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
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
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.