Spiral galaxies are a type of galaxy that are disc-like in shape but have their disc of stars spun into a set of spiral arms where there is a higher concentration of nebulae and large, bright stars. It is believed that stars will regularly "pile up" inside these arms like a traffic jam, and then leave the arm to travel to the next one. Like elliptical galaxies, they vary in size from millions of stars to trillions.
Spiral galaxies can be subdivided from Sa to Sd, with later letters indicating a looser and more diffuse spiral. The Sa subcategory is usually reserved for the tightest galaxies, with Sd being the most diffuse.
| E · S0 · SA · SB · SM · Irr |
dSph · dE · dIrr