The word galaxy is derived from the Greek term galaxias, which means “milky circle” and is a collection of solar systems, stars, planets, moons, black holes, asteroids, dust and nebulae (a cloud of gas and dust) held together by gravity.
They can be small, containing only a million stars or large with more than a trillion stars. The current estimate is that there are over 100 billion galaxies in the known universe.
Galaxies are considered to have been created from irregular large clouds of helium and hydrogen, the gases created in the first few seconds of the birth of the universe.
Sections of the cloud were thought to be denser than others and because of this; gravity caused them to collapse and then start to cool. Pieces of the collapsing cloud then formed into even smaller denser regions which created the first stars.
When some of the earlier massive stars reached the end of their life they exploded heating the surrounding gas which slowed the collapse of the galaxy clouds. These events also produced heavier elements such as carbon and nitrogen into the galaxy clouds. The cycle of collapse, the formation of stars, along with slowing down eventually formed balanced and stable galaxies.
“Newer” stars have a different metal composition than older stars since they were created later in a galaxies evolution.
There are two theories as to how elliptical and spiral galaxies were formed:
Spiral: Firstly, that the spin of the collapse of the cloud forming the galaxy created its shape and secondly, that they were created due to a pronounced spin which got even faster as the cloud collapsed.
Elliptical: Firstly, that these galaxies were formed from clouds which did not have the spin of spiral galaxies, so formed a more rounded structure and secondly, that they were formed by collisions of spiral galaxies.
The second elliptical formation theory is supported by the suggestion that in the early formation of the universe galaxies were closer together than they are now so collisions were probably common and also that these galaxies occur in areas of galaxy clusters where collisions are more likely to occur.
Elliptical galaxies also do not have as much interstellar gas compared to spiral galaxies which suggests the collisions of spirals ignited much of the gases turning them into stars.
Types of Galaxies
There are three main types of galaxies: Elliptical, Irregular and Spiral.
Elliptical galaxies are shaped like a spheroid. Looking at one from earth we only see two of their three dimensions and they resemble oval shaped discs.
Their surface brightness decreases as you move out from their centre. They are classified according to how much their elongation corresponds to a perfect circle. This is known as their ellipiticity. The larger the number assigned the more elliptical a galaxy is.
The classification scale ranges from E0 to E7. A classification of E0 means that the galaxy appears to be a perfect circle, and E7 would indicate an extremely flattened galaxy.
Irregular galaxies are galaxies with no definite structure and are considered to be formed by galaxies colliding or coming close enough to one another so that their gravitational forces interact. It is also thought that they may be young galaxies that have not yet reached a symmetrical state.
Irregular galaxies by no regular or symmetrical structure and are divided into two groups. The first group, Type I irregular, are regions of hydrogen gas with many young stars and a luminous nebulae.
Despite the name, Type I irregulars do display some form of structure and are most closely related to spirals, having discs and bumps. They do not however show any signs of having a spiral structure. These galaxies can be regarded as primal as they are lacking in “heavy” elements (elements with metallic properties such as carbon and iron).
The second group, Type II irregular, are those with large amounts of dust created by to two or more galaxies colliding, merging or interacting gravitationally. This dust causes most of the light from their stars to be blocked. They can be formed in a number of ways; One of the most common being the gravitational interaction or collision with another nearby galaxy.
Spiral galaxies have three main parts; a bulge, a disc and a halo. The bulge is spherical and is found in the centre of the galaxy and normally contains older stars. The disc consists of dust, gas and younger stars and forms arm like structures. The halo is a loose, spherical structure located around the bulge and part of the disc, and contains clusters of stars called globular clusters.
Spiral galaxies are also classified into two groups; barred and ordinary, according to how tightly their arms are wound around the structure.
The barred group, which has a bar of material running through the nucleus where the arms emerge from, are identified by the letters and the ordinary group, which are normal spirals whose arms originate directly from the nucleus or bulge.
Another type of galaxy is the S0 galaxy (also known as a Lenticular galaxy) which is classed as an intermediate type of galaxy between a flattened galaxy and a true spiral. Their shape consists of a bulge and a flat disc but no spiral structure.
Galaxy clusters are classified into four different types; irregular and regular; rich and poor.
Regular galaxy clusters are spherically shaped and, as they normally have thousands of galaxies, are usually classed as rich.
These are massive systems which are mostly elliptical or irregular in shape and often contain large amounts of hot gas. They tend to form relatively spherical clouds and have very large galaxies at their centre.
Irregular galaxy clusters have no specific shape, and as they usually only comprise of a few hundred galaxies or less, are generally classed as poor.
These can be small groups or large spread out groups, and have many different shapes and densities. They do not contain as many galaxies as the regular type; an example being our own galaxy which is part of an irregular cluster of about 30 galaxies known as the “Local Group”.
Galaxy clusters are usually part of an even larger cluster called a Supercluster. They span vast regions of the universe and have large areas of empty space in between them.
@ 2013 Brian McKechnie (aka WorldEarth)