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:
- An elliptical galaxy (E0-7) has 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.
- A lenticular galaxy (S0 and SB0) appears to have a disk-like structure with a central spherical bulge projecting from it, and does not show any spiral structure.
- A spiral galaxy (Sa-d) has 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.
- A barred spiral galaxy (SBa-d) has 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.
- An irregular galaxy (Irr) can be of type Irr-I, which shows spiral structure but is deformed in some way, and Irr-II for any other galaxy that does not fit into another category.
| Galaxy Type | Mass ( Solar Masses) | Luminosity (Solar Luminosity) | Diameter (kpc) | Stellar Populations | Percentage of Observed Galaxies |
|---|---|---|---|---|---|
| Spiral
/ 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 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.
| 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 |