Definition

Emission nebulae are clouds of gas such as the Orion Nebula which are perceived because they emit light. Planetary nebulae are emission nebula associated with individual stars, which often have the telescopic appearance of a planet.

Mechanism

The mechanism by which light is created in emission nebula is that the gas is heated by the star, causing its constituent atoms to break. The gas is heated specifically by ultraviolet radiation coming from the nearby star(s). These photons interact with the gas and cause the atoms to eject one or more electrons. The electrons gain energy from the ultraviolet radiation. The electrons eventually recombine with the atoms and re-emit the energy, some of it in the form of light. It is this light which enables the nebula to be seen. The free electrons in the gas also lose energy in the form of radio waves, so radio telescopes can also detect emission nebula.

Composition

The most abundant element in interstellar gas is hydrogen, which is very susceptible to ionization by ultraviolet light: nearby to a hot star which emits ultraviolet light essentially all the hydrogen is split into its components – a free nucleus and a free electron. The symbol for this is H II (pronounced “H-two”) and emission nebulae produced in this way are therefore also called H II regions.

>Hot stars that emit ultraviolet light are usually young stars, recently formed by the collapse of part of the gas cloud; usually many stars are formed simultaneously and form a star cluster. Thus hot stars in clusters are associated with emission nebula – the association between tsars and gas clouds is not by chance (as is usually the case with reflection nebulae) but is generic. The connection between the Orion Nebula and the Trapezium stars is of this kind.

Emission nebula can derive their energy from sources other than hot stars. Gas clouds that collide, such as the remnants of Supernovae, re-radiate the kinetic energy of the collision. They are called collisionally-excited emission nebula.

Examples

IC 417


Trifid Nebula


The Omega Nebula


The Lagoon Nebula