Galaxies are collections of hundreds of billions of stars, dust and gas, orbiting a central point and held together by gravity. They are often visible from Earth, despite their distance from us due to the combined brightness of the stars in each galaxy.
Our galaxy, the Milky Way is just one of hundreds of billions of galaxies in the universe. These galaxies come in all shapes and sizes, from simple ellipses and spirals to complicated shapes. They have been the focus of study of several different astronomers over time, particularly Charles Messier in the 18th century and Edwin Hubble in the first half of the 20th century who produced the Messier Catalogue and the Hubble sequences respectively.
How do we Classify Galaxies?
In 1926, the American astronomer Edwin Hubble set out the Hubble sequence (see below) for classifying galaxies based on their shape. This system (often referred to as the tuning fork) sets out 4 categories for galaxies to fall under - elliptical, spiral, spiral barred and irregular. The first three of these are then broken down into E0 - E7 for elliptical galaxies, Sa - Sc for spiral galaxies and SBa - SBb for spiral barred galaxies.
The Local Group
Whilst most galaxies are very distant and moving ever further away, a handful of galaxies are close enough to our galaxy to be gravitationally bound. The main objects in the Local Group, besides the Milky Way, are the Andromeda Galaxy, the Triangulum Galaxy (see right) and the Large and Small Magellanic Clouds. The galaxies in the Local group are moving closer together, with the Andromeda Galaxy set to collide with the Milky Way in approximately 4 billion years.
Galactic Rotation and Dark Matter
Most galaxies rotate around a central point. The speeds at which different parts of galaxies rotate can be found by observing the Doppler shift of the 21 cm radio waves emitted by hydrogen atoms in the arms. These 21 cm radio waves are the only radiation we obtain directly from the hydrogen atoms so are a particular focus of observers of galaxies. When the expected rotation curve (the relationship between orbital speed and distance from the centre) is compared to the observations made from the 21 cm radio waves, there is a significant difference. Whereas in the prediction where matter closer to the edge of the galaxy moves slower than that at a certain peak distance from the centre, the matter closer to the edges of the galaxy actually moves faster than at the point where the peak orbital speed should be. The best explanation we have for this is that there is invisible "dark" matter on the edges which drags the matter nearer the edges to faster speeds.