The Hubble classification system is an observational system for classifying galaxies according to their morphologies. In it, Edwin P. Hubble divided the galaxies into two distinct types: spiral galaxies and elliptical galaxies. He further divided the spirals into regular spirals and barred spirals -- spiral galaxies whose central bulges were replaced by a bar-like structure. These three distinct types made up Hubble's tuning fork diagram -- the ellipticals made up the handle, and the spirals made up the two tines of the fork.

The original classifications came from Hubble's book, The Realm of the Nebulae published in 1936. After publication of the book, Hubble continued to take photographs of large galaxies as examples, and these images were published in full by Allan Sandage in 1961, along with a new introduction to the Hubble classification system. As a result, it is often known as the Hubble-Sandage classification scheme.

The ellipticals were subdivided into types E0 through E7, with E0 ellipticals being essentially circular, and becoming progressively more elliptical to E7. The number after the E is determined by

10 × (1 - a/b)

where a and b are the semi-minor and semi-major axes of the elliptical galaxy.

Both the normal spirals (class S) and barred spirals (class SB) were subdivided into subclasses 0, a, b, and c.

S0 and SB0 are what are called lenticular galaxies. They have flattened, disk-like shapes and perhaps a weak central bulge/bar, but no spiral arms to speak of. They also have no star forming regions visible, and their light comes almost entirely from old stars. The S0,SB0 galaxies are given a subscript 1, 2, or 3 depending upon whether they have no dust, some dust, or a lot of dust obscuring the central parts of the galaxy.
Sa and SBa have large central bulges/bars, very tightly wound spiral arms, and few star forming regions visible, so that most of their light also comes from older stars.
Sb and SBb have moderate sized bulges/bars and clear spiral arms. Star forming regions are also visible within the spiral arms, and much of the galaxian light comes from younger, hotter, and bluer stars within the arms. Our own Milky Way is an example of an Sb galaxy.
Finally, the Sc and SBc have very small central bulges/bars, and very loose, broad spiral arms (so-called grand design spiral galaxies). The spiral arms are full of star forming regions, and much of the galaxian light comes from young, hot, and blue stars within these spiral arms.

The main problem with Hubble's classification scheme is that it classifies only large galaxies and ignores most of the dwarf galaxies. Most galaxies in the Local Group are dwarf galaxies, and the dwarf galaxies probably make up a large fraction of the galaxies in the universe (though they're hard to detect outside the Local Group because they are so faint). However, the Hubble-Sandage classification does include another category, namely the irregular galaxies, given the designation Irr. The Large Magellanic Cloud is an example of an irregular galaxy.

It is important to note that Hubble considered his tuning fork to be an evolutionary diagram. He believed that galaxies started out as spherical ellipticals (E0), slowly turned into E7/S0, and evolved into spirals -- through the ``early type'' Sa into the ``late type'' Sc. We now know this to be impossible. Spirals are rich in gas from which they form stars, while ellipticals are gas-poor. It would also be difficult to force the stars within an elliptical galaxy -- orbiting randomly around the center of gravity -- to start orbiting coherently in a plane, let alone to do so locked into spiral arms. However, it may be that evolution goes the other way -- spirals may turn into ellipticals over time. This may occur through the gravitational interaction of two or more galaxies, which serves to break up the ordered, spiral structure.

Sources: Galactic Astronomy, by D. Mihalas and J. Binney; The Milky Way as a Galaxy by G. Gilmore, I. King, and P. van der Kruit.

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