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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