Emission nebulae . A cloud of interstellar gas which is glowing because it absorbs, and then re-emits, electromagnetic radiation from hot stars embedded within it. The predominant wavelength of radiation that is released by the hydrogen gas is 656 nm which is in the red end of the electromagnetic spectrum. Thus the emission nebulae are a characteristic pinkish-red in colour. As the solar winds from the ionising star blow away the central regions, the nebula dissipates.
Emission nebulae are usually isolated regions of much larger interstellar clouds .
A quantity which defines the amount of 'work' a body or a system can perform. Some common forms of energy are kinetic energy which is embodied by a moving object, potential energy which is embodied by an object suspended in a gravitational field or radiant electromagnetic energy which is carried by photons . All forms of energy are interchangeable, e.g. nuclear energy to heat to light. Energy level . The energy associated with a quantum state .