Alpha Centauri

Alpha Centauri (α Centauri, abbreviated Alf Cen or α Cen) is the star system closest to the Sol System, being 4.37 light-years from Earth. It consists of three stars: Alpha Centauri A and Alpha Centauri B which form the binary star Alpha Centauri AB, and a small and faint red dwarf, Alpha Centauri C (also named Proxima Centauri). To the unaided eye, the two main components appear as a single point of light with an apparent magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and is the third-brightest star in the planetary night sky of Earth, outshone only by Sirius and Canopus.

Alpha Centauri A has 1.1 times the mass and 1.519 times the luminosity of the Sun, while Alpha Centauri B is smaller and cooler, at 0.907 times the Sun's mass and 0.445 times its luminosity. The pair orbit about a common center with an orbital period of 79.91 years. The elliptical orbits have a marked eccentricity, so that the distance between them varies from nearly that between Pluto and the Sun (35.6 astronomical units), to that between Saturn and the Sun (11.2 AU).

Though not visible to the naked eye, Proxima Centauri is the closest star to the Sun, at a distance of 4.24 light-years. The distance between Proxima Centauri and Alpha Centauri AB is about 13,000 astronomical units, equivalent to about 430 times the radius of Neptune's orbit. Aeros, an Earth-sized exoplanet in the habitable zone of Proxima Centauri, was discovered in 2016. It was visited in 2109 and colonized in 2173.

Alpha Centauri A

Alpha Centauri A, otherwise known as Rigel Kentaurus A, Toliman A, Bungula A, Gliese 559 A, HD 128620 and HIP 71683, is the principal member, or primary, of the binary system Alpha Centauri. It is a solar-like main-sequence star with a similar yellowish colour, whose stellar classification is spectral type G2 V; it is slightly larger and more luminous than Sol. From the determined mutual orbital parameters, Alpha Centauri A is about 10 percent more massive than the Sun, with a radius about 22 percent larger. When considered among the individual brightest stars in the sky (excluding Sol), Alpha Centauri A is the fourth brightest at an apparent magnitude of +0.01, being slightly fainter than Arcturus at an apparent magnitude of −0.04.

The type of magnetic activity on Alpha Centauri A is comparable to that of the Sun, showing coronal variability due to star spots, as modulated by the rotation of the star. However, since 2005 the activity level has fallen into a deep minimum that might be similar to the Sun's historical Maunder Minimum. Alternatively, it may have an over long stellar activity cycle and is slowly recovering from a minimum phase.

Alpha Centauri B

Alpha Centauri B, otherwise known as Rigel Kentaurus B, Toliman B, Bungula B, Gliese 559 B, HD 128621 and HIP 71681, is the companion star, or secondary, of the binary system. It is a main-sequence star of spectral type K1 V, making it more an orange color than the primary star; it is slightly smaller and less luminous than the Sun. Alpha Centauri B is about 90 percent the mass of the Sun and 14 percent smaller in radius. Although it has a lower luminosity than component A, star B emits more energy in the X-ray band. The light curve of B varies on a short time scale and there has been at least one observed flare. Component B is more magnetically active than Alpha Centauri A, showing a cycle of 8.2±0.2 yr compared to 11 years for the Sun, and about half the minimum-to-peak variation in coronal luminosity of Sol.

In 2012, a planet around Alpha Centauri B was detected. After that, scientists met up to discuss an unmanned mission to the planet. Eventually, in 2048, the Bullet probe (equipped with a small robotic lander and rover) was launched and arrived at Alpha Centauri Bb (later known as Helion) in 2057.

The existence of a planet, Alpha Centauri Bc, was announced in 2013. It has an estimated orbital period of approximately 12 Earth days – smaller than that of Mercury – with a semimajor axis of 0.10 AU and an eccentricity smaller than 0.24.