How do spiral and elliptical galaxies differ?

Spiral galaxies have a flat, spinning disc with a bulge in the centre, while elliptical galaxies are round or elongated without a disc.

Spiral galaxies, as the name suggests, have a distinctive spiral structure. They consist of a flat, spinning disc containing stars, gas, and dust, along with a central bulge composed of older stars. Around the outside of the disc, there are usually several arms that spiral outwards. These arms are regions of high star formation, giving them a blue colour due to the presence of hot, young stars. Our own galaxy, the Milky Way, is an example of a spiral galaxy.

On the other hand, elliptical galaxies are round or elongated spheroids, without a disc or spiral arms. They are generally composed of older, low-mass stars, with a minimal amount of interstellar matter, which means they have little ongoing star formation. This gives them a reddish or yellowish colour, as the stars they contain are older and cooler. Elliptical galaxies can range in size from small dwarf galaxies to giant ellipticals which are much larger than typical spiral galaxies.

The differences between these two types of galaxies extend to their dynamics as well. Spiral galaxies rotate, which gives them their flat disc shape, while the stars in elliptical galaxies have more randomised orbits, resulting in a more three-dimensional shape. The formation and evolution of these different types of galaxies are areas of ongoing research in astrophysics, with various theories suggesting that interactions and mergers between galaxies can lead to the transformation from one type to another.