Page 385 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch14_351-376.indd Page 362 9/3/10 6:16 PM user-f465
tiL12214_ch14_351-376.indd Page 362 9/3/10 6:16 PM user-f465 /Users/user-f465/Desktop

see more galaxies than individual stars in any direction, each
Rotation axis Beamed
radiation galaxy with its own structure of billions of stars. Yet there are
similarities that point to a common origin. Some of the similari-
+ –
Magnetic ties and associations of stars will be introduced in this section
field lines along with the Milky Way galaxy, the vast, flat, spiraling arms
of stars, gas, and dust where the Sun is located (Figure 14.14).
Magnetic axis
+
+
+ THE MILKY WAY GALAXY
Away from city lights, you can clearly see the faint, luminous
band of the Milky Way galaxy on a moonless night. Through a
ticles
telescope or a good pair of binoculars, you can see that the lu-
minous band is made up of countless numbers of stars. You may
– also be able to see the faint glow of nebulae: concentrations of gas
– and dust. There are dark regions in the Milky Way that also give
+
an impression of something blocking starlight, such as dust. You
+ can also see small groups of stars called galactic clusters. Galactic
–
– clusters are gravitationally bound subgroups of as many as
1,000 stars that move together within the Milky Way. Other
clusters are more symmetrical and tightly packed, containing as
Energetic
charged particles many as a million stars, and are known as globular clusters.
Viewed from a distance in space, the Milky Way would
appear to be a huge, flattened cloud of spiral arms radiating out
FIGURE 14.13 The magnetic axis of the pulsar is inclined from the center. There are three distinct parts: (1) the spherical
with respect to the rotation axis. Rapidly moving electrons in the concentration of stars at the center of the disk called the galactic
regions near the magnetic poles emit radiation in a beam pointed
nucleus; (2) the rotating galactic disk, which contains most of
outward. When the beam sweeps past Earth, a pulse is detected.
the bright, blue stars along with much dust and gas; and (3) a
spherical galactic halo, which contains some 150 globular clus-
ters located outside the galactic disk (Figure 14.15). The Sun is
nothing. Evidence for the existence of a black hole is sought by located in one of the arms of the galactic disk, some 25,000 to
studying X rays that would be given off by matter as it is acceler- 30,000 light-years from the center. The galactic disk rotates, and
ated into a black hole. the Sun completes one full rotation every 200 million years.
Evidence of the existence of a black hole has been provided The diameter of the galactic disk is about 100,000 light-
by photographs from the Hubble Space Telescope. Hubble years. Yet in spite of the 100 billion stars in the Milky Way, it
pictured a disk of gas only about 60 light-years out from the is mostly full of emptiness. By way of analogy, imagine reduc-
center of a galaxy (M87), moving at more than 1.6 million km/h ing the size of the Milky Way disk until stars like the Sun were
(about 1 million mi/h). The only known possible explanation reduced to the size of tennis balls. The distance between two of
for such a massive disk of gas moving with this velocity at the these tennis-ball-size stars would now compare to the distance
distance observed would require the presence of a 1 to 2 billion across the state of Texas. The space between the stars is not actu-
solar-mass black hole. This gas disk could only be resolved by ally empty since it contains a thin concentration of gas, dust, and
the Hubble Space Telescope, so this telescope has provided the molecules of chemical compounds. The gas particles outnumber
12
first observational evidence of a black hole. the dust particles about 10 to 1. The gas is mostly hydrogen,
and the dust is mostly solid iron, carbon, and silicon compounds.
14.3 GALAXIES Over 40 different chemical molecules have been discovered in
the space between the stars, including many organic molecules.
Stars are associated with other stars on many different levels, Some nebulae consist of clouds of molecules with a maximum
6
3
from double stars that orbit a common center of mass, to groups density of about 10 molecules/cm . The gas, dust, and chemical
of tens or hundreds of stars that have gravitational links and a compounds make up part of the mass of the galactic disk, and
common origin, to the billions and billions of stars that form the stars make up the remainder. The gas plays an important role
the basic unit of the universe, a galaxy. The Sun is but one of in the formation of new stars, and the dust and chemical com-
an estimated 100 billion stars that are held together by gravita- pounds play an important role in the formation of planets.
tional attraction in the Milky Way galaxy. The numbers of stars
and the vastness of the Milky Way galaxy alone seem almost
beyond comprehension, but there is more to come. The Milky OTHER GALAXIES
Way is but one of billions of galaxies that are associated with Outside the Milky Way is a vast expanse of emptiness, lacking
other galaxies in clusters, and these clusters are associated with even the few molecules of gas and dust spread thinly through the
one another in superclusters. Through a large telescope, you can galactic nucleus. There is only the light from faraway galaxies

362 CHAPTER 14 The Universe 14-12