Transcript: Black holes have few observable properties; after all, there’s nothing to see. Mass is a fundamental property, and it can be measured in principle by the gravitational interactions of a black hole with a nearby star. Angular momentum is another property. As with neutron stars, black holes have collapsed by a large factor from a normal stellar state, so they must be rotating very rapidly. A black hole also has a measurable surface area at its event horizon, and it has an amount of electric charge. We cannot say what a black hole is made of because information of the material that went in to make it is lost as soon as that material passes the event horizon. The idea of information being lost is connected to the fact that the entropy of black holes is very large, about 100 million times larger than the equivalent entropy of a star like the Sun.