Galaxies are objects like our galaxy, the Milky Way.They are gravitationally bound systems of stars, stellar remnants,
interstellar gas and dust, and dark matter. The nearest major galaxy to the Milky Way is the Andromeda Galaxy (Messier 31),
which is about 2.5 Mio light years away. Its one of the most distant objects that can be seen with the naked eye.
Galaxies can be subdiveded to the following three classes:
Spiral galaxies consist of a flat, rotating disc containing stars, gas and dust, and a central concentration of stars
known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters.
Spiral galaxies are named for the spiral structures that extend from the center into the galactic disc.
The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young,
hot OB stars that inhabit them.
An elliptical galaxy is a type of galaxy having an approximately ellipsoidal shape and a smooth,
nearly featureless brightness profile. Unlike flat spiral galaxies with organization and structure,
they are more three-dimensional, without much structure, and their stars are in somewhat random orbits around the center.
An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy.
The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence,
and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure.
A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own
Milky Way's 200-400 billion stars. An example is the LMC, an irregular dwarf galaxy in the neighborhood of the Milky Way.
Most galaxies are nor isolatet, but form groups and clusters. The Milky Wayis a member of thes so called “Local Group” -
also M 31 and M 33 are members of this galaxy group.
A famous galaxy cluster is the “Virgo Cluster” which consits of maby more than 2000 galaxies.