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What is the Hubble Sequence?

An irregular galaxy is... irregular. With no definitive shape or structure, these galaxies are often chaotic in appearance and are thought to make up 20-25% of all galaxies in the universe. In many cases they are thought to have started out as spiral or elliptical galaxies but were then gravitationally disrupted by a galactic collision/near miss, or by being gravitationally influenced by a larger nearby galaxy. This type of galaxy is classified into three groups: Irr 1, an irregular galaxy with some structure, closely related to spiral galaxies in that they often have disks and bulges, but without any sort...

Unlike the defined disks and spiraling arms of spiral galaxies, an Elliptical galaxy is one without very much structure. Having a rather bland appearance, elliptical galaxies (hence the name) take on a featureless ellipsoidal shape. This type of galaxy is the most abundant in the universe and can vary greatly in size, some dwarf elliptical galaxies can get as small as 10% the size of the Milky Way, while others such as M87 are identified as some of the largest galaxies in the universe with over ten trillion stars! Most ellipticals are composed of very old stars with a very thin interstellar medium leading...

A spiral galaxy is one that contains a central bulge surrounded by a rotating flattened disk of young, bright stars and interstellar medium. Due to density waves, this rotating material separates itself into two or more spiraling arms, slowing the rotation of stars and molecular gas and compressing it to form new stars. The central bulge of these galaxies, composed of old dim stars, is thought in most cases to contain a supermassive blackhole. Although never directly observed, the presence of these blackholes can be inferred by detecting their effect on nearby matter such as stars and interstellar medium. Spiral galaxies can be...

     As the title suggests, a supernova remnant is the remains of a supernova, a stellar explosion. An event caused by one of two ways, either a massive star's core suddenly collapses after running out of enough fuel to generate fusion energy, or when a white dwarf star accumulates too much matter from a star it's in orbit with, becoming unstable and exploding. What are left after these spectacular phenomena are supernova remnants, highly luminous structures expanding their stellar guts at as much as ten percent the speed of light, replenishing the interstellar medium and carrying with them elements such...