 Black holes are categorized by their mass. There are currently three categories. The smallest are called stellar mass black holes. They range from three to fifty 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 X1. It has twenty-one times the Sun's mass. We'll examine this system in detail later in the segment. The largest are called supermassive 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 star is the one at the center of our galaxy. It has four point one 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 Sagittarius A star later in the segment. The third are called intermediate mass black holes, or IMBH. They have from one hundred to one hundred thousand 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 the segment, we'll cover one of the best candidates. It is indicated by the white circle.