Galaxy Classification

By looking at galaxies closely you can start to see similar features. Sorting schemes have been created to separate galaxies into groups based on how they look. The most famous of these was made by Edwin Hubble in 1936. It is often known as the Hubble tuning-fork because of the shape of it.

You can use Hubble's scheme to try and group galaxies yourself in our Galaxy Classification workshop

Hubble Tuning Fork
Hubble Tuning Fork Diagram
Credit: NSO

There are 3 broad classes of galaxies based on how they look:

Elliptical Galaxies
Ellipticals are given the letter E. They are also given a number based on how squashed they seem. If they look like balls (circular) they are classified as E0. The more squashed they look, the higher the number it is given. This makes them appear more like rugby balls. The most flattened are given the number E7.

Lenticular Galaxies
These are given the tag S0. They have a bright central bulge which is surrounded by a disk structure. Unlike spiral galaxies, the disks of lenticulars have no visible spiral shape. It does not appear that stars are still forming in these galaxies.

Spiral Galaxies
Standard spiral galaxies are given the symbol S. At least half of spirals seem to have a bar structure in the middle, and these are given the symbol SB

Spirals are given another letter which is based on how tight the spiral arms are, and how big the central bulge is. Sa (and SBa) galaxy types have tightly wound arms and large central bulges. Sc's (and SBc's) have loosely wound arms and a small central bulge. Sb and SBb's lie somewhere in the middle.

Since this scheme was made galaxies have been studied in much more detail. These broad classes have grown to allow for more detail. Other types of galaxy have also been discovered. There are now also irregular galaxies which have no obvious structure at all, and dwarf galaxies. Dwarf galaxies have only a few million stars and can be spiral, elliptical or irregular in shape.