A spiral galaxy like the Milky Way has 3 basic components to its visible matter:
(1) the disk (containing the spiral arms), (2) the halo, and
(3) the nucleus or central
bulge. These components are indicated schematically in the adjacent figure.
The halo and the nucleus are also referred to collectively as the
component of the galaxy since they have an approximately spherical
distribution with respect to the center of the galaxy.
In addition to these visible
components, the galaxy also contains at least three other components that are
"invisible": the galactic magnetic field, charged particles trapped in the galactic
magnetic field, and a halo of "dark matter" that is of unknown composition but that
makes itself felt by its gravitational influence on the visible matter.
The Disk of the Galaxy
Most of the gas and dust of the Milky Way is contained in the disk. This
material between the stars is often
interstellar medium. The gas is primarily
hydrogen and helium, and the dust makes many regions of the disk opaque. For
example, we cannot see the center of the galaxy in visible light because of
intervening dust clouds in the disk.
The adjacent image shows the nearby Andromeda Galaxy, which also has the Messier designation
M31. It is a spiral galaxy much like our own, and the dark dust lanes between the spiral arms are
quite prominent in this image
The disk is not a clearly defined thing, because it depends on what
objects that we use to define it, and because portions of it are blocked from our
view at visible wavelengths. In addition, as one goes vertically in the disk
(perpendicular to the plane),
there is no sharp boundary for the disk.
Rather, the density of stars gradually gets smaller.
Therefore, astronomers sometimes refer to more than one disk for the
galaxy. For example, the "disk" defined by the hottest young stars and their
associated dust clouds is about 5-10 times thinner than the "disk" defined by older
stars like the Sun, which is approximately 1000 parsecs thick.
The disk is quite prominent in our own galaxy and in other spiral galaxies because
which contain many hot young stars and therefore is luminous.
These younger stars are often contained in associations, which are groups
of typically 10-100 young stars that are moving together through space because they
have been recently formed from the same nebula, and
open clusters, which contain 100-1000 stars and
are more strongly bound together gravitationally than associations.
The Halo of the Galaxy
The halo of the galaxy is rather spherical in shape and contains little gas, dust, or
star formation. It is difficult to measure precisely, but the halo appears to extend
beyond the disk. The clusters found in the halo are
globular clusters (approximately 100 of
them), so the halo is population II, and contains very old stars. Dating of globular
clusters by their turnoff points indicates that they may be as old as 15 billion
years and are the oldest components of the galaxy. This implies that the galaxy
at least 15 billion years old.
The stars in the disk of the galaxy are on nearly circular orbits lying in the
plane of the galaxy, but the stars of the halo are on more elliptical orbits that
are randomly oriented. Thus, the halo stars pass through the disk and the nucleus
of the galaxy, but spend the majority of the time far above or far below the plane
of the galaxy.
The Nucleus of the Galaxy
The nuclear bulge or core contains the highest density of stars in the
galaxy. Although some hot young stars may be found in the nucleus, the primary
population of stars there is similar to the old stars found in the halo. Although
at visible wavelengths
the core of the galaxy is obscured by dust, gas, and stars, we
can observe it at other wavelengths and there is some evidence that violent processes
may be taking place there. As we shall discuss later, many galaxies may contain very
massive black holes
at their cores, and our own galaxy may be no exception.
The Galactic Magnetic Field and Cosmic Rays
The disk of the galaxy is permeated by a magnetic field. This field is weak, being
only about 1/50,000 of the strength of the Earth's magnetic field at the surface, but
it influences the motion of charged particles in the galaxy. One important
consequence of the magnetic field is that it can bend the path of and even trap the
high-energy charged particles that we call cosmic rays. Thus the galaxy is filled
with cosmic rays and because of the effect of the magnetic field we cannot tell with
certainty where they come from, though the strongest arguments favor supernova
explosions for their source.
The Dark Matter Halo
As we shall discuss in more detail later, there is abundant evidence that the vast
majority of matter in the Universe does not show up in our telescopes but we can
infer its presence by its gravitational influence. We refer to this as
and at present we do not know
what it is, though there is fairly strong evidence that it is not the ordinary matter
of stars, gas, dust, and planets. In our own galaxy, the observed
rotation of the stars and gas clouds
indicates that the visible
matter is surrounded by a halo of this dark matter containing the major portion of
the total galaxy mass and extending very far beyond the visible matter.
Some indirect means suggest that the dark matter halo may extend as far as 100,000
parsecs from the center of the galaxy.