Starship Asterisk*

I wonder what causes the brightness of the center of M101. Lots of stars, with more Population II (old) stars with a red hue than Population I (young) stars with a blue hue. Also, while most of the galaxy is thin and disc-like, the center bulges and is more sphere-like. So not only is the star dens...

As a totally uninformed person when it comes to astronomy (and most other topics, if the truth is to be known), I wonder what causes the brightness of the center of M101. Thanks