Notes from "Gravity's Engines" - How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos by Caleb Scharf
"Black holes" form when enough "stellar remains ... implode" into a sphere and collapse "inwards to a single point that is , to all intents and purposes, of infinite density - an inner singularity." These places in the universe can be detected by the enormous amount of energy that is released as stars and other matter are absorbed across the surrounding "accretion disk".
There are "baby black holes only a few times the mass of our sun". However, others also occur up to 10 billion times the solar mass; which is the largest yet observed, and may be a maximum size.
The author, Caleb Scharf, says that compared to the at least 100 billion galaxies in the universe, our Milky Way is one of the largest that has been observed and it contains about "200 billion stars{that} amount to a mass approximately 100 billion times that of our Sun, and its disk stretches across a diameter of 100,000 light-years." And every "210 million years, we complete another circumnavigation of the Milky Way." And every year our galaxy creates a few new stars of about 3 solar masses total.
In spite of being a rather large galaxy, the mass at the center of the Milky Way beyond the event horizen and around the singularity can be measured! It is believed to be only about 4 million solar masses.
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