Shell stars are variable stars with a disk of gas at their equator, causing irregular changes in brightness. They are classified by spectral characteristics, size, and emission spectra. The mechanism behind the gas disk and variability is not fully understood.
A shell star is thought to be surrounded by a disk of gas at its equator. These stars are also known as Gamma Cassiopeiae variables because the earliest example was found in the constellation Cassiopeia. A shell star is called a “variable” because of the irregular changes, or variability, in brightness caused by the surrounding gas cloud. This process is not fully 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 W end of the spectrum, called blue stars, are the hottest. Stars at the other end of the spectrum are the cooler 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 AF range, or white and yellow-white stars. Most shell stars are in range B.
The letter “e” is often applied as a secondary classification to a star to indicate increased emissions, and in most cases, a shell star will be designated as such. The increased emissions come from the hydrogen portion of the emission spectrum, meaning these stars display greater amounts of hydrogen, in a higher energy state, than other stars. A shell star can also show increased emission spectra for other elements such as iron, helium and calcium, among others.
Stars are also classified by size, which also corresponds to overall luminosity or luminosity. Most shell stars are among the largest types of stars. Roman numerals are used to indicate this classification, with I stars being the largest and V stars being the smallest. Our sun is a V-size star. Most shell stars are in the III-IV range, but some are in the V class.
The variability of the brightness and emission spectra of shell stars make it difficult to fully understand or accurately classify them, as they may appear to fall into different categories according to accepted criteria, at different times. Extremely fast rotation is thought to play a large role in this variability, but does not fully explain changes in brightness or emissions. As of 2011, astronomers are still trying to explain the mechanism behind the gas disk and its relationship to the variability of shell stars.
Protect your devices with Threat Protection by NordVPN
