A shell star is thought to be surrounded by a disc of gas at its equator. These stars are also known as Gamma Cassiopeiae variables because the first example was found in the constellation Cassiopeia. A shell star is called "variable" because of the irregular changes, or variability, in luminosity, caused by the surrounding cloud of gas. This process is not entirely understood, but may be related to the rapid rotation of all known stars of this type.
Stars are classified according to their spectral characteristics and assigned a letter, and shell stars are divided into four groups based on this classification. The letters are, in order, O, B, A, F, G, K, and M. Stars at the O end of the spectrum, called blue stars, are the hottest. Stars at the other end of the spectrum are the coolest stars and are classified as red stars. Three types of shell stars fall under the O and B, or blue and blue-white, portions of the spectrum, and the fourth group belongs to the group of stars in the A-F range, or white and yellow-white stars. A majority of shell stars fall in the B range.
The letter "e" is often applied as a secondary classification to a star to indicate heightened emissions, and in most cases, a shell star will be denoted as such. The increased emissions are from the hydrogen portion of the emission spectrum, meaning that these stars exhibit larger amounts of hydrogen, in a higher energy state, than other stars. A shell star may also show increased emission spectra for other elements such as iron, helium, and calcium, among others.
Stars are also classified according to size, which also corresponds to overall luminosity or brightness. Most shell stars are among the larger star types. Roman numerals are used to indicate this classification, with I stars being the largest and V stars the smallest. Our own sun is a size V star. A majority of shell stars fall into the III-IV range, but some are found in the V class.
The variability in luminosity and emission spectra for shell stars makes it difficult to fully understand them or to classify them accurately, as they may appear to fall into different categories according to accepted criteria, at different times. The extremely fast rotation is thought to have a major role in this variability but does not fully explain the changes in luminosity or emissions. In 2011, astronomers are still trying to explain the mechanism behind the gas disk and its relation to shell stars' variability.