The last major instability region in the HR diagram is the one in the upper right. It is occupied by a variety of interesting objects, known collectively as RED VARIABLES. These occupants include numerous small-range variables with periods of a few weeks, conspicuous large-range variables with periods of a few months and rare supergiant variables […]
There is one type of pulsating variable that lies nowhere near the Cepheid instability strip in the HR diagram: the ? CEPHEI (or ? Canis Majoris) stars. These hot, massive stars have periods of a few hours and ranges of a few hundredths of a magnitude in visible light. The ranges in ultraviolet light are […]
Below the RR Lyrae variables in the Cepheid instability strip, we find the DWARF CEPHEID VARIABLES (sometimes called AI Velorum stars). As their name implies, they are smaller and fainter than Cepheids (or RR Lyrae stars). They have periods of 0.05 to 0.3 day, and ranges of 0.3 to 0.7 magnitude. Those with the larger […]
Even more conspicuous in globular star clusters are the RR LYRAE variables. Whereas a globular cluster may contain a handful of Population II Cepheids at the most, it may contain several hundred RR Lyrae variables. These variables are also found throughout our Galaxy, especially in the halo. RR Lyrae variables are giant stars, larger, hotter […]
In the upper part of the Cepheid instability strip lie the CLASSICAL CEPHEIDS. These stars gave the instability strip its name; they are so called because they resemble the prototype 8 Cephei. This star can be found on almost any star map, and its brightness variations can normally be seen easily by the unaided eye. […]
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 […]
The stars that vary irregularly in brightness are called T TAURI VARIABLES, after their prototype in the dark clouds of Taurus. The light curve of one such variable. T Tauri stars have absorption-line spectra typical of F, G or K giants, consistent with their location above the main sequence. The lines are broadened, probably due […]
Variability begins a* soon as a star forms by the gravitational contraction of an interstellar gas and dust cloud. Within the contracting cloud, a core of dense gas forms, heats up and begins to produce energy. This transition from a dark cloud to a self-luminous star would certainly be visible were it not for the […]
Once a variable has been discovered and announced in an astronomical journal, it is catalogued arid named according to the fol¬lowing system. Stars lacking a proper name (like Polaris) or a Greek letter name (like 8 Cephei) are designated by one or two capital letters, followed by the genitive case of the constellation’s Latin name. […]
INTRINSIC VARIABLE STARS are stars that vary in brightness or other respects for reasons that are internal to the star rather than Aside from their pathological interest they provide important information about the structure, evolution and proper-of the stars. For instance, the timescale of the variation may be related with the luminosity of the star. […]