Hubble's "Tuning Fork" Diagram
See reference: http://cse.ssl.berkeley.edu/Segwayed/lessons/classifying_galaxies/tunfork.htm
Today we classify galaxies mainly into two major groups following Hubble's
examples, as shown above. Elliptical galaxies range from round shapes
(E0) to oval shapes (E7).
Spiral galaxies have a pinwheel shape and
are classified according to their bulge, as well as how tightly their
arms are wrapped around the bulge. They range from Sa, which has a large bulge and tight, smooth arms,
to Sc, which has a small bulge and loose, lumpy arms.
galaxies, classified as SB, are pinwheel-shaped and have a distinct "bar"
of stars, dust, and gas across their bulge. They range from an SBa,
which has a bar across its large bulge and tight, smooth arms, to an
SBc, which has a bar across its small bulge and loose, lumpy arms.
Irregular galaxies have no definite shape but still contain new stars, gas, and
dust. The chart below summarizes the properties of the main classes
Properties of the main classes of galaxies
|Shape and structural properties
||Have disks of stars, gas, and dust containing spiral arms that
attach to a central bulge. Sa and SBa have the largest bulges. SB
galaxies have central bars.
||Have neither disk nor arms. Stars are distributed evenly from
near circular to oval (football).
||Have no definite structure.
||Have both young and old stars. Halos consist of old stars only.
||Contain mostly old stars.
||Contain both young and old stars.
|Gas and dust
||Disks contain gas and dust. Halos contain little gas or dust.
||Have little or no gas or dust.
||Have lots of gas and dust.
||Stars form largely in spiral arms.
||Have little or no star formation.
||Have lots of star formation.
||Gas and stars rotate around the center of the galaxy.
||Stars move on randomly oriented orbits like a swarm of bees.
||Stars and gas have irregular orbits.