a star whose outer layers are observed to depart significantly from equilibrium. This nonequilibrium manifests itself by variations in the brightness or spectrum of the star and by the presence of emission lines in the spectrum. Strictly speaking, stationary stars do not exist. Our sun constantly experiences departures from equilibrium in its outer layers in the form of the ejection of prominences. However, the stellar characteristics of the sun, such as luminosity, spectrum, diameter, and mass, remain constant over a very long period of time; thus, the sun is a stationary star.
A particular, but very common, type of nonstationary star is the intrinsically variable star, which is characterized by a variation in brightness. The most common manifestations of stellar instability are ejection of material from the star into the surrounding space, pulsations, and shells.
Material may be ejected at a constant rate for a certain period of the star’s life. This is observed to be the case for various classes of stars, such as Wolf-Rayet stars and P Cygni stars. Material may also be ejected all at once in the form of an explosion, as happens in novae, supernovae, and certain nova-like stars.
In the case of pulsations, the star periodically contracts and expands; however, material does not leave the star. This phenomenon is characteristic of cepheids and other pulsating variables; if such variables have constant periods, they are called quasi-stationary stars. Stellar shells, which can be detected by the presence of emission lines in the spectrum, are formed as a result of the ejection of material from the star and imply a past or present nonstationary state. The causes of nonstationary states of stars are very diverse. We can distinguish internal and external causes; an example of the latter is the influence of one star on the other in a close binary.
From the viewpoint of the modern theory of stellar evolution, all stars pass through a number of nonstationary stages as they evolve. At such times, a variation in the star’s energy sources and the associated rearrangement of the star’s structure lead to substantial variations in the star’s characteristics, which are observable as a variation in the brightness and spectrum of the star.
V. P. ARKHIPOVA