Geography Reference
In-Depth Information
Although it is our sun and we love it, the sun is a rather ordinary star. What seems to set it apart, of
course, is the peculiar little planet called Earth that so far has proven to be the only place capable of gen-
erating and maintaining life.
The sun—and its tidy little solar system whirling about space with clockwork regularity—is but one of
a large number of stars within a
galaxy.
Our galaxy is the Milky Way, which also explains the source of the
word
galaxy
, derived from the Greek word
galaxias
for “milky.” On a summer night, the reason for this
name is apparent, as the Milky Way looks like a large, white puddle of milk, spilled across the sky.
Just as the sun is an average star, the Milky Way is in many ways a typical galaxy. It has about
100
billion
stars. Shaped somewhat like a child's pinwheel, the Milky Way spins through space with four spiral
arms radiating out from the center. The closer to the center of the pinwheel, the more densely packed the
stars. Our solar system lies within the Milky Way's Orion arm. It takes light a hundred thousand years to
cross from one side of the Milky Way to the other.
Although humanity has known about the stars of the Milky Way for centuries, and philosopher Im-
manuel Kant speculated about the existence of other galaxies more than two hundred years ago, the dis-
covery of galaxies was a recent one. Until 1923, in fact, other galaxies were thought to be clouds of gas.
Edwin Hubble, one of the greatest of American astronomers, resolved the issue when he was able to dis-
cern individual stars in the Andromeda galaxy, the Milky Way's nearest galactic neighbor. Light from
the Andromeda galaxy takes about two million years to reach our galaxy. Hubble also began a system of
galaxy classification: the egg-shaped, or elliptical; the irregular; and the spiral, the form of the Milky Way.
Strewn across the empty darkness of space are perhaps a hundred billion galaxies, each containing mil-
lions or billions of stars. The galaxies, in turn, are collected in groups and superclusters. The Milky Way
and nearby Andromeda belong to the friendly-sounding Local Group, which at twenty or thirty galaxies is
fairly small potatoes in the intergalactic big picture. This Local Group lies at the fringes of the Virgo Su-
percluster, which consists of about five thousand galaxies. Lying between the superclusters are voids—vast
areas of space where no stars shine. Unknown until the 1980s, the voids separating the superclusters of
galaxies are like vast Saharas of space, except that they are millions of
light years
across.
So how big is the universe? To put it bluntly, it is an unimaginably big place. Astronomers can see out
to distances of 10 billion light years, but that may be only part of the universe, the size of which is literally
unknown. If you're still not impressed with space numbers, try and count the stars. The latest estimate for
stars in the galaxies of the
visible
universe is 1 sextillion, or 1 followed by
21 zeroes.
And you thought the
United States budget deficit was impressive. Wait till Washington, DC, hears that there is a number called
sextillion.
How Far Is a Light Year?
Apart from our star, the sun, the star closest to earth is Proxima Centauri, a faint companion to the better-
known Alpha Centauri. These stars are a mere 4.2 light years away, meaning it takes a little over four years
for light from these stars to reach the earth. Or, to put it another way, that's more than
25 trillion
miles.
That distance, by the way, is the average distance between most stars.
The distance between stars is so great that conventional measurements of the distance between them in
familiar units is impractical. A more manageable astronomical unit, called the
light year
, was created to
simplify matters. A light year is the distance it takes light to travel in one year at a speed of about 186,200
miles per second. One light year is equivalent to about 6 trillion miles.
