] A protoplanetary nebula forms when a red giant star
] ejects most of its outer layers, and is observable when
] dust within the nebula reflects light from the central
] star.
]
] Subsequently, as the very hot core (six or more times
] hotter than the Sun) gets further exposed, the cloud of
] ejected material becomes bathed with ionizing ultraviolet
] light, which makes it glow. At this later stage these
] objects are known as planetary nebulas.

RedNova