Hubble sequence
The Hubble sequence is a classification of galaxy types developed by Edwin Hubble in 1936. It is also called the tuning-fork diagram as a result of the shape of its graphical representation. Galaxy types are divided as follows: - Elliptical galaxies (E0-7) have an ellipsoidal form, with a fairly even distribution of stars throughout. The number gives the degree of eccentricity: E0 galaxies are nearly round, while E7 are greatly flattened. The number indicates only how the galaxy appears on the sky, not its true geometry.
- Lenticular galaxies (S0 and SB0) appear to have a disk-like structure with a central spherical bulge projecting from it. They do not show any spiral structure.
- Spiral galaxies (Sa-d) have a central bulge and an outlying disk containing spiral arms. The arms are centered around the bulge, and vary from tightly wound (Sa) to very loose (Sc and Sd). The latter also have less pronounced central bulges.
- Barred spiral galaxies(SBa-d) have a similar sort of spiral structure to spiral galaxies, but instead of emanating from the bulge, the arms project out from the ends of a "bar" running through the bulge, like ribbons on either end of a baton. Again, SBa to SBd refer to how "tightly wound" these arms are.
- Irregular galaxies (Irr) are divided into Irr-I, which show spiral structure but are deformed in some way, and Irr-II for any other galaxy that does not fit into another category.
Hubble based his classification on photographs of the galaxies through the telescopes of the time. He originally believed that elliptical galaxies were an early form, which might have later evolved into spirals; our current understanding suggests that the situation is roughly opposite, however,
this early belief left its imprint in the astronomers' jargon, who still speak of "early type" or "late type" galaxies according to whether a galaxy's type appears to the left or to the right in the diagram. More modern observations of galaxies have given us the following information about these types: - Elliptical galaxies are generally fairly low in gas and dust, and are composed mostly of older stars.
- Spiral galaxies generally have plentiful supplies of gas and dust, and have a broad mix of older and younger stars.
- Irregular galaxies are fairly rich in gas, dust, and young stars.
From this, astronomers have constructed a theory of galaxy evolution which suggests that ellipticals are, in fact, the result of collisions between spiral and/or irregular galaxies, which strip out much of the gas and dust and randomize the orbits of the stars. See galaxy formation and evolution.
See also Gérard de Vaucouleurs
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