This permitted classification of galaxies with magnitudes down to 17.5.

In the course of the work, Hubble set up the classification of galaxies displayed in his famous 'tuning-fork' diagram, which has been used ever since.

Galaxies - The introduction of Hubble Law and discussions of the classification of galaxies are among the topics of lesson 13.

Of all present day astronomers who quote Hubble's classification of galaxies, Hubble's law or the Hubble constant, one questions whether any have read a line of Hubble's writing.

Among his contributions are the observational proof that many nebulous objects are actually galaxies beyond our own Milky Way galaxy, the classification of galaxies according to the Hubble sequence, and the development of Hubble's Law relating a galaxy's observed red shift to its distance away.