Page 322 - NS-2 Textbook

ASTRONOMY 317

Clusters are classified both by their appearance and available supply of raw material from space and are, rel-
their "population." A moving cluster contains a few stars atively speaking, near the end of their lives as luminous
that travel in parallel lines. Opell clusters are loosely stars.
grouped stars, often found in areas where there are glow- In May 1997 scientists analyzing data from a gamma
ing masses of dust and gas. Most open clusters are found ray observatOlY satellite orbiting in 1991 armounced the
in the Milky Way, so they often are called galactic clus- discovery of a giant antimatter cloud of positrons some
ters. Globular clusters contain thousands of stars-too 3,000 light-years wide near the center of the Milky Way
many to cmmt, even with the best photography. They galaxy. When such positrons merge with normal elec-
may contain as many as 100,000 stars. Star clouds are clus- trons they annihilate each other, releasing energy in the
ters in which the stars are so thick that they look like process. The density of the cloud, however, is so thin that
glowing clouds. it would pose no danger to anything in the galaxy that it
might encounter.
There is a large number of other galaxies, each con-
GALAXIES
taining billions of stars. Each galaxy is separated from
On a clear night you can see what appears to be a wispy neighboring galaxies by oceans of space. Many of the
cloud extending across the northern sky. It is in fact a galaxies have cepheid variables in the outer regions of
vast band of stars called the Milky Way-our own galaxy. their formations, as ,,",Tell as temporary stars and some
A galaxy is a huge collection of stars, star clusters, dust, supernovae. The cepheids enable astronomers to calcu-
and gas, all held together by gravitation. late roughly how far distant these galaxies are from us.
The Milky Way is shaped like a giant disc or pin- Galaxies may be classified, according to their shapes, into
wheel, with more stars in the center than at the edges. It three different groups (1) ellipsoidal galaxies, which have
is estimated to be 80,000-100,000 light-years from one rather clearly defined, symmetrical shapes, ranging from
edge to the other. From top to bottom it is 10,000-15,000 spheres to ellipsoids; (2) spiral galaxies, which have a dis-
light-years thick. The Milky Way contains over 100 bil- tinct nucleus with one or more spiral arms; and (3) irreg-
lion stars revolving about a common center in the con- ular galaxies, which have no regular shape.
stellation Sagittarius. These stars revolve at fantastic Observations by the Hubble and Spitzer space tele-
speeds. scopes and others have led astronomers to conclude
Our Stm is located about two-thirds of the distance that there are massive black hales in the centers of most
from the center of the galaxy to its outer rim. The Sun galaxies, including our own. A black hole is a theoreti-
and the rest of our solar systelll revolve arollild the cen- cal concentration of mass so great and so dense that not
ter of the galaxy, moving at a speed of about 150 miles even light can escape its gravitational pull. Astrono-
per second. Still, it takes about 225 million years for us to mers believe that black holes might be formed from
conlplete one circuit. inlploding remnants of dying supergiant stars. The
The galaxy looks much like a spiral nebula. In fact, masses of black holes are proportional to the masses of
earlier astronomers thought the Milky Way was a nebula. the galaxies containing them. The one in the center of
But modern telescopes clearly show that the galaxy is our galaxy may have a mass equal to 2 million tfiles
composed of billions of stars too far away to be distin- our Sun. III
guished as separate points of light. The Milky Way is best
seen on a clear summer night, running across the sky
from north to south. CRITICAL THINKING
As crowded as the stars in the Milky Way appear, we
see only a fraction of the actual number because of the 1. Research the various ways that astronomers and as-
huge amount of gas and dust fogging up the space be- trophysicists have of determining whether there are
tween the stars. Most of the stars in the center of the bodies such as planets in orbit around distant stars.
galactic swirl are thus blocked from view. 2. Research how cosmologists have determined the
All the stars in our galaxy can be placed in two dis- probable age of our tuUverse.
tinct groups. These groupings are called Populatioll I and
Populatioll II. TIle basis of these classifications is their lo-
cation in the galaxy. Population I stars are fowld in re- Study Guide Questions
gions where there is a great deal of dust and gas. These
are young stars that are still forming, growing, and 1. What are the stars?
adding mass. The stars in the neighborhood of our Sun 2. After the Sun, what star is closest to Earth?
belong to Population I, as do the open clusters. 3. A. What is the most common mlit of astronomical
Population II stars are older stars, located in regions distance?
essentially free of dust and gas. They have used up the B. hl nilles, what distance does it represent?