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Identification
Galaxies are disks of stars that usually take an elliptical or spiral shape. However, when a galaxy has no identifiable structure, it is a lenticular galaxy. Most are round like spiral galaxies, but they lack spiral arms, according to Sloan Digital Sky Survey. Like elliptical galaxies, they tend to have bright central bulges.
Formation
Astrophysicist David Palmer, writing for NASA's Goddard Space Flight Center, explains that a lack of interstellar gas gives lenticular galaxies their unusual shape. Interstellar gas is what forms new stars, which make up the arms of spiral galaxies. Lenticular galaxies, which have no arms or new stars, may have experienced collisions with other galaxies that left them without this star-forming gas.
Composition
Lenticular galaxies are composed of old stars. The online Messier Catalogue of deep sky objects states that the lack of new stars means old stars in lenticular galaxies have time to settle into position with even spacing. This gives them their subtle, round shape.
Considerations
Lenticular galaxies are often mistaken for elliptical galaxies due to their similar shapes. Like lenticular galaxies, elliptical galaxies have little interstellar gas left, few new stars, and no spiral arms. However, lenticular galaxies are actually an intermediate stage between spiral galaxies and elliptical galaxies, according to Richard Gelderman, a member of the faculty of the department of physics and astronomy at Western Kentucky University.
Examples
Several famous lenticular galaxies exist. An example is a bright galaxy in the Virgo Cluster called Messier 84, which Charles Messier discovered in 1781. Another is MCG-6-30-15, which looks plain from a distance of 130 light years, but actually contains spinning black holes, according to Discover Magazine.