Cross.
Moving fast
Being so near, Alpha Centauri drifts quickly against the far background. By this site's calculation from its catalogued proper motion, it moves 3.7 seconds of arc a year: the width of the full Moon in under 500 years.
Where on Earth, and when
It rises from anywhere south of about 29°N.
South of about 29°S, it never sets.
It climbs at least 20° high, the finder's threshold for a suggestion, from anywhere south of about 9°N.
It is highest in the evening sky, at about 9 p.m. local time, in mid-June, from either hemisphere, wherever it rises. With daylight saving time, that is nearer 10 p.m. on the clock.
Worked out by this site from the Bright Star Catalogue, for a flat, clear horizon. Hills, buildings, haze and city light all cut into this; the air also lifts objects near the horizon by about half a degree. For your own place and night, use the sky finder.
Sources
- Astronomy NASA Science, Our Nearest Celestial Neighbor? An Exotic 3-Star System (opens in a new tab) for the three stars, Proxima and its planet
- Astronomy ESA, Hipparcos: The 50 Brightest Stars (opens in a new tab) for rank and combined distance
- Astronomy IAU Working Group on Star Names, IAU Catalog of Star Names (opens in a new tab) for the IAU names
- History Ian Ridpath, Star Tales: Crux (opens in a new tab) for the Pointers
- Astronomy Bright Star Catalogue, 5th Revised Ed. (Hoffleit & Warren 1991), as computed by this site's star engine (opens in a new tab) for its proper motion