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

Galaxies are systems of stars, dust, gases, dark matter, and stellar remnants that are gravitationally bound. While there are many different kinds of galaxies, such as irregulars, spirals, and ellipticals, they all share the characteristic of being systems of billions or hundreds of billions of stars.

Stars, planets, gas, dust, and dark matter are all held together by gravity and make up a galaxy.

The components necessary to classify a grouping of stars and planets as a galaxy include a significant number of stars (ranging from millions to trillions), interstellar gas and dust, held together by gravity, and a distinct structure such as a disk, spiral arms, or an elliptical shape. Additionally, galaxies are typically massive structures that exhibit rotation and emit various forms of electromagnetic radiation.