Tom knew that bright nebulae are divided into emission nebulae and reflection nebulae, depending on their type of luminescence.
Emission nebulae are nebulae that can emit light themselves. Usually, the interstellar dust molecules that make them up are ionized by a huge celestial body, and then release light of various colors.
For example, the Eagle Nebula. There is an extremely famous structure inside this nebula, which is the Pillars of Creation.
Several huge pillars, tens of light-years long, extend from this nebula. The gas clouds forming these pillars collapse inward under the influence of external disturbances and their own gravity, eventually condensing into stars.
Dozens to hundreds of huge stars are born in this stellar nursery. It is precisely because of the birth of these stars that the dust forming this nebula is ionized and heated, and thus emits light.
