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magazine / ja01
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July/August 2001 issue |
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Meet the neighbours
Map and text by Steven Fick
Standing under a starry sky on a clear summer night, it is hard to believe
that our part of the galaxy is filled mainly with stars which
are too dim for our eyes to see. The fact is, though, that the
stars in such familiar configurations such as the Big Dipper and
the Northern Cross are immensely more distant and brilliant celestial
objects than the sun's closest stellar neighbours.
This three-dimensional map (below) shows our neighbours within 11
light-years — 104 trillion kilometres. (By contrast, the Earth
is only 8.3 light minutes or 150 million kilometres from the sun.)
These stars are fairly typical of our galaxy, the Milky Way. Most
are red and orange dwarfs — smaller than giant stars, relatively
cool and quite faint.
 The
sun is a dwarf star, sometimes referred to as
a yellow dwarf because its surface temperature of 6,000°C produces
a soft yellow-white glow. It formed about 4.5 billion years ago and is
roughly halfway through its 10-billion-year lifespan. When it burns out,
it will become a white dwarf. Stars much larger than the sun move through
stellar evolutionary changes more rapidly. Some have already exploded
as a supernova and collapsed into a neutron star — so dense that
the atoms are crushed — or a black hole, with gravity so intense
that even light cannot escape. As far as astronomers know, orange and
red dwarfs, like most of our nearest neighbours, have not yet had time
to burn out.
 Proxima
Centauri is the closest star to the sun — 4.3 light-years away.
Even so, travelling at the speed of a Concorde jet, it would take two
million years to reach it. A red dwarf, it is bound by gravitation to
the double star Alpha Centauri A and B, which it orbits over millions
of years.
 Alpha
Centauri A is a yellow star like our sun and one of the few in the
sun's immediate neighbourhood that is visible to the naked eye. Its
partner, Alpha Centauri B, is an orange-red dwarf. They revolve
around each other every 80 years.
 Sirius
A (a.k.a. Alpha Canis Majoris), nine light-years from the sun, is
the brightest star in the night sky. Not surprisingly, its Greek name
means scorching. Also known as the "dog star," its first appearance
in the early morning signalled for the Romans the onset of the hottest
time of the year and for the Egyptians the imminent flooding of the Nile.
It forms a double star with Sirius B, the two revolving around each other.
Sirius B has shrunk into a white dwarf whose mass, similar to our sun's,
has been crushed into a sphere only three times the Earth's diameter,
making it one ten-thousandth the brightness of Sirius A.
 Epsilon
Eridani, an orange dwarf, is also discernible to the naked eye and
was catalogued by the ancient Greeks before the invention of modern telescopes.
Barnard's
Star is the fastest-moving of our close neighbours, which suggests
that it is an extremely old star — a remnant from the early history
of our galaxy, when its shape was more spherical.
Scientists believe that 85 percent of our galaxy is made up of faint stars,
such as Ross 128, CD-36 15693, Wolf 359, BD+36 2147, Ross 248, Ross 154,
Barnard's Star and the star pair L726-8A and L726-8B — all examples
of red and orange dwarfs.
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