NOTABLE STARS A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site Alpha Centauri Alpha Centauri. Image by Claus Madsen/ESO The nearest star system to the Sun. Alpha Centauri lies at a distance of 4.395 light-years (1.349 parsecs) in the constellation Centaurus. Alpha Centauri consists of two reasonably Sun-like stars orbiting closely about each other and possibly a third, red dwarf star, much further out. The two brightest components, A and B, revolve around each other once every 80 years and are separated by about 25 astronomical units (3.75 billion kilometers, or 2.3 billion miles). Alpha Centauri A is a yellow G star similar to the Sun but about 1.5 times as bright; B is a smaller, orange K star with about half the Sun's luminosity. The third possible member of the system, Proxima Centauri, is a red dwarf, about 7,000 times fainter than the Sun, which, if it is truly associated with the main pair, moves in a very wide orbit with a period of millions of years.   A B Proxima Spectral type G2V K1V M5Ve Apparent magnitude -0.01 1.35 11.01 Absolute magnitude 4.34 5.70 15.45 Luminosity (Sun=1) 1.519 0.500 0.000138 Mass (Sun=1) 1.100 0.907 0.1 Temperature (K; Sun=5770) 5790 5260 3040 Radius (Sun=1) 1.227 0.865 0.145 Age (million yrs; Sun=4650) 4850 4850 4850 Hydrogen (%; Sun=73.7) 71.5 69.4 69.5 Helium (%; Sun=24.5) 25.8 27.7 27.8 Heavy elements (%; Sun=1.81) 2.74 2.89 2.90 Planets in the Alpha Centauri system? An artist's concept of an Earthlike planet in the Alpha Centauri system. Image by Mark Fisher Whether or not the Alpha Centauri system contains any planets is undetermined but there has been much speculation about how, if it did, a planet would have to move in order to have the best chance of supporting life.1, 2, 3 If it circled widely around either A or B, it would not only lie outside the habitable zone (where liquid water can exist on a planet's surface) of either of these stars but be subject to complex gravitational influences which would tend to destabilize its orbit. An even wider path, on the other hand, that took a planet around both main stars, might be stable but would fall even further outside the habitable zones. The best chance for life (as we know it) might be on a world which circled closely around A or B and enjoyed similar levels of light and warmth to those on Earth. To an inhabitant of such a world, the other star would appear about 1,000 times brighter in the sky than our Moon and be visible for half the year in the daytime and half the year at night. A and B would make a striking color contrast of orange and yellow, while Proxima would appear as a dim red point of light barely visible without a telescope. In 2008 a team of researchers led by astronomers at the University of California, Santa Cruz, completed a study which suggests that rocky planets similar to Earth may exist in the Alpha Centauri system and could be detected using existing techniques. Javiera Guedes used computer simulations of planet formation to show that terrestrial planets are likely to have formed around Alpha Centauri B and be orbiting in the habitable zone. The researchers then showed that such planets could be observed using a dedicated telescope. Coauthor Gregory Laughlin, professor of astronomy and astrophysics at UCSC, said a number of factors converge to make Alpha Centauri B an excellent candidate for finding terrestrial planets. The Doppler detection method, which has revealed the majority of known extrasolar planets, measures shifts in the wavelength of light from a star to detect the tiny wobble induced by the gravitational tug of an orbiting planet. Factors that favor the use of this technique for Alpha Centauri B include the brightness of the star and its position in the sky, which gives it a long period of observability each year from the Southern Hemisphere. Detecting small, rocky planets the size of Earth is challenging, however, because they induce a relatively small wobble in their host stars. According to Laughlin, five years of observations using a dedicated telescope would be needed to detect an Earth-like planet around Alpha Centauri B. Coauthor Debra Fischer of San Francisco State University is leading an observational program to intensively monitor Alpha Centauri using the 1.5-meter telescope at the Cerro Tololo Inter-American Observatory in Chile. The researchers hope to detect real planets similar to the ones that emerged in the computer simulations. To study planet formation around Alpha Centauri B, the team ran repeated computer simulations, evolving the system for the equivalent of 200 million years each time. Because of variations in the initial conditions, each simulation led to the formation of a different planetary system. In every case, however, a system of multiple planets evolved with at least one planet about the size of Earth. In many cases, the simulated planets had orbits lying within the habitable zone of the star. [Text adapted from UCSC press release.] See also Orion, Project. References Benest, D. "Planetary Orbits in the Alpha Centauri System," Bulletin of the American Astronomical Society, 18, 810 (1986). Benest, D. "Stable Planetary Orbits Around One Component in Nearby Binary Systems," Celestial Mechanics, 43, 47 (1988). Harrington, R. "Planetary Motion in Alpha Centauri," Journal of Astronautic Science, 35, 359 (1987). Related entries    • nearest stars    • brightest stars Related category    • NOTABLE STARS External site Alpha Centauri: possible planets and life Alpha Centauri: a family portrait (ESO) Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP