- noun a galaxy consisting of an older set of stars in the centre from which two spiral arms of gas, dust, and newer stars extend
- The most common type of galaxy in the Hubble classification, with spiral arms emerging from a central core. In recent years much work has been carried out on the formation of the spiral structure, which seems to be due to pressure waves moving around the galaxy. Our own galaxy, of about 100 billion stars, is a comparatively large member of the class. Spiral nebula is an obsolete synonym.