Seismology of the Sun and Stars
Ground-based observations of the spectrum of solar oscillations have already permitted us to put tight constraints on the hydrostatic stratification of the sun, and to infer the angular velocity throughout much of the solar interior. From this information we have identified an error in earlier calculations of stellar opacity, and we have evidence that