Galaxies (Important Facts) {Part-2}

A GALAXY IS A HUGE MASS OF STARS, nebulae, and interstellar material. The smallest galaxies contain about 100,000 stars, while the largest galaxies contain up to 3,000 billion stars.

There are 3 main types of galaxy, classified according to their shape:

(i) Elliptical - Oval shaped

          The Sombrero Galaxy 

(ii) Spiral - Having arms spiralling outwards from the central bulge.

This is the galaxy in which we are living.

     Milky Way Galaxy

(iii) Irregular - Having no obvious shape.

       Antennae Galaxy 

Sometimes,the shape of a galaxy is distorted by a collision with another galaxy.
Quasars (quasi-stellar objects) are thought to be galactic nuclei but are so far away that their exact nature is still uncertain.They are compact, highly luminous objects in the outer reaches of the known Universe: while the furthest known "ordinary" galaxies are about 12 billion light years away,the furthest known quasar is about 13 billion light years away.

  Quasar

Active galaxies such as Seyfert galaxies and Radio galaxies, emit intense radiation.In a Seyfert galaxy, this radiation comes from the galactic nucleus, in a radio galaxy, it also comes from huge lobes on either side of the galaxy.

          Seyfert galaxy NGC 1097

       Radio Galaxy Centaurus A

The radiation from Active galaxies and Quasars is thought to be caused by material falling into central black holes.