The giant spiral assemblage of several billion stars that is
home to the Sun and its family of planets, including Earth, is
only one of the billions of star systems known to exist in the
universe (see EXTRAGALACTIC SYSTEMS).  Because it is our star
system, however, it is usually called the Galaxy.  The Milky
Way, another name for it, is the portion visible to the naked
eye.
 
In the 20th century, astronomers determined that the Galaxy is
a disk-shaped object, far larger than most of the galaxies in
its neighborhood (see LOCAL GROUP OF GALAXIES).  Its visible
disk is about 100,000 LIGHT YEARS wide but only about 2,000
light-years thick.  A halo of other materials, including star
clusters, surrounds the disk.
 
The total mass of the Galaxy can be measured by studying the
motions of individual stars and clouds of hydrogen gas in
different parts of the galaxy and by applying CELESTIAL
MECHANICS to calculate a total mass that will account for the
observed motions.  The mass can also be determined from the
motions of the Galaxy's small satellite galaxies, especially
the nearby dwarf elliptical galaxies, and globular clusters.
Recent computations by both methods agree that the Galaxy's
mass is possibly 1,000 to 2,000 billion times the mass of the
Sun.  As the Sun's mass is about average for a star in the
Galaxy, the total number of stars must also be of this order.
Most of these stars are invisible from the Earth, however,
because the solar system lies in the dense plane of the Galaxy, 
where interstellar dust obscures all but its nearer parts.
 
The area around the Galaxy is populated by about twenty
galaxies that make up a small cluster called the local group.
Most of these neighbors, such as the MAGELLANIC CLOUDS, are far 
smaller and less luminous than the Galaxy.  The only other
large galaxy is the ANDROMEDA GALAXY, which is more than 2
million light-years away, is somewhat larger and more luminous
than our own galaxy and is visible to the naked eye.  The
Andromeda spiral differs slightly in shape from our galaxy,
having a larger smooth, amorphous central bulge and spiral arms 
that are less patchy.
The Sun lies a little more than 30,000 light-years from the
center of the Galaxy.  From our vantage point, the Galaxy
appears thicker toward its center, in the direction of the
constellation Sagittarius, and somewhat thinner in other
directions.  However, because of the obscuration by dust, which 
limits our view in all directions, it is difficult to realize
from observation that we are not near the center of the system.

DETERMINATION OF GALACTIC STRUCTURE
 
Until the 1920s it was thought that the system of stars
outlined by the Milky Way was the entire universe;  early
attempts to understand the structure of the Galaxy were thought 
of as studies of the universe itself.  In 1784, Sir William
HERSCHEL attempted to determine the structure of the Milky Way
(he referred to his work as exploring the "construction of the
heavens") by making extensive star counts through telescopes,
recording the number of stars in various directions, and
plotting the results in a series of maps.  Assuming that all
stars had approximately the same brightness and that the Galaxy 
was uniformly dense, Herschel calculated the extent of the
system of stars in various directions and concluded that we
live in the central region of a flat, round arrangement of
stars that extends far along the Milky Way.
 
A much more accurate view of the Galaxy resulted from Harlow
Shapley's studies of globular clusters, begun in 1914.  Shapley 
realized that dust obscured large numbers of stars along the
Milky Way and discovered that making star counts was not as
good a way of gauging the size of the Galaxy as determining the 
size of the system of globular clusters that lie above and
below the obscuration of the Milky Way plane.  Using this
method, he determined that the Galaxy is about ten times larger 
than previously thought, and that the Sun is located at a
considerable distance from the center.  He found that the
clusters make up a thin, spherical halo that surrounds a bright  
flat disk.  The detailed structure of the disk was difficult to 
discover because of the dust, but several astronomers,
especially Jacobus C. Kapteyn in Holland and Bart J. Bok in
the United States, pursued the task of plotting the
distribution of stars to try to find a pattern, particularly a
pattern of spiral arms like those seen in many other galaxies.
Only fragments of structure emerged, however, and they were
found to resemble scraps of spiral arms only when stellar
associations were discovered during the late 1940s.
 
