Nebula is a huge cloud of gas and dust in space.From it stars will be born in long time...

Planetary nebula is the gases puffed out from a star which almost finish its hydrogen in the core..Pull of gravity to center and pressure of fusion pushes out wards..THis remains in equilibrium. When fusion stops the gravity reduces and outer layers are lost into space.These gases in a ring shape are called a planetary nebula.

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