For instance, Sagittarius A* (pronounced “Sagittarius A Star”) in the center of our own Milky Way galaxy.
Astronomers have detected many active galactic nuclei like quasars around the universe, but Sagittarius A* is on the quiet side despite being four million times more massive than the sun.
However, it became apparent over the course of about two and a half hours that the source was variable and was, in fact, Sagittarius A*.
At its peak, Sagittarius A* was 75 times brighter than usual in infrared.
Scientists have been watching Sagittarius A* for decades, but no one was sure what to make of it for much of that time; it was just a strong X-ray source deep in the Milky Way.
In the late 1990s and early 2000s, studies of objects near Sagittarius A* demonstrated it had a strong gravity explained best by a supermassive black hole.
Today, the evidence for Sagittarius A* as the gravitational center of the Milky Way is quite solid.