Elliptical galaxies are characterised by an absence of spiral arms. Most are deficient in dust and few possess regions of current star formation. From observations it is seen that eighty per cent of galaxies are elliptical. One theory states that ellipticals are produced by galactic mergers of spiral galaxies . See also cD galaxy .
Emission nebulae .
A cloud of interstellar gas which is glowing because it absorbs, and then re-emits, electromagnetic radiation from hot stars embedded within it. The predominant wavelength of radiation that is released by the hydrogen gas is 656 nm which is in the red end of the electromagnetic spectrum. Thus the emission nebulae are a characteristic pinkish-red in colour. As the solar winds from the ionising star blow away the central regions, the nebula dissipates. Emission nebulae are usually isolated regions of much larger interstellar clouds .