GALAXY...
Galaxies take on many shapes, from huge round clouds of stars to intricate whirlpool shapes with defined arms full of stars. They may stretch from anywhere between a few thousand light-years wide, to more than 100,000 light-years across. Milky Way itself is just one among hundreds of billions of other galaxies. The Milky Way is part of a group of galaxies named the Local Group, which also contains the Andromeda (M31) Galaxy and held together by gravity, and it is this same force that in turn links clusters of galaxies together form to Virgo Supercluster.
Types of Galaxies...
There are four main types of galaxies: spiral, elliptical, lenticular, and irregular, as shown below. Edwin Hubble prefixed a number between 0 and 7 to indicate how far they deviate from a perfect sphere (0 being a perfect circle).
Spiral Galaxy
Spiral galaxies account for around a third of the galaxies in the nearby Universe. Their central core of old stars is surrounded by spiral arms full of bright young stars. These galaxies are usually rich in regions of star formation, as they contain lots of gas and dust. Barred spirals, such as NGC 6217 , have a "bar" of stars across their central regions.
Lenticular Galaxy
Lenticular galaxies typically possess a large, roughly spherical core of old stars surrounded by a disc of gas and stars, giving them an overall lens-like shape. The disc consists of stars and gas, as found in spiral galaxies, but ful these lenticulars - such as one in the Coma Cluster - do not have spiral arms, young stars, or dusty, glowing star-forming nebulae.
Elliptical Galaxy
Elliptical galaxies appear as large "blobs" of stars and are usually full of old red and yellow stars. Unlike spiral galaxies, they contain hardly any star-forming nebulae. The largest of these galaxies are nearly perfect spheres. In M 87, a powerful jet is being ejected by a supermassive black hole at the centre of the giant elliptical galaxy.
Irregular Galaxy
Irregular galaxies do not have a distinct shape. They typically contain lots of gas, dust, and hot, blue stars, but do not take any particular form. Some show signs of structure, such as central bars, and they often feature vast, pink, star-forming nebulae. NGC 2366 is a dwarf galaxy with a star-forming region 10 times bigger than that of the much-larger Milky Way.