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Where do these objects come from, and why are there so many types?
- gas clouds are collapsing, exploding and colliding.
- stars are forming, evolving and dying.
They gather into galactic groups due their gravitational attraction – these processes take millions of years or more to happen, so no human lifetime is determiner to observe the evolution of a star.
But even our local patch of the Universe is large enough that we can see all the stages of these processes happening in one place or another – it is similar to walking down the street and seeing adults, children of all sizes, and pregnant women; pretty soon the picture of how humans are born and grow up becomes apparent without actually witnessing a person pass through every stage.