Here's a breakdown:
* Spiral Galaxies: These galaxies have a flattened disk shape with spiral arms. They contain a significant amount of gas and dust, which is concentrated in the disk and spiral arms. This gas and dust fuels star formation, giving them a young, blue appearance.
* Elliptical Galaxies: These galaxies are smooth, oval-shaped, and lack a defined disk. They contain much less gas and dust than spiral galaxies, and therefore have less ongoing star formation. They tend to be older and redder in color.
* Irregular Galaxies: These galaxies have no defined shape or structure. They often result from collisions or interactions between other galaxies. They can have a wide range of gas and dust content, depending on their origin.
So, while shape and size provide an initial classification, the presence and distribution of gas and dust is a key factor in understanding the properties, evolution, and activity of each galaxy type.
