Black holes are categorized by their mass. There are currently three categories. The smallest are called ‘stellar mass black holes.’ They range from 3 to 50 times the mass of the Sun. They are formed by the explosion and collapse of a star. In 1971, the first black hole ever discovered was a stellar mass black hole (Cygnus X-1). It has 21 times the Sun’s mass. We’ll examine this system in detail later in the segment.
The largest are called ‘super massive black holes’ or SMBH for short. They have millions to billions of times the mass of the Sun. These are mostly found at the center of large galaxies. Sagittarius A* is the one at the center of our galaxy. It has 4.1 million times the mass of the Sun. Being the SMBH closest to us, it provides the most detailed information about these kinds of black holes. We’ll be doing a deep dive into Sag A* later in the segment.
The third are called ‘intermediate-mass black holes’ or IMBH. They have from 100 to 100,000 times the mass of the Sun. They are thought to form by the merging of stellar mass black holes or the runaway collision of massive stars in dense stellar clusters that collapse into black holes. Several IMBH candidate objects have been discovered. But to date, none have been confirmed. Later in this segment we’ll cover one of the best candidates [named 3XMM J215022.4−055108]. It is indicated by the white circle.