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[en] Light-curve observations of the double-mode Cepheid TU Cas obtained by 10 different groups of observers on several photometric systems over a time span of 67 years have been carefully studied to determine the fundamental and first-overtone periods and their amplitudes on the V magnitude scale. The presence of a second-overtone radial pulsation is discussed, and it is concluded that a previous detection of this mode was spurious because of the lack of a proper zero-point corretion for two groups of observations. It is possible that the amplitude of one of the two modes varies during the entire observing period, with the fundamental-mode amplitude constant at 0.69 +- 0.03 and the overtone amplitude decreasing from 0.4 to 0.25 mag. If this Cepheid displays the two pulsation modes because it is mode switching, this switching time scale may be less than a few hundred years
[en] Nine variable blue stragglers have been found in the globular cluster NGC 5466. The six dwarf Cepheids in this cluster coexist in the instability strip with other nonvariable stars. The three eclipsing binaries are among the hottest of the blue stragglers. The hypothesis is discussed that all blue stragglers in this cluster have undergone mass transfer in close binaries. Under this hypothesis, rotation and spin-down play important roles in controlling the evolution of blue stragglers in old clusters and in affecting some of their observational properties. 14 refs
[en] Determination of the age of the universe from the age of the oldest stars yields values exceeding 15 billion years; it is based on the knowledge of the development of star clusters. Age determination based on radioisotope abundance depends on a number of presumptions and is little accurate. It gives values of 8 to 19 billion years. The cosmological method determines age by the expansion of the universe. At first the Hubble constant only allowed 2 billion years for the age of the universe. More recent values of the constant give a range of 10 to 20 billion years for the age of the universe. (M.D.). 3 figs., 3 tabs
[en] Results are presented of a search for RR Lyrae variables in the SMC clusters NGC 121, Lindsay 1, Kron 3, Kron 7, and Kron 44. The techniques used rediscovered the four RR Lyraes in NGC 121, but no variables were found in the other clusters. It is concluded that RR Lyraes do not occur in SMC clusters younger than about 11 Gyr. 18 refs
[en] A survey of variable-period RR Lyrae field stars of type RRab was conducted. The data for these stars were culled from the fourth edition of the General Catalogue of Variable Stars (Kholopov 1985, 1987). The survey includes stars known to exhibit the Blazhko effect, stars that possibly exhibit the Blazhko effect, stars known only to have variable periods, and stars that possibly have variable periods. A correlation between the length of the pulsation period and the presence of the Blazhko effect in RRab stars was searched for by comparing the period distributions of each of the aforementioned groups of stars to the period distribution of a sample of nonvariable period RRab stars. From the analysis of the data, the mean periods for each of these groups appear to be significantly different from the mean period of the nonvariable period RRab stars. 10 refs
[en] A photometric and spectroscopic study of the Type II cepheid AU Pegasi indicates it is a member of a binary with a period < or approx. =50 days. Constraints on the orbit indicate the companion is likely to be more massive than AU Peg and may be a compact object. The light curve, pulsation-velocity curve, and spectrum of AU Peg are consistent with its cepheid nature. However, the colors are peculiar and the large period changes remain to be explained
[en] The age of the universe is the time that has elapsed since the Big Bang. To calculate the age, the expansion rate of the universe and distance to the galaxies must be determined. Unfortunately, it appears that the expansion rate is not constant but is decelerating. In the 1920's and 30's, Edwin Hubble set out to estimate the age of universe based on the expansion rate and distance to the galaxies. His method is described along with its flaw. Since that time several others have estimated the age of the universe. Their methods as well as results vary. These are discussed in the article. The ages determined from the various methods range from 10 to 20 billion years. There are two independent ways to determine the age of the universe. What they actually do is determine the age of our galaxy which would give a lower limit to the age of the universe. The first method calculates age of globular clusters which yields as age range from 8 to 18 billion years. The second method involves observing the speed at which radioactive substances decay. This also yields and age greater than 10 billion years. It is clear that there is still a lot of work to do before the true age of the universe can be determied
[en] We present low-resolution spectra for variable stars in the Cepheid period range from the ROTSE-I Demonstration Project and the All Sky Automated Survey, some of which were previously identified as type II Cepheid candidates. We have derived effective temperatures, gravities, and metallicities from the spectra. Based on this, three types of variables were identified: Cepheid strip stars, cool stars that lie along the red subgiant and giant branch, and cool main-sequence stars. Many fewer type II Cepheids were found than expected and most have amplitudes less than 0.4 mag. The cool variables include many likely binaries as well as intrinsic variables. Variation among the main-sequence stars is likely to be mostly due to binarity or stellar activity.
[en] The observed modulation of the period and amplitude of Polaris, the short-period classical Cepheid, may be cyclical, though irregular, and superimposed on the continuing evolutionary changes of this star. If so, it curiously resembles the Blazhko effect seen in RR Lyrae stars, as Evans and her colleagues have noted. The present author's recent theory of the Blazhko effect based on a solar-like magnetoconvective cycle in the stellar envelope is here applied to Polaris, with some limited success. The theory may also explain the slow cycle seen in another short-period Cepheid, V473 Lyr. It is therefore possible to predict, tentatively, an entire new class of short-period 'Blazhko Cepheids'.
[en] Ages of 156 classical cepheids in the Galaxy are calculated. A semiempirical period - age relation for cepheids with small (P < or approximately 10 days) and large periods is obtained. The relation is consistent with that derived theoretically and verified by means of cluster cepheids