Galaxies

Galaxies:
A galaxy is a collect of stars, gas and dust bound together by their common gravitational pull.
Galaxies range from 10,000 to 200,000 light-years in size and between 109 and 1014 solar
luminosities in brightness.
The discovery of `nebula', fuzzy objects in the sky that were not planets, comets or stars, is
attributed to Charles Messier in the late 1700's. His collection of 103 objects is the first galaxy
catalog. Herschel (1792-1871) used a large reflecting telescope to produce the first General
Catalog of galaxies.

Before photographic plates, galaxies were drawn by hand by the astronomer.

Galaxies have certain features in common. Gravity holds the billions of stars together, and the
densest region is in the center, called a core or bulge. Some galaxies have spiral or pinwheel arms.
All galaxies have a faint outer region or envelope and a mysterious dark matter halo.

The contents of galaxies vary from galaxy type to galaxy type, and with time.
Almost all galaxy types can be found in groups or clusters. Many clusters of galaxies have a large,
supergiant galaxy at its center which has grow by cannibalizing its neighbors. Our solar system is
located in outer regions of a spiral galaxy we call the Milky Way. The nearest neighbor galaxy is
Andromeda Galaxy (M31).

Above is a 3D plot of most of the Local Group of galaxies, the population of galaxies within 1000
kpc if the Milky Way. Clustering of dwarf satellite galaxies around the great Milky Way and
Andromeda spirals can be seen.
Hubble sequence :
Almost all current systems of galaxy classification are outgrowths of the initial scheme proposed
by American astronomer Edwin Hubble in 1926. In Hubble's scheme, which is based on the
optical appearance of galaxy images on photographic plates, galaxies are divided into three
general classes: ellipticals, spirals, and irregulars.