An important breakthrough occurred in 1951, when Harvard
scientists Harold Ewen and Edward Purcell made the first radio
detection of the 21-cm emission line of neutral hydrogen gas in 
the Milky Way.  By 1954, Dutch and Australian radio astronomers  
were ready to assemble a radio map of the Galaxy.  Since radio
waves pass through the dust unimpeded, this map was far more
accurate than those based on visual observations.  The result
clearly showed a complex and beautiful spiral structure, very
much like that of the giant galaxy Messier 101 or the Whirlpool 
Galaxy, Messier 51.
 
Our present view of the Galaxy is based on highly detailed
radio maps of neutral atomic hydrogen gas and other sources,
including hot gas clouds and gas-dust complexes that emit
radiation from various molecules and parts of molecules, such
as water, carbon monoxide, and hydroxyl.  The Galaxy consists
of a slightly warped, scalloped disk of heavy-element-rich
stars, gas clouds, and dust, surrounded by a tenuous spherical
halo of old, heavy-element-poor stars and star clusters.  The
halo extends to about 85,000 light-years from the center, and
is enveloped by the corona, which reaches to at least 200,000
light-years.
 
In recent years astronomers have begun to examine the core of
the Milky Way in other wavelengths (see INFRARED ASTRONOMY;
RADIO ASTRONOMY;  X-RAY ASTRONOMY).  Infrared studies have
revealed a small number of fast-moving red supergiant stars
within 5 light-years of the center.  Strong radio and X-ray
emissions from the same area suggest that a BLACK HOLE may
exist in that region, and that it may be generating the
extremely hot gases spiralling around the galactic center at
speeds of up to 700,000 kilometers per hour.  Heavy-metal
synthesis, which accompanies the formation of new stars, is
also thought to occur in that region.  Farther out are dramatic 
radiowave-emitting filaments perpendicular to the galactic
plane;  these arcs of matter, approximately 150 light-years
long, suggest that a huge magnetic field exists around the
galactic core.  Unrelated to these arcs are three bizarre,
threadlike structures, uniformly bright and about one
light-year wide and more than 100 light-years long, that cut
across the central galactic regions.  These threads remain
unexplained but may be magnetic in structure.
 
COMPOSITION OF THE GALAXY
 
Our galaxy, like most well-studied spiral galaxies, chiefly
consists of stars, gas, and dust.  A census of the visible part 
of the Galaxy indicates that most of the mass is in the stars,
with only about 2% in the form of gas (mostly hydrogen) and
about 0.01% in the form of dust.
 
The stars of the Galaxy have been divided into two kinds,
called Population I and Population II.  Population I stars are
prevalent in the spiral arms and include stars of all ages,
from over ten billion years to only a few hundred thousand
years old.  They all contain elements heavier than helium, in
amounts comparable to those found in the Sun.  Population II
stars, on the other hand, are found in the bulge around the
galactic nucleus and in the spherical halo, which includes both 
the thin envelope of stars surrounding the disk and the
globular clusters.  All Population II stars are approximately
12 to 15 billion years old, and all are deficient in their
amounts of heavy elements, some by factors of more than a
hundred.  These are the Galaxy's oldest inhabitants, and they
are frequently offered as evidence that the Galaxy itself is 15 
billion years old.
 
The total amount of stars, gas, and dust in the Galaxy does not 
quite equal the total measured mass.  Although there has been
considerable recent controversy about the matter, it may be
necessary to account for the difference by suggesting that the
Galaxy contains matter in some undetected, invisible form, such 
as molecular hydrogen, black holes (collapsed and invisible
stars), or meteoroids.  Recent investigations suggest that the
"missing mass" of the Galaxy might be found in the corona.  The 
composition of the corona is unknown, but it is estimated that
it contains between 100 and 200 billion solar masses.
 
