They are somewhat poorly understood, most likely binary star systems in which mass exchange is ongoing or has recently happened, and most likely consist of at least one B-type main-sequence stars in a dust envelope. They undergo irregular light variations over long periods of time, with magnitude variation of approximately 2 magnitudes. The hydrogen emission lines from these stars are much stronger than in normal Be stars, and certain forbidden lines are present in the spectrum as well so they are classified as B[e] stars.
FS CMa stars are rare and it has been difficult to determine their properties well enough to determine what causes their unusual character. Several have been detected in massive star clusters and this has ruled out some theories about their origin. The preferred explanation is still for a binary origin, possibly as the result of stellar mergers.
Serpens ("the Serpent", Greek Ὄφις) is a constellation of the northern hemisphere. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the 88 modern constellations defined by the International Astronomical Union. It is unique among the modern constellations in being split into two non-contiguous parts, Serpens Caput (Serpent Head) to the west and Serpens Cauda (Serpent Tail) to the east. Between these two halves lies the constellation of Ophiuchus, the "Serpent-Bearer". In figurative representations, the body of the serpent is represented as passing behind Ophiuchus between Mu Serpentis in Serpens Caput and Nu Serpentis in Serpens Cauda.
The brightest star in Serpens is the red giant star Alpha Serpentis, or Unukalhai, in Serpens Caput, with an apparent magnitude of 2.63. Also located in Serpens Caput are the naked-eye globular cluster Messier 5 and the naked-eye variables R Serpentis and Tau4 Serpentis. Notable extragalactic objects include Seyfert's Sextet, one of the densest galaxy clusters known; Arp 220, the prototypical ultraluminous infrared galaxy; and Hoag's Object, the most famous of the very rare class of galaxies known as ring galaxies.
Part of the Milky Way's galactic plane passes through Serpens Cauda, which is therefore rich in galactic deep-sky objects, such as the Eagle Nebula (IC 4703) and its associated star cluster Messier 16. The nebula measures 70 light-years by 50 light-years and contains the Pillars of Creation, three dust clouds that became famous for the image taken by the Hubble Space Telescope. Other striking objects include the Red Square Nebula, one of the few objects in astronomy to take on a square shape; and Westerhout 40, a massive nearby star-forming region consisting of a molecular cloud and an H II region.