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:

			/	S0	Sa	Sb	Sc
E0	...	E7									Ir
			\	SB0	SBa	SBb	SBc

where E0 to E7 are elliptical galaxies, S0 and SB0 are transitional types, Sa to Sd are spiral galaxies, SBa to SBd are barred spiral galaxies, and Ir are irregular galaxies.

• Elliptical galaxies have an ellipsoidal form, with a fairly even distribution of stars throughout. The number gives the degree of eccentricity: E0 galaxies are nearly spherical, while E7 are greatly flattened.
• 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 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 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 show no clearly discernable or regular shape, and therefore, they do not fit into this simple Hubble classification sequence.

Known Properties of Galaxies

Galaxy Type	Mass (Solar Masses)	Luminosity (Solar Luminosity)	Diameter (kpc)	Stellar Populations	Percentage of Observed Galaxies
Spiral and Barred Spiral	109 to 1011	108 to 1010	5-250	disk: Population I halo:Population II	77%
Elliptical	105 to 1013	105 to 1011	1-205	Population II	20%
Irregular	108 to 1010	107 to 109	1-10	Population I	3%

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 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.

Elliptical Galaxies examples

Name	Right Ascension	Declination	Hubble Type
M49 (NGC 4472)	12h 29.8m	8° 00'	E4
M59 (NGC 4621)	12h 42.0m	11° 39'	E3
M60 (NGC 4649)	12h 43.7m	11° 33'	E1
M84 (NGC 4374)	12h 25.1m	12° 53'	E1
M86 (NGC 4406)	12h 26.2m	12° 57'	E3
M89 (NGC 4552)	12h 35.7m	12° 33'	E0
M110 (NGC 205)	00h 40.4m	41° 41'	E6