DYNAMICS OF THE GALAXY
 
In the early 20th century, as the mystery of the structure of
the Galaxy was being unraveled, the motions of stars were also
being determined.  Astronomers recorded the slow perceived
position changes of stars (proper motion) and their motions
toward or away from Earth (radial velocities);  the latter are
easily measured by the Doppler shift of the stars' spectral
lines.  In 1904, Kapteyn found that stars did not move at
random but in two streams flowing in opposite directions along
the Milky Way, one converging in the constellation of Orion and 
the other converging 180 deg away, in Scutum.  The Swedish
astronomer Bertil Lindblad showed that this streaming motion is 
simply the result of the rotation of the Galaxy.  Stars
traveling in nearly circular orbits around the galactic center
with the Sun will have larger motions relative to the Sun
either toward or away from the center--depending on whether
they are approaching the nearest or farthest points in their
elliptical orbits--than in the direction of motion.  Therefore, 
we preferentially see motions towards us or away from us in
these directions.
 
In 1927, Jan H. Oort of the Netherlands showed that the
motions of stars in different parts of the Milky Way could be
used to derive the properties of the rotation of the Galaxy,
including the speed of the Sun through space.  When modern
values are used in Oort's equations, it is found that the Sun's 
velocity is approximately 250 km/sec in its orbit around the
galactic center.  The velocity for stars at larger distances
from the center is smaller;  in the inner part of the Galaxy
the velocities are similar to those of solid bodies.  These
velocity differences cause differential rotation in the disk,
and they may be the primary cause of the spiral shape of the
arms (and also, incidentally, of the rotation of the bodies in
the solar system, including the Earth).
 
The dynamics of the spiral arms are still only imperfectly
understood.  Differential rotation will make spiral arms out of 
almost any structural feature in a galaxy, but the arms should
only last a fraction of the age of the galaxy.  It would lead
to a rapid winding-up of the arms in the 50 or so rotations
that have occurred since the Galaxy was formed.  One possible
explanation is that the arms are not constant physical
entities, but are waves of high star density moving more slowly 
than the stars.  Stars slow down and pile up temporarily in an
arm because of its higher gravitational field, then pass out of 
the arm and proceed until they encounter the next arm.
 
Determinations have been made of the Galaxy's movement as a
whole, relative to the rest of the universe.  For example,
high-altitude measurements were made of the universe's
BACKGROUND RADIATION, the residual glow of the so-called "big
bang" that is assumed to have occurred in the first moments of
the universe (see COSMOLOGY).  The measurements indicated that
the Galaxy is moving, relative to the universe, in the same
direction as the constellation Leo lies relative to the Earth,
and with a velocity of more than 600 km/sec (373 mi/sec).  The
galaxy is also moving at about 100 km/sec (62 mi/sec), relative 
to the center of mass of the local group of galaxies.  The
local group, in turn, is moving at a comparable velocity
relative to the supercluster of galaxies to which it belongs.
Some astronomers have proposed that the flow is moving toward a 
huge, distant region of space that has been called the Great
Attractor, but others have since disputed this theoretical
structure.
 
RADIATION FROM THE GALAXY
 
From a distance, the Galaxy could be detected by a wide variety 
of means, since it emits radiation at almost all possible
wavelengths:  it is optically bright, emitting the equivalent
of approximately 200 billion Suns in optical (visible)
radiation;  it is a strong source of radio-line emission,
especially from its large mass of neutral hydrogen;  it is a
source of radio continuum noise, both from the hot gas clouds
in its arms and from its dense, hot nucleus;  its huge, dark,
cool complexes of dust and gas emit infrared emission;  it
shines in the ultraviolet region of the spectrum because of its 
large number of very hot, recently formed stars;  and it gives
off X radiation from many sources.

Paul W. Hodge
Bibliography:  Bok, Bart J.  and Priscilla F., The Milky Way,
5th ed.  (1981);  Catchpole, R. M., "A Window on Our Galaxy's
Core," Sky & Telescope, February 1988;  Corwin, H. G., and
Bottinelli, L., eds., The World of Galaxies (1990);  Hodge,
Paul W., Galaxies (1986);  Shuter, W. L., Kinematics, Dynamics
and Structure of the Milky Way (1983).

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