These astronomical objects are fully described by mass, charge, and angular momentum, and have an event horizon; what are they?

• A. Neutron stars
• B. Quasars
• C. White dwarfs
• D. Black holes

Answer: (D)

The idea being tested is that black holes are uniquely determined by just three quantities: mass, electric charge, and angular momentum. This comes from the no-hair idea, which says that once matter collapses into a black hole, all other details of the original object are lost, and the hole’s external gravitational and electromagnetic fields depend only on those three parameters. The event horizon is the boundary surrounding a black hole from within which nothing, not even light, can escape, which is a hallmark feature of these objects.

In practice, astrophysical black holes are thought to have negligible net charge, so they’re typically described mainly by mass and angular momentum (the Kerr solution), though charge is allowed in theory (the Kerr–Newman solution). The other options don’t fit: neutron stars and white dwarfs have solid surfaces and no event horizon, so they aren’t fully described by mass, charge, and angular momentum alone. Quasars are extremely bright energy sources powered by accretion onto black holes, not standalone objects defined by those three parameters. So the correct description is a black hole.