The nebula is a visible interstellar cloud of gas and dust. Nebulae are always as large as hundreds of world years in the first place, and they can contain stars and star clusters. Despite its majestic appearance, the mass of nebulae is extremely sparse. We cannot imitate such a density even in laboratories on Earth.
Stars can form from so-called star-forming nebulae by gravitational collapse. After its extinction, part of the star’s mass begins again with a nebula. Even during their lifetime, stars have a great influence on nebulae: they enrich their mass with part of the stellar wind, or, conversely, they influence their slow evaporation due to photon erosion.

Nebula Orion

HII regions are the birthplaces of stars. These form when clouds are highly scattered, molecules otherwise affect their own gravity, often under the influence of a nearby supernova explosion.
Nebulae can be classified according to the way they shine:
Emission nebulae are clouds built to radiate by stars. The star causes the gas to ionize, which then emits its own light. E.g. NGC 7000, NGC 1976, NGC 2070, NGC 1499, …
Reflective nebulae shine due to the reflection of light from adjacent stars and do not emit their own light. E.g. NGC 6334, NGC 605, M78, …

Nebula NGC 1977

Dark nebulae are not illuminated. They can be observed when the light is the light of distant stars or other nebulae. E.g. NGC 6611, NGC 2264, Barnard 33, …
The most common molecule in nebulae is molecular hydrogen (H2), in which a maximum of 10% of the nebula’s hydrogen atoms are bound. The energy to form molecular hydrogen gives atomic hydrogen ultraviolet radiation, which also splits them into individual atoms.
Hydrogen makes up 70% of nebulae, 28% helium and 2% other elements.

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