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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 SaSbSc
E0...E7 Ir

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.

Known Properties of Galaxies
Galaxy Type Mass (Solar Masseses) 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
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:

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