While observing the night sky in a low bortle, you may be able to see a fuzzy band of light stretching across the sky. What you are looking at is the plane of our galaxy- The Milky Way. Our galaxy is home to an estimated 400 billion stars, with most younger stars lying in the centre of the plane of the galaxy- in the spiral arms where most of the galaxy's dust and gas, which clump together to create sites of star formation, is concentrated; relatively older stars are generally found in the outer areas of the plane- though some can be found in 'halos' of globular clusters around the outside of the Milky Way.

The reason as to why there are so many stars is that the radius of the galaxy is around 15kpc, and contains a central nuclear bulge with evidence of a black hole (Saggitarius A) located right in the middle. This is the same black hole that everything in the galaxy, including the solar system, orbits. The milky way is one of the 50 members of The Local Group all held togather by mutual gravitation- this group stretches across approximately 3Mpc.

 

By far the most interesting glaxies are active galaxies- these have an acive galactic nucleus (AGN), containing a supermassive black hole at the centre! Fun fact: The estimated mass of these black holes are about 10^6 to 10^9 times greater than black holes formed during a supernova. Anyway, the nucleus has such a huge gravitational influence on nearby stars, that it forms an accretion disc in which stellar matter accelerates into the black hole, and galactic jets of electrons and positrons are emmitted into two beams almost at the speed of light. There are three types of active galaxies- Seyfert galaxies, quasars (these are really cool), and blazars.

Seyfert galaxies were discovered in 1943 by astronomer Carl Seyfert. These have bright nuclei and emit strongly in the infrared, ultraviolet, and X-ray regions. Quasars were discovered in 1964 by Allen Sandage, and also emit strongly in the UV and X-ray, but also emit radio waves. Quasars appear similar to a star on images and often have high redshift. One thing that is special about quasars is that they can emit thousands of times the energy output of our galaxy- making them extremely bright, and one of the most powerful objects in the whole universe. Blazars are compacted quasars which have a high speed and a high energy, but their galactic jets are pointing at us.