Extragalactic astronomy

Representation of collision galaxies.

The study of objects outside the Milky Way is called extragalactic astronomy.

Extragalactic astronomy was born as such when Edwin Hubble discovered the Cepheids in the Andromeda Nebula, confirming that due to its distance it should be outside our galaxy and that due to its size it should be a galaxy comparable to or even larger than ours.

Later it was found that galaxies are not isolated, but rather form groups of different sizes. Furthermore, there is a hierarchical organization where smaller groupings are part of larger groupings.