What is the difference between a galaxy and a nebula? How do astronomers tell them apart?
Galaxy is a system of widely separated star systems, held together gravitationally. Nebula is a misty cloud of dust and gas, capable of hiding a single or a system of space bodies.
The definitions tell astronomers which is which.
Galaxies are widely separated.
Nebulae can exist within a galaxy.
They are visible in the night sky as a cloud of gas and dust, in outer
space. like a defective patch, in contrast to luminous bodies..