This tuning-fork style diagram of the Hubble sequence was created on the base of the Wikimedia Commons file: Hubble sequence photo.png. [commons.wikimedia.org/wiki/File:Hubble_sequence_photo.png]
"The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1936. It is often known colloquially as the Hubble tuning fork diagram because of the shape in which it is traditionally represented.
Hubble’s scheme divides regular galaxies into 3 broad classes - ellipticals, lenticulars and spirals - based on their visual appearance (originally on photographic plates). A fourth class contains galaxies with an irregular appearance. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy." [Hubble sequence. Wikipedia]
The astronomical diagram example "Tuning-fork style diagram of the Hubble sequence" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Astronomy solution from the Science and Education area of ConceptDraw Solution Park.