The Hubble Tuning-Fork

In 1926, the American Astronomer Edwin Hubble developed a classification scheme of galaxies based on their shapes and structure, or in other words, on their morphology. This scheme is called the Hubble Sequence or Hubble tuning-fork diagram because the shape in which it is usually shows resembles a tuning-fork, like the images below.

In the Hubble tuning-fork, galaxies are roughly separated into two categories: elliptical and spiral galaxies. Ellipticals were assigned numbers from zero to seven, referring to how elliptical the galaxy is (E0 is very round, E7 is very elliptical).

Hubble assigned spiral galaxies letters from a to c, which tell us how compact the spiral arms are. Spiral galaxies are then divided into two sub-categories: unbarred and barred spiral galaxies, referring to whether or not the spiral galaxy has a bar of stars running through its centre. Barred spiral galaxies have a B in their classification.

Lenticular galaxies, classified as S0, are a type of galaxy intermediate between elliptical and spiral galaxies.

Finally, some galaxies do not fit into any category. These galaxies are generally referred to as irregular galaxies, classified as Irr.

Elliptical and lenticular galaxies are often referred to as early-type galaxies, while spirals and irregular galaxies are referred to as late-type. These names are a result of the common misbelief that the Hubble sequence was supposed to be an evolutionary sequence, from elliptical galaxies through lenticulars to either barred or regular spirals. Current evidence suggests the otherwise: the early Universe appears to be dominated by spiral and irregular galaxies!

Click the buttons below to learn more about each type of galaxy!