A lenticular galaxy shows a structure that shares some features with a spiral galaxy, and some with an elliptical galaxy. Like a more conventional spiral galaxy, a lenticular galaxy has the shape of a flattened disc, commonly with a central bulge and bright nucleus. This general structure gives rise the name 'lenticular', which means 'lens-shaped'.
Like an elliptical galaxy, a lenticular galaxy shows little or no sign of interstellar gases, and this means that the disc of a lenticular galaxy generally appears as featureless, though in some cases residual dust can form patterns across the disc. Spiral arms are not usually detectable, but some lenticular galaxies show a central bar analogous to those seen in full barred spiral galaxies.
The origins of the lenticular form is not understood in detail, but in most cases a lenticular galaxy probably represents a later development of a spiral galaxy of more familiar form. On this model, the interstellar gas within the galaxy has been entirely consumed, leaving an undifferentiated population of older, redder stars. The loss of this interstellar medium means that young bluestars can no longer be formed. As an alternative, it has been suggested that some lenticular galaxies may be the ultimate result of the merging together of other galaxies under certain specific conditions.
