By Louis Varricchio, M.Sc.
Earth’s nearest star is, of course, our Sun located 93 million miles away. But common references to Earth’s “nearest star” usually refer to Alpha Centauri A, a star in a nearby stellar system consisting of three suns. The Alpha Centauri system is located in the constellation Centaurus.
While Alpha Centauri A is considered the nearest star to Earth—at just over four light years distant—its smaller companion, the red dwarf star Alpha Centauri C or Proxima Centauri, can be a tad closer due to the long, slow dance of orbital mechanics. Overall, the Alpha Centauri system is approximately 4.2 light years from us (a light-year is the distance a beam of light travels in 365 days—9,460 billion kilometers or 5,880 billion miles).
Alpha Centauri A is the third brightest star in our night sky although to see it you must travel to either southern Florida or Texas. The star is best seen during the month of May when it is highest in the southern sky.
In recent decades, some astronomers have begun referring to Alpha Centauri A as “Rigil Kentaurus”, which literally means “foot of the centaur” in Greek. However, both names, Alpha or Rigil, are still correct to use for this Sunlike star.
Alpha Centauri A is nearly a twin of our Sun although it is slightly larger and brighter (spectral type G2 with an apparent magnitude of +0.01). Alpha Centauri B is a yellow-orange star, slightly smaller and cooler than our Sun.
Because Alpha Centauri A is so Sunlike, some astronomers have speculated that there might be Earth-like planets orbiting it although none have been detected so far. And because it is so similar to our Sun, Alpha Centauri has been a popular destination in space-age mythology—from author A.E. Van Vogt’s classic 1944 science-fiction tale of suspended-animation star trekking, titled “Far Centaurus”, to television’s long wandering “Lost in Space” Robinson family.
Let’s put some things into stark perspective: Using current chemical rocket technology, a crewed voyage to Alpha Centauri B would take 32,000 years! Such an impossibly long trek would require a multi-generation starship and vast amounts of fuel; in effect, such a starship would be a miniplanet in its own right with vast living areas, hydroponic gardens and livestock to nurture and sustain many generations of people in flight.
In 1987, NASA and the U.S. Naval Academy proposed Project Longshot, an ultra-fast robot probe intended to fly to Alpha Centauri powered by nuclear-pulse propulsion. As proposed, Longshot could reach the Alpha Centauri system within a century. This advanced rocket technology could be scaled up to power an ultra-fast manned mission—but then who would finance such a bold voyage?
Because Alpha Centauri A is a member of a triple star system, it appears as a single point of light from Earth. Both Alpha Centauri A and B are too close together to distinguish them as individual stars. Alpha Centauri C is simply too dim to be seen with the naked eye from Earth.
Alpha Centauri A and B orbit a shared center of gravity once every 80 years. Approximately 3.6 billion kilometers (2.2 billion miles) seperate the two stars—about the distance between our Sun and the planet Uranus. Trio member Alpha Centauri C orbits A and B at 1,500 billion kilometers (930 billion miles) taking several million years to circle its distant stellar companions. Because C is so distant from its larger companions, an inhabitant living on a planet orbiting stars A or B, would have no idea that Proxima was even part of their stellar system.
NASA computer models suggest that Earth-like planets could form close to either Alpha Centauri A and B. The Alpha Centauri system may hold the best chance for finding extraterrestrial life beyond our solar system.
Former NASA science writer Lou Varricchio, M.Sc., is a member of the NASA-JPL Solar System Ambassador program in Vermont.