Emission nebulae like M42 occur only near stars that emit large amounts of ultraviolet radiation.

Here's why:

* Ultraviolet radiation is the key: The intense ultraviolet radiation from hot, young stars ionizes the hydrogen atoms in the nebula.

* Ionization and light emission: When an electron is stripped from a hydrogen atom, it becomes ionized. When the electron recombines with the proton, it releases energy in the form of light, creating the characteristic glow of emission nebulae.

Think of it like this: The ultraviolet radiation acts like a powerful energy source, exciting the atoms in the nebula and making them emit light.