General Astronomy/Types of Galaxies

Estimates of the number of galaxies in the universe range from 10 billion to over 100 billion. According to Hubble's Law, galaxies are red-shifted. This means that they're moving away from us. Young galaxies are more blue, while old galaxies are more red. As spiral galaxies mature, the center becomes red. The arms are bluish.

The Hubble Sequence
The most common form of classification for galaxies is based on a system which categorises them by their visible structure. This is known as the Hubble Sequence, and was developed by Edwin Hubble in the 1920s. Galaxies are organized in a form resembling a tuning fork, often called the Hubble Tuning Fork Diagram. It is typically drawn with elliptical galaxies on the left, lenticular galaxies in the middle, and two branches of spiral galaxies on the right: one for un-barred spirals, and one for barred spirals. Elliptical galaxies, also known as "early-type" galaxies, have an ellipsoidal form, with a fairly even distribution of stars throughout. They are mostly featureless and have no form of visible disk. Examples of ellipticals include M87, whose black hole was the first to be imaged in high-resolution, and ESO 383-76, which is one of the largest galaxies ever discovered. They are denoted by the letter E, with the number giving the degree of eccentricity: E0 galaxies are nearly spherical, while E7 are greatly elongated.

Lenticular galaxies appear to have a disk-like structure with a central spherical "bulge" projecting from it, but they do not show any spiral structure. They were originally introduced as a theoretical intermediate class between ellipticals and spirals before being confirmed by observations. An example of a lenticulat galaxy is NGC 2787. These are given the class S0.

Spiral galaxies, also known as "late-type" galaxies, have a central "bulge" and an outlying "disk"; the disk is notable for having spiral "arms" within it, centered on the bulge. Sa galaxies have very "tightly wound" arms, while Sc galaxies are very loose spirals. 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 SBc refer to how "tightly wound" these arms are. Examples of spiral galaxies include: the Milky Way, our home galaxy; Andromeda (M31), the closest galaxy to us; and the Pinwheel Galaxy (M101).

Finally, there are irregular galaxies, which show no clearly discernable or regular shape. Some examples of these include the Large and Small Magellanic Clouds, which can be seen from the Earth's southern hemisphere. These are given the class Irr.

Links to Galaxy Evolution
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.

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.