A nearby star can make a nebula shine, either through reflected starlight, or because energy from the star makes the nebula itself glow. The plural is nebulae. Neutron stars are very dense, only ten or twenty kilometres across, but more massive than our Sun. The smallest 'unit' of light. A pulsar is a neutron star that is emitting a beam of radio waves, and spinning, and so the effect is like that of a lighthouse.
Pulsars keep exceptionally good time.
The smallest 'unit' of energy. A quantum of light is called a photon. They are now accepted to be the exceedingly bright centres of certain galaxies, tiny central cores that put out the power of perhaps a thousand ordinary galaxies. The word 'radiation' has two different meanings, which can be confusing. First, there is electromagnetic radiation.