What do spiral galaxies have a nucleus of?

A spiral galaxy nucleus contains a mass of stars in a "bulge" and is thought that many contain supermassive black holes at the very core of the nucleus.

-In galaxies with tightly wound spiral arms the bulge generally contains old, red stars with low metal content (called population II stars); whereas galaxies with more loose arms generally have younger, blue stars (called population I stars). Some bulges have similarities to the elliptical galaxies and others are similar to disk galaxies.

-Evidence of supermassive black holes have been discovered in many galaxies and theorized to exist in many more galaxies (at least the larger ones). Our own Milky Way Galaxy contains a supermassive black hole at its core.