* Nebulae: Nebulae are vast clouds of gas and dust in space. They are not stars themselves, and they don't produce their own light. However, they reflect light from nearby stars, which makes them visible. Some nebulae also emit light as a result of ionization - when the gas is energized by the radiation from nearby stars. This is often referred to as "emission nebulae".
* Gravity: Gravity is a force, not a source of light. It's an invisible force that pulls objects towards each other. It doesn't emit any light itself.
In summary:
* Nebulae are illuminated by nearby stars and sometimes emit light due to ionization.
* Gravity does not produce light.
