Question

What components are necessary to classify a grouping of stars and planets as a galaxy?

Answer 1

The group of stars and planets have to be gravitationally bound.

Explanation:

galaxies are gravitationally bound systems of stars, dust, gases, dark matter and stellar remanents.
Although there are a variety of different types of galaxies like ellipticals, spirals and irregulars, their common feature is that they are all gravitationally bound systems of billions or hundreds of billions of stars.