Here's why:

* Spiral galaxies are characterized by their spiral arms that extend from a central bulge.

* Nuclear bulges are dense, spherical regions of stars that are found in the center of many spiral galaxies. They are thought to be composed of older, redder stars than those found in the spiral arms.

While some elliptical galaxies can also have a central bulge, it's not as characteristic as it is for spiral galaxies.

Let me know if you'd like to learn more about galaxies!