There are many other catalogs that are used, and they are all listed on the first page in the Pictures section of this site. Classification of Stars Stars are generally classified by a spectral class. Spiral galaxies are also given a subclassification of a, b, or c. Sa galaxies are spiral galaxies that have loose, poorly defined arms, while Sc galaxies have well defined arms. Barred spiral galaxies look sort of like spiral galaxies, except there is a bar extending through the nucleus, and the arms spiral off of that bar. Barred spiral galaxies are given the same subclassification as spiral galaxies, a through c, and each subclass stands for the same thing.
Lenticular galaxies look like Spiral galaxies, but without the spiral arms.
Because of this, they are sometimes referred to as S0 galaxies. Elliptical galaxies are galaxies that are shaped like ellipses. Irregular galaxies are galaxies with an irregular shape, and therefore don't fit into any of the above categories. Elliptical NGC 205 Lenticular Irregular Large Magellenic Cloud Classification of Nebulae Nebulae are large clouds of gas. Reflection nebulae reflect the light from nearby stars.