The bright band of stars stretching across the night sky. Our Galaxy is sometimes called the Milky Way Galaxy. An area of dust and gas in space. 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.