Last modified on 26 June 2003.
Galaxies are entities which consist of stars, interstellar gas,
and dust. They are clearly separate from other such entities in
their composition and in their internal kinematics. Some galaxies
have many young stars while others are predominantly old. Some
galaxies have lots of gas and/or dust while others have only
little. The larger the galaxy is the more variation there usually
is. The smallest galaxies have only a hundred thousand stars while
the largest ones have thousands of billions of stars. There are
different types of galaxies: elliptical, lenticular, spiral,
irregular, and peculiar galaxies.
Elliptical galaxies: Typically elliptical galaxies have
ellipsoidal shape, little interstellar matter, no recent star
forming activity, and, thus, very old stars. Most of the galaxies
Lenticular galaxies: Lenticular galaxies are intermediate
between elliptical and spiral galaxies, having a shape of a
Spiral galaxies: The most striking feature of the spiral
galaxies are their spiral arms. Spiral galaxies have several
components: a central bulge, a disc, spiral arms, and a halo. The
central bulge is a spherical area in the centre of the galaxy where
there are a lot of stars. A disc is a flat, thin circular structure
that contains most of the interstellar matter and young stars of
the spiral galaxy. Spiral arms are (thought to be) density waves
that compress the matter of the disc into a spiral shape around the
centre of the galaxy. The halo is a large spherical region around
the galaxy where the density of matter is very low. The halo stars
are the oldest of the galaxy. Sometimes there is also a central bar
in a spiral galaxy. The bar extends through the bulge and the
spiral arms start from the ends of the bar.
Irregular galaxies: All the galaxies that do not belong
to any other class are irregular galaxies. They have an irregular
shape and often show signs of a recent gravitational interaction
with other galaxies.
Peculiar galaxies: Some galaxies are very peculiar. They
don't fit in the Hubble classification of
galaxies and are unusually active or interact closely with nearby
See also a table of galaxy classes and types.
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Rami T. F. Rekola