The most common type of galaxy, ranging in shape from nearly spherical (classified as E0) to greatly elongated (classified as E7). Elliptical galaxies vary greatly in size and include some of the largest and smallest known galaxies. They do not have spiral arms and have considerably less interstellar gas and dust than spiral galaxies, with little or no star formation taking place within them. Their stars follow individual elliptical orbits around the center of the galaxy. Long thought to be older galaxies which had used up all the material for star formation, it has also been suggested that elliptical galaxies actually form from collisions between spiral galaxies. Compare irregular galaxy, lenticular galaxy, spiral galaxy. See more at Hubble classification system.