1. Shape: This is the most obvious and fundamental property. Galaxies are categorized into three main types:

* Spiral galaxies: These galaxies have a flat, disk-like structure with spiral arms.

* Elliptical galaxies: These galaxies are smooth, oval-shaped, and lack spiral arms.

* Irregular galaxies: These galaxies lack a defined shape and don't fit into either of the previous categories.

2. Luminosity: This refers to the total amount of light a galaxy emits. Galaxies vary greatly in luminosity, from dwarf galaxies that are barely visible to giant galaxies that outshine all the stars in their host galaxy.

3. Composition: This refers to the types of stars, gas, and dust that make up a galaxy. For instance, young galaxies tend to have more gas and dust, while older galaxies have more stars.

These three properties provide a basic framework for understanding the diversity of galaxies in the universe.