EMBARGOED FOR Measuring the size of objects in space is so difficult that until late astronomers were uncertain of the precision of their estimates of the how large, or small, the stars closest to Earth were.
The three nearest known stars are gravitationally bound in a system commonly called Alpha Centauri. The two larger stars, said to be Sun-like, are named Alpha Centauri A and B. The nearest to us is the littlest and is called Proxima Centauri. It is classified as a red dwarf and contains just a fraction of the mass of our Sun.
The three-star system is 4.36 light-years away, meaning light requires 4.36 years to travel from the stars to Earth, and so we see them as they existed 4.36 years ago.
On Saturday, March 15, astronomers announced that Alpha Centauri A is now calculated to be 1,061,000 miles wide (1,708,000 kilometers), or 1.227 times the size of the Sun. The B-star is 748,100 miles across (1,204,000 kilometers), or 0.865 times the Sun's diameter.
Best known stars now
The diameters were determined with a new high-tech setup at the European Southern Observatory's Paranal Observatory in Chile. Using a complex technique called interferometry, researchers combined light from two telescopes separated by a distance nearly equal to the length of a football field at the Very Large Telescope Interferometer. The scheme creates one effectively larger and more precise light-gathering device. Previously known distances to the stars were also factored in to calculate the sizes.
The observations represent the first direct measurements of the two stars' diameters. Proxima Centauri was measured by the same facility late last year.
Astronomers said the refined measurements agree with diameter projections made using indirect methods, and they also support theories about how stars evolve. Because the two nearby stars are Sun-like, astronomers say they are particularly useful in helping to model the past and future of the star we orbit.
"Alpha Centauri is not only the nearest stellar system -- thanks to these studies, it is now also the best known one," said study participant Frederic Thevenin of the Nice Observatory.
The ESO's Pierre Kervella is lead author of a paper on the results that will be published in the European journal Astronomy & Astrophysics.
Alpha Centauri A and B orbit each other at a distance of about 2.2 billion miles (3.6 billion kilometers), a bit more than the distance from the Sun to planet Uranus. It takes 80 years for them to complete an orbit. Proxima Centauri is nearer to Earth than the other two stars, by the rather large distance -- roughly 10,000 times the distance from Earth to the Sun.
Astronomers figure Proxima Centauri orbits the other two stars in a huge circle that takes millions of years to complete.
More stars? Maybe planets?
Binary star systems are known to be common -- about half the points of light in the night sky actually represent double stars. And triple star systems may be more common than astronomers know, a study released in early 2002 showed.
It is even possible that the Alpha Centauri system, which resides in the constellation Centaurus in the Southern Hemisphere, harbors more stars, very dim objects that have yet to be detected. Astronomers say Alpha Centauri A is a good candidate for having planets, too, and the Hubble Space Telescope may be tasked to look for them soon.
All three known stars in the system were born about 4.85 billion years ago, astronomers believe. Our Sun began shining about 4.6 billion years ago. The A and B stars are both about the same temperature as the Sun.
Proxima Centauri is about seven times smaller than the Sun. It contains just enough mass to cause hydrogen to burn, and it is much cooler and, intrinsically, only about 1/150th as bright as the Sun.
This small star is barely a star at all, in fact. Its mass is just above that of brown dwarfs, a class of object that seems to straddle the definition between stars and planets. Though 150 times more massive than Jupiter, Proxima Centauri is only about 1.5 times bigger than the planet.
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