A galaxy is a massive,
gravitationally bound system that consists of stars and
stellar remnants, an interstellar gas and dust, and an
important but poorly understood component tentatively
dubbed dark matter.
Typical galaxies range from
dwarfs with as few as ten million stars up to giants
with a hundred trillion stars, all orbiting the galaxy's
center of mass. Galaxies may contain many multiple star
systems, star clusters, and various interstellar clouds.
The Sun is one of the stars in the Milky Way galaxy; the
Solar System includes the Earth and all the other
objects that orbit the Sun.
Galaxies are
categorized according to their apparent shape. A common
form is the elliptical galaxy, which has an
ellipse-shaped light profile. Spiral galaxies are
disk-shaped with dusty, curving arms. Galaxies with
irregular or unusual shapes are known as irregular
galaxies, and typically result from disruption by the
gravitational pull of neighboring galaxies. Such
interactions between nearby galaxies, which may
ultimately result in galaxies merging, or colliding
galaxies.
Although it is not yet well understood,
dark matter appears to account for around 90% of the
mass of most galaxies. Observational data suggests that
supermassive black holes may exist at the center of
many, if not all, galaxies.
There are probably
more than 170 billion galaxies in the observable
universe. Most galaxies are 1,000 to 100,000 parsecs in
diameter and are usually separated by distances on the
order of millions of parsecs, or megaparsecs.
Other less known galaxy types are Peculiar Galaxies,
Lenticular Galaxies, and Dwarf Galaxies.
Peculiar galaxies
are galactic formations that develop unusual properties
due to tidal interactions with other galaxies. An
example of this is the ring galaxy, which possesses a
ring-like structure of stars and interstellar medium
surrounding a bare core. A ring galaxy is thought to
occur when a smaller galaxy passes through the core of a
spiral galaxy.
A
lenticular galaxy is an intermediate form that
has properties of both elliptical and spiral galaxies.
These are categorized as Hubble type S0, and they
possess ill-defined spiral arms with an elliptical halo
of stars.
Dwarf
Galaxies - Despite the prominence of large
elliptical and spiral galaxies, most galaxies in the
universe appear to be dwarf galaxies. These galaxies are
relatively small when compared with other galactic
formations, being about one hundredth the size of the
Milky Way, containing only a few billion stars.
Ultra-compact dwarf galaxies have recently bee`fdn
discovered that are only 100 parsecs across.
Information
obtained from the Wikipedia web site. Wikipedia the free
encyclopedia.
