Alpha Centauri

Alpha Centauri, closest star system to Earth, appearing as the third brightest star visible in the night sky. Alpha Centauri is located in the constellation Centaurus and is sometimes called Rigil Kentaurus, which literally means “foot of the centaur.”

Alpha Centauri is actually a triple star system that appears as a single point of light because its two largest and brightest members, Alpha Centauri A and Alpha Centauri B, are too close together for the naked eye to tell them apart. The two stars have a combined visual magnitude of about -0.28 on a logarithmic scale in which lower numbers are brighter, with negative numbers being brighter than positive numbers. The system’s third member, Alpha Centauri C, is too small and dim to be seen without a telescope.

The Alpha Centauri system is only visible from the Southern Hemisphere and the southernmost portion of the Northern Hemisphere. In Texas, Louisiana, and Florida, for example, it appears very low in the southern sky and is most easily visible in May.

Alpha Centauri A and Alpha Centauri B lie about 4.35 light-years from Earth. A light-year is the distance light travels in a year, equal to about 9,460 billion km (5,880 billion mi). The two stars circle their common center of gravity, a point in space between them, about once every 80 years. The average distance between A and B is about 3.6 billion km (2.2 billion mi), which is a bit more than the average distance between the Sun and the planet Uranus.

Alpha Centauri C orbits A and B at a tremendous distance—about 1,945 billion km (1,209 billion mi)—so far out that C takes about half a million years to circle the two larger stars. Alpha Centauri C is also called Proxima Centauri; in the current portion of its orbit it is the closest star to our solar system, at a distance of 4.2 light-years.

Alpha Centauri A is a yellow star, slightly larger and brighter than the Sun, of spectral type G2, with an absolute magnitude or intrinsic brightness of +4.34 compared to +4.85 for the Sun. Its visual or apparent magnitude if viewed alone is -0.01. Spectral type indicates a star’s surface temperature and the predominant color of the light it gives off. Absolute magnitude is how bright a star would appear if viewed from a standard distance. Apparent magnitude is a measure of how relatively bright a star appears in Earth’s sky—a small, nearby star may appear just as bright as a much larger star that is farther away. The lower the apparent magnitude, the brighter the star appears.

Alpha Centauri B is a yellow-orange star somewhat smaller and cooler than the Sun, of spectral type K1, with an absolute magnitude of +5.7 and an apparent magnitude of about +1.35. Proxima Centauri is a red dwarf star of spectral type M5, much smaller and cooler than the Sun.

The different orbits of the Sun and Alpha Centauri around the center of the Milky Way galaxy mean that many thousands of years from now Alpha Centauri will be nearer to Earth and will appear even brighter in the night sky. It will also move northward as seen from Earth and become visible to much of the Northern Hemisphere.

Astronomers continue to look for planets that may possibly orbit any of the stars in the Alpha Centauri system (see Extrasolar Planets). Theories of planet formation allow for rocky planets in stable orbits around Alpha Centauri A or Alpha Centauri B almost out to the distance between Mars and the Sun. Planets at much longer distances might orbit around the common center of gravity between the two stars. Planets might also orbit Proxima Centauri.