Conversation so far:
what celestial object lies at the center of the milky way galaxy
The Galactic Center is the rotational center of the Milky Way, a supermassive black hole which powers the compact radio source Sagittarius A-Star.
how much mass is contained in a supermassive black hole
A supermassive black hole contains a mass of the order of hundreds of thousands to billions of times the mass of the Sun.
do all galaxies have a supermassive black hole
Observational evidence indicates that nearly all large galaxies contain a supermassive black hole, located at the galaxy's center.
what kind of radiation is emitted by supermassive black holes
Hawking radiation is black-body radiation that is predicted to be released by black holes.

Question: what else can be at a galaxy's center
Self-contained rewrite: what else can be at a galaxy's center besides a supermassive black hole