With these interferometers, radio astronomers were able to identify an exceptionally bright, small spot buried in the heart of Sagittarius A. The astronomers were so surprised by this spot that they named it Sagittarius A* (or Sgr A*), applying the asterisk usually used to denote an excited state of an atom.
Equipped with clearer and more detailed observations, astronomers were able to start placing estimates on the size and mass of the object emitting the radio waves. They found that it was both smaller than our solar system and millions of times more massive than the Sun.
There’s only one kind of object in the universe that fits that description: a supermassive black hole.
Daredevils in the dark
The supermassive black hole at the center of the Milky Way doesn’t emit light itself. It is a black hole, after all. But all the gas and dust surrounding that monster is perfectly capable of emitting light. As it violently swirls and compresses on its way to the event horizon, the material can reach scorching hot temperatures, approaching 18 million degrees Fahrenheit (10 million Celsius). At those temperatures, the material, which quickly forms a thin, rapidly spinning accretion disk, emits intense amounts of radiation across the entire electromagnetic spectrum.
Most of that radiation gets absorbed by the gas and dust within the galactic core, with only X-rays and radio emissions making their way across the galaxy to our planet. (Since we developed radio telescopes before X-ray astronomy, we are more familiar with Sagittarius A* as a radio source rather than an X-ray source).