Pulsating Variables (Variable Stars)
Pulsating variables brighten and fade because of a cyclic expansion and contraction of the whole star. As a result, the outer layers of the star alternately approach us and recede from us. These motions produce a periodic Doppler shift in the lines in the spectrum of the star. Using a spectrograph, the astronomer can measure this shift and plot a VELOCITY CURVE – a graph of radial velocity against time – for the star.
These velocity variations were first observed in the pulsating variable 8 Cephei, by A.A.Belopolsky in 1894. At that time, the nature of the variations was unknown, and they were attributed to orbital motion of the star about an invisible companion. By 1914, Harlow Shapley had proved that the variations in brightness, velocity and temperature were not consistent with orbital motion, but could best be explained by the hypothesis of pulsation .
When a star pulsates, it expands past its average size but is pulled back by gravity. Like a swinging pendulum, it overshoots again, and continues to contract. Gas pressure then pushes it back out again. Dissipative or frictional forces should eventually bring the pulsation to a stop; the fact that they do not tells us that some process is constantly feeding mechanical energy into the star. The pulsation motions are quite large in the outer layers of the star: typically 40km s”1 but as much as 200 km s”1 in a few cases. In some stars, the expansion velocity may exceed the escape velocity from the star, and the outer layers may be ejected completely.
A star is a sphere of gas. confined in the centre and free at the surface. The pulsation of a star is similar to the vibration of the column of gas in an organ pipe, closed at one end and open at the other. In both cases, the gas can vibrate in one or more of several MODES. The simplest mode is called the FUNDAMENTAL MODE. In this mode, the amplitude of vibration decreases smoothly from the free end to the closed end. There arc also HARMONIC MODUS, which differ from the fundamental mode in that the periods are shorter and in that there are nodes or stationary points within the gas, The mode in which the star (or the organ pipe) vibrates depends on how and where the driving mechanism the source of mechanical energy – is applied.
Pulsating variables are not distributed at random in the UK diagram: most lie in a vertical band called the CEPHEID INSTABILITY STRIP, or the GREAT SEQUENCE. This merges into a broad instability region in the upper right. Within these regions, many if not most stars pulsate. Furthermore, the period, range and shape of light curve are correlated with the position of the variable in the HR diagram. The largest, most luminous stars have the Iongest periods.