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Stars


Variable Stars

Variable Stars The changes of brightness in (intrinsic) variable stars indicate that something is happening to them. The cause must be truly physical, because changes of color, spectrum, magnetic field, and radial velocity accompany the changes in light. Some variable stars display a more or less regular rhythm, or period, and are known as the periodic variables. Others, only roughly periodic, are know as the cyclic or semiregular variables, and then there are stars whose variations show no obvious pattern, the irregular variables. Far more spectacular are the changes shown by some stars that undergo some sort of explosion - the so-called cataclysmic variables. These include the novae (new stars), and the

Figure 08-07 HR, Variable Stars [view large image]

supernovae, which undergo the largest changes and attain the greatest luminosities recorded for any variable or nonvariable stars. Figure 08-07 shows the various types of variable stars in the HR diagram. Table 08-02 summarizes the properties of all the types.
It seems that these variable stars represent stars with different mass in various evolutionary stages. In brief, the T Tauri stars occur during an early stage of stellar evolution. While near the main sequence, stars do not tend toward variability, except for local flares. After evolution removes them from the main sequence, stars may become pulsating variables in the "instability strip" (see Figure 08-07) or other types of variable stars. Most of the variable stars have evolved to late stages in their life cycles and are located above and to the right of the main sequence on the HR diagram. Pulsation of some variable stars is related to the initiation of helium and various nuclear burning after running out of hydrogen fuel (see Figure 08-05 a and b). In the final stage the star becomes the cataclysmic variables.

Type Period Range Mag.
Range
Spectral Types Mean Abs. Mag. Spatial Distribution
Classical Cepheids 2 - 8 d 1 F, G superg. -3 Dust-filled galactic plane
RR Lyrae 0.1 - 1 d 1 A, F giants 0 Dust-free galactic nucleus
Type II Cepheids
(W Vir, RV Tau)
1 - 100 d 1 F-G, G-K -2 High galactic latitude, halo
Long Period 90 - 600 d 3 - 6 M,S,R,N (em) -1, 0 Dust-free galactic plane
Semiregular ~ 100 d 1 M,S,R,N -2 Dust-filled galactic nucleus
Irregular   0.1 M,S,R,N -2 Dust-filled galactic nucleus
Beta Cepheids (CM) 3 - 6 h 0.1 B -3 Dust-Filled regions
Dwarf Cepheids 1 - 3 h 0.2 - 1 A - F +2 Dust-filled regions
Magnetic or Spectrum 0.5 - 1 d 0.1 A 0  
R Coronae Borealis Stars irrg. (fading) 6 G, K, R (em) -3 Low galactic lat., carbon stars
Flare Stars irrg. 6 K, M (em) +10 Lower main sequence stars
T Tauri Stars irrg. 1 - 3 G, K - M +5, +2 Dark clouds of dust & gas

Table 08-02 Types of Intrinsic Variables

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