In this corner of the ring, we have a compact object, let's welcome neutron star. And in this corner of the ring, we have a black hole let's welcome stellar mass. And three, two, one, orbit. When a black hole is in a binary system, we're able to deduce how massive it is, but that alone isn't enough to really say what a black hole is like. We need to explore the way that black holes are classified. Any black hole astrophysicist will tell you about three basic weight classifications, stellar mass, intermediate mass, and super-massive black holes. There are other ways of classifying black holes as well. Like whether they have charge, or spin, but there might be some black holes that don't fit into these categories at all. Stellar mass black holes are the light weights of the black hole family. They're produced at the end stages of a star's life, and range between three solar masses up to about 100. This is due to the 20 to 30 solar mass minimum mass for a dying star that scientists theorize is required to create a black hole. Also a main sequence stars only get about as big as 100 solar masses. Since stellar mass black holes require the collapse of a star, they are sometimes called collapsars. During the collapse, lots of gas will be expelled before the black hole forms. The resulting black hole will therefore have a much lower mass than the progenitor star. The collapse of a supergiant star might produce a three solar mass black hole. The Schwarzschild radius equation tells us that a three solar mass black hole would have an event horizon roughly nine kilometers in radius. That's an easy ratio to remember. For every solar mass, the radius of the event horizon gets roughly three kilometers larger. A nine kilometer radius black hole would be about the same height as Mount Everest. Since the diameter is double the radius, a three solar mass black hole would have a diameter of 18 kilometers, which would make its diameter roughly twice the height of Earth's highest mountain Mount Everest. If instead you measure 18 kilometers across the ground, you could walk across a stellar mass black hole in a little more than three hours. As long as you could survive the extreme gravity. Intermediate mass black holes are the middle weights of the black hole family. These black holes aren't the result of a stellar collapse, but of an existing stellar mass black hole growing by consuming gas, dust, stars, and other black holes to become more massive. Intermediate-mass black holes are classified in a range between 100 and 100,000 solar masses. At 100 solar masses, the lightest of the intermediate-mass black holes would be roughly 300 kilometers in radius, which means that they would span the orbital height of the International Space Station which orbits Earth 400 kilometers above the surface. Supermassive black holes, the heavyweights of the black hole family occupy everything above a 100,000 solar masses. These are the black holes that reside at the center of galaxies, and are some of the oldest objects in the universe. Weighing 100,000 solar masses, the diameter of the smallest supermassive black hole's event horizon would be about 600,000 kilometers. That's 40 percent as big as the sun. A black hole eclipse would sure look strange. The biggest black holes that astrophysicists theorize are limited to no heavier than about 50 billion solar masses. That would make the largest mass black holes about 300 billion kilometers across. At that size, the largest black holes would still be smaller than our own solar system. If you consider that it goes well beyond the orbit of Pluto to the edges of the Oort Cloud, which are five quadrillion kilometers from the Sun. A photon traveling at the speed of light would take 58 days to cross a 50 billion solar mass black hole. We only calculated the smallest black hole in each of the categories in order to illustrate where the boundaries are in the range of sizes. In reality, these boundaries are more like scientific guidelines than they are rules. The black holes that we have observed and measured fall all across this scale. The first black hole merger detected by LIGO created a stellar mass black hole weighing 60 solar masses. We suspect that intermediate-mass black holes could be found at the center of smaller dwarf galaxies, which can be found in orbit around larger galaxies like the Milky Way. They fall into the intermediate category, but below 10,000 solar masses, they are just below our guidelines for supermassive black holes. The black hole at the center of our galaxy is Sagittarius A-star. It weighs just over four million solar masses, our galaxy's heavyweight contender.