A lenticular is a lenticular galaxy, which is a galaxy that is in between a spiral (galaxy) and an elliptical (galaxy), as described by its morphology (which is just a fancy word for shape).
Lenticulars have discs, like spirals, and may have weak spiral arms; however, they show no (or very little) sign of ongoing star formation … there are no HII regions, the spiral arms are smooth (not clumpy), what little dust there is (if any) is near the nucleus, and so on. In the Hubble classification scheme – in which ellipticals are E and spirals S – lenticulars are S0. Lenticulars may have bars or rings (or both), but, like any spiral arms they may have, they will be weak. Unless they are (almost) edge on, lenticulars may be very hard to distinguish from ellipticals. Ellipticals and lenticulars are called early-type galaxies, spirals (and irregulars) late-type.
Among the Messier objects which are galaxies, four are lenticulars, M84, M85, M86, and M102 (NGC 5866); the brightest, and perhaps most spectacular, lenticular is NGC 5128, or Centaurus A (Cen A). While Cen A is a lenticular, by morphological definition, it is quite unlike most lenticulars … it seems to be a galaxy collision, between a spiral and an elliptical, and there is a great deal of star formation going on!
The second-brightest lenticular, NGC 3115 (also known as the Spindle galaxy) is much more typical – clearly a disc galaxy, but no sign of star formation (lots of old, Population II, stars, as seen by its color), and no obvious dust lane. In other words, a spiral galaxy in which star formation has stopped, because most of the gas and dust has been turned into stars, or driven out of the galaxy, or stripped from it.
So which of these three mechanisms – or combo – for removing the gas and dust from a spiral accounts for how most lenticulars came to be lenticular? It’s an active area of research, in extra-galactic astronomy; for example, spirals which pass through the center of a rich cluster should have their gas and dust stripped, by the interaction with the hot plasma at the center of such clusters.
Universe Today stories about lenticulars often reflect their special place in galaxy evolution; for example GALEX Spies a Ghost – And It’s Alive!, and Big Galaxies, Older Stars. Some others simply feature magnificent images of lenticulars, e.g. Hubble View of NGC 5866, and Hubble Zooms In On Coma Galaxy Cluster.
Two Astronomy Cast episodes are worth a listen, Galaxies, and The Story of Galaxy Evolution.
Sources: Wikipedia, Caltech Cool Cosmos, SEDS.org, Atlas of the Universe
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