A star that cannot emit light is known as which of the following?

A star that cannot emit light is primarily known as a black hole. In astrophysics, black holes are regions of spacetime where the gravitational pull is so strong that nothing, including light, can escape from them. This inability to emit or reflect light is what gives them their name. A black hole forms when a massive star collapses under its own gravity after exhausting its nuclear fuel, resulting in a singularity surrounded by the event horizon, beyond which no information can escape. Thus, the defining characteristic of a black hole is its complete opacity to light, making it invisible against the backdrop of space.

In contrast, the other options represent types of stellar objects that can emit light. Main sequence stars, such as our Sun, are stable and actively fuse hydrogen into helium, producing light and heat. Brown dwarfs, while unable to sustain hydrogen fusion like main sequence stars, can emit some light in the infrared spectrum due to residual heat from their formation, but are not as luminous as stars. Red giants are in a later stage of stellar evolution and can radiate significant amounts of light as they fuse heavier elements. Thus, the unique property of a black hole—its inability to emit light—is what categorizes it as the correct answer to this question.