Single stars

As was noted in Section 1.1, two basic methods have been used to measure rotational velocities of single stars. One of them consists of extracting rotational broadening from a spectral line profile, from which one infers the projected equatorial velocity v sin i along the line of sight. The other one consists of determining the modulation frequency of a star's light due to the rotation of surface inhomogeneities (such as spots or plages) across its surface. If observable, this modulation...

Historical development

The study of stellar rotation began at the turn of the seventeenth century, when sunspots were observed for the first time through a refracting telescope. Measurements of the westward motion of these spots across the solar disk were originally made by Johannes Fabricius, Galileo Galilei, Thomas Harriot, and Christopher Scheiner. The first public announcement of an observation came from Fabricius (1587-c. 1617), a 24-year old native of East Friesland, Germany. His pamphlet, De maculis in Sole...

Ekman pumping and secondary circulation

Away from the ground, the atmosphere adjusts to a geostrophic equilibrium in which the pressure-gradient force balances the Coriolis force associated with a steady flow along the surfaces of constant pressure. If this motion extends to the ground, the effect of turbulent friction is to disrupt this geostrophic balance, thus producing a flow across these surfaces from high to low pressure. Hence, work is being done on the fluid within the surface boundary layer by the pressure-gradient force....

Rotation of evolved stars

Among the many problems that beset the theory of rotating stars, the redistribution of angular momentum in stellar interiors during evolution is by far the least understood. As we know, the post-main-sequence evolution of a star is accompanied by a strong contraction of its helium-rich core and by a corresponding expansion of the surrounding envelope. Unless there exists a very efficient transport of angular momentum from the core to the envelope, it is evident that the former has to spin up...

Close binaries

In Section 1.1 we pointed out that the early-type components of close binaries rotate more slowly than the average of single stars of the same spectral type. In contrast, whereas the rotational velocities of single main-sequence stars of spectral type F5 and later are quite small (i.e., less than 10 km s-1), appreciable rotations are common among the late-type components of close binaries. It has long been recognized that the distribution of rotational velocities in the close binaries is caused...

Contact binaries The astrostrophic balance

Rossby Number Binary Stars

In Sections 4.6 and 8.2-8.4 we have considered detached close binaries, in which the tidal distortions are relatively small and where components display physical characteristics that are similar to those of single stars. When the two components are separated by a few radii only, these tidal distortions may become, however, quite large. This is well illustrated by eclipsing binaries that exhibit sinusoidal-type light curves, and for which the first-order scheme of approximation adopted in the...

P dm dm

Many useful properties can be deduced from these equations. For this purpose, let us define the effective gravity d V 2 d V g U Q2m 1z, 3.18 where 1m and 1z are the unit vectors in the m and z direction. Equations 3.16 and 3.17 become It follows at once that the effective gravity is everywhere orthogonal to the surfaces of constant pressure i.e., the isobaric surfaces . This is a general property, which is valid no matter whether one has Q Q m or Q Q m, z . Let us now assume the star rotates as...