 A galaxy is made of billions of
stars, dust, and gas all held together by gravity. Galaxies are scattered
throughout the Universe. They vary greatly in size and shape. Not all
galaxies look alike.
Among the galaxies, there are
apparently three main categories, according to their appearance: the disk
galaxies (`cosmic frisbies' according to P. Murdin, D. Allen, and D.
Malin), consisting of a huge disk of star s and
interstellar matter, which may form interesting patterns, the elliptical
galaxies (`cosmic footballs') which are uniformly looking, ellipsoidal
agglomerations of stars, and the irregular galaxies (`cosmic misfits')
which cannot be integrated in this scheme.
Physically, it is not necessary so
clear (at least in the opinion of the present author) if this
classification is real, because there exist intermediate types even
between ellipticals and spirals, i.e. spiral
 galaxies often
have an ellipsoidally formed "bulge" which may be very luminous (as in
case of the Sombrero galaxy M104) or rather inconspicuous; some spirals
seem to lack this component at all. A heavy bulge is often connected with
the presence of a big ellipsoidal core. On the other hand, at least some
ellipticals seem to house a disk component also; ths most conspicuous
example of such a galaxy is probably Centaurus A (NGC 5128), a prominent
galaxy in the Southern hemisphere which is not a Messier object because of
its southern declination, but forms a group with the beautiful spiral M83.
Centaurus A is regarded as peculiar. One may speculate that e.g. the disk
around the center of M87, which is often regarded as the accretion disk
around the supermassive object in that galaxy's nucleus, may be a smaller
manifestation of the same disk phenomenon.
Focussing on the disk (or disk
dominated) galaxies, these often show beautiful and conspicuous patterns
in the form of spiral arms and/or luminous bars. These structures have
been a mystery for a long time, it was thought
 that there may be physically
different classes of disks (e.g., "normal" and "barred" disks), but now it
seems as if they are all the consequences of gravitational interactions
with neighboring galaxies. Encounters with neighbors cause inhomogenities
and un-symmetries in the gravitational field within the disk, which tends
to compress the gas in some regions. If the density of the gas in these
regions exceeds a certain critical value (which depends on parameters as
the temperature), star formation can take place, resulting in the
formation of red diffuse emission nebulae and blue clusters of hot young
stars, which slowly change their color to the yellow when they come to
age, and their hottest stars have disappeared (i.e., exploded as
supernovae).
The star forming regions tend to be
aligned along spiral arms, as the denser regions in the interstellar
matter apparently prefer to form such patterns. When getting older, they
sometimes stay conspicuous as yellowish "fossil arms", which can be traced
in several galaxies.
 Galaxies are classified,
according to their appearance, in the so-called Hubble scheme (after its
inventor, Edwin Powell Hubble; see e.g. our illustration of the Hubble
Scheme with Messier galaxies). This scheme defines the classes listed
above, i.e. spiral, elliptical and irregular galaxies, and is especially
interesting for spirals: Those with pronounced bar structures are called
"barred spirals" and classified "SB", while normal spirals are simply
called "S" or sometimes "SA"; some authors take "SAB" or "S(B)" for mixed
types. Spiral galaxies, "normal" and barred, with conspicuous bulges
(especially near their center) are classified "Sa" or "SBa", those which
have prominent bulges and pronounced arms are clssified "Sb" or "SBb", and
those which are dominated by the arms are "Sc" or "SBc". If the core and
bulge seems to be lacking, a galaxy is classified "Sd" or "SBd", and those
which have no pronounced core and show irregularities are classified as
"Sm"; these represent a type between disk and irregular
galaxies.
 Some of the galaxies, mostly
those who had no closer encounters for a longer period of time, and those
who have lost most of their interstellar matter for some reason, do not
show any conspicuous pattern within their disks; these are often called
"S0" or "lenticular" galaxies. Although they are disks, they can often
hardly be distinguished from ellipticals from their appearance, and have
often been misclassified in the past. This misclassification happened to
all the four Messier lenticulars in the past, and to many other galaxies
of this type.
When undergoing a heavy interaction,
or collision, with a massive neighbor, disk galaxies may be distorted very
peculiarly, and then are often classified as irregular; this is the case
for the only Messier irregular, M82.
All disk galaxies have a very
different appearance, depending from what direction they are seen, or
under which
 angle toward the line of
sight (to us) their disk is inclined. According to this situation, they
are either seen from their edge (or "edge-on"), or from near their
equatorial plane, as thin, flat, linear and elongated patches, often with
dusty structures along their equators, or almost from their poles so that
we can see their disks "face-on". Tom Polakis has featured some edge-on
galaxies.
Our Milky Way is one of the big and
more massive spiral galaxies, and is of Hubble type Sbc, or perhaps SBbc
if it should have a bar. It consists of about 200 billion stars, with our
own Sun being a fairly typical specimen. It is a fairly large spiral
galaxy and it has three main components: a disk, in which the solar system
resides, a central bulge at the core, and an all encompassing
halo.
The disk of the Milky Way has four
spiral arms and it is approximately 300pc thick and 30kpc in diameter. It
is made up predominantly of Population I stars which tend to be blue and
are reasonably young, spanning an age
 range between a million and
ten billion years.
The bulge, at the centre of the
galaxy, is a flattened spheroid of dimension 1kpc by 6kpc. This is a high
density region where Population II stars predominate---stars which tend
toward red and are very old, about 10 billion years. There is growing
evidence for a very massive black hole at its centre.
The halo, which is a diffuse
spherical region, surrounds the disk. It has a low density of old stars
mainly in globular clusters (these consist of between 10,000 - 1,000,000
stars).The halo is believed to be composed mainly of dark matter which may
extend well beyond the edge of the disk.
|
 |
|
