Galaxies are collections of stars, interstellar dust and gas, and other matter.
They contain anywhere from millions to trillions of stars. They are extremely large compared
to our solar system. If our solar system could fit into a coffee cup, then our galaxy
(the Milky Way) would be the size of the North American Continent!
During the 1920's when most astronomers thought the Milky Way was the whole of the Universe,
Edwin Hubble realized that the Milky Way was only one of a Universe full of galaxies. He
classified them into a system that is known as the "tuning fork."
Elliptical galaxies are sorted by how round or or stretched out they appear. An E0 galaxy is very round
and an E7 galaxy is very elliptical. The higher the number, the more eccentric the ellipse.
Edwin Hubble 1889-1953
Some galaxies have a spiral shape, and a bright bar running across the middle.
Hubble named these galaxies "barred spiral galaxies."
Galaxies that have the spiral shape without the central bar are classified simply as "spiral galaxies."
Both barred and non-barred spiral galaxies are more finely sorted by how tightly their arms are
wound to the core. Type "a" have arms wound very tightly to the core and have large central bulges.
Type "c" have arms wound loosely and have small central bulges.
Some galaxies appear to be between the elliptical and spiral galaxies, they are
labeled S0 on the tuning fork. These are called "lenticular galaxies" which means "lens-like."
Lenticular galaxies have a central bulge and a disk like the spiral galaxies
but no spiral arms.
A third class is called "irregular." Irregular galaxies are difficult to classify as spiral or
elliptical, and can have almost any shape. Many of these probably result from two
galaxies colliding, or by being reorganized by the gravity from a near miss. Galaxies that are
extremely odd are sometimes classified as "peculiar."