| Summary: | It is a well-known observational fact that the velocity distribution of a group of stars is related to the spectral classes of the stars. In particular, a large vertex deviation exists for stars of early spectral types, which disappears for stars of later spectral types. Also, the velocity dispersions tend to increase with later spectral types.
An examination of the nearby stars yielded relations
between the velocity distribution and the spectral type. Since it was possible to estimate ages for a number of stars, the dependence of the velocity dispersions on age could also be determined.
It was proposed that the observed effects are due to spiral density waves. If account was taken also of the fact that the orbits of the younger stars are not yet well-mixed, then it was found that the predicted values of the vertex deviation agree quite well with the observational
values. The increase in the velocity dispersion can be explained if the spiral pattern has dissolved and reformed a number of times. From a comparison of the theoretical age-dependence of the velocity dispersion and the observational curve it was possible to estimate the number of spiral patterns which have existed, their amplitudes
and their decay rates. === Science, Faculty of === Physics and Astronomy, Department of === Graduate
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