Caption: A Hubble tuning fork diagram illustrating the Hubble sequence which is a galaxy morphological classification scheme developed by Edwin Hubble (1889--1953) in 1926 (see Wikipedia: Hubble sequence).

The Hubble sequence is conventionally illustrated on a Hubble tuning fork diagram as shown here.

Hubble introduced the Hubble tuning fork diagram by 1936 at the latest (see The Realm of the Nebulae, Edwin Hubble, 1936, p.45; The Realm of the Nebulae, Edwin Hubble, 1936, p. 45, partially online; No-509).

Features:

1. The Hubble sequence as shown on the Hubble tuning fork diagram includes the following galaxy types (divided into galaxy subtypes as indicated):
   1. Ellipticals labeled E0 through E7 are the handle. E0's are spherical and with increasing number the ellipticals become increasingly elongated a seen projected on the sky. The non-E0's may be either oblate or prolate.

   2. Unbarred spiral galaxies labeled Sa, Sb, Sc are the upper prong. Going from a to c, the bulges get smaller and the spiral arms less tightly wound.

   3. Barred Spirals labeled SBa, SBb, SBc are the lower prong. Going from a to c is the same as for the spirals.

   4. Intermediate spiral galaxies: These are NOT shown and have no conventional place on a Hubble tuning fork diagram. (They are conventionally shown on the Hubble tuning fork diagram for the de Vaucouleurs system.) The intermediate spiral galaxies are intermediate between being unbarred and barred and are given symbol SAB. The Milky Way and NGC 6744 in constellation Pavo are classified as SAB's.

   5. Lenticulars which are labeled SO. These are like spirals without spiral arms. They can have bars in which case they are SBO's.

   6. Irregular Galaxies: These are NOT shown on this diagram, but they are connventionally located between the ends of the tuning fork prongs and are labeled Irr.

2. Classification by the Hubble sequence is somewhat affected by human subjectivity.

   But it's still highly useful to look at a galaxy and know with some degree of certainty what type of galaxy it is.

3. By the by, Edwin Hubble did NOT assume that Hubble sequence was an evolutionay sequence (see Wikipedia: Hubble seqence: Physical significance). However, it may have been thought of that way for some decades after its advent by some. In fact, it is NOT an evolutionay sequence in any simple sense. Galaxy formation and evolution is a complex process.

   Confusingly, ellipticals and lenticulars are collectively referred to as early-type galaxies and spirals as late-type galaxies. Just accept this misleading terminology.

4. One bit of galaxy formation that can be mentioned here is that ellipticals probably are usually (almost always?) formed by galaxy merger. Merging galaxies can be of any type including two spiral galaxies.

   A galaxy merger disorders the orbital planes of the stars so that they are no longer concentrated in a galactic disk, but form an approximate spheroid in many cases: i.e., the merging galaxies become an elliptical.

   Ellipticals by the way are usually red sequence galaxies (AKA quenched galaxy) which means they have little or no star formation because their interstellar medium (ISM) is too hot for molecular cloud to form. For theories of galaxy quenching, see Wikipedia: Galaxy formation and evolution: Galaxy quenching.

5. There are actually many other galaxy types, NOT included in the Hubble sequence. For a non-exhaustive list of these, see the insert below---which is absent if this is NOT the first time this figure (galaxy_hubble_sequence.html) has been used in the current html web page. In any case, for a non-exhaustive list of galaxy types, see Galaxy Classification Systems and Types of Galaxies below (local link / general link: galaxy_types.html):
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