(1) Elliptical galaxies
(2) Spiral galaxies
(3) Irregular galaxies
(4) S0 galaxies
Explanation:
(1) Elliptical galaxies
Elliptical galaxies
These systems exhibit certain characteristic properties. They have complete rotational symmetry; i.e., they have figures of revolution with two equal principal axes. Third smaller axis is presumed axis of rotation. The surface brightness of elliptical at optical wavelengths decreases monotonically outward from a maximum value at the centre, following a common mathematical law of the form:
I = I0( r/a +1 )−2,
where I is the intensity of the light, I0 is the central intensity, r is the radius, and a is a scale factor.
(2) Spiral galaxies
Spiral galaxies are classified into two groups; ordinary and barred. The ordinary group is designated by S or SA, and the barred group by SB. In normal spirals, the arms originate directly from the nucleus, or bulge, where in the barred spirals, there is a bar of material that runs through the nucleus that the arms emerge from. Both types are given a classification according to how tightly their arms are wound. The classifications are a, b, c, d ... with "a" having the tightest arms. In type "a", the arms are usually not well defined and form almost a circular pattern. Sometimes you will see the classification of a galaxy with two lower case letters. This means that the tightness of the spiral structure is halfway between those two letters.
(3) Irregular galaxies:
Irregular galaxies have no regular or symmetrical structure. They are divided into two groups, Irr I and IrrII. Irr I type galaxies have HII regions, which are regions of elemental hydrogen gas, and many Population I stars, which are young hot stars. Irr II galaxies simply seem to have large amounts of dust that block most of the light from the stars. All this dust makes is almost impossible to see distinct stars in the galaxy.
(4) S0 galaxies
These systems exhibit some of the properties of both the elliptical and the spirals and seem to be a bridge between these two most common galaxy types. Hubble introduced the S0 class long after his original classification scheme had been universally adopted, largely because he noticed the dearth of highly flattened objects that otherwise had the properties of elliptical galaxies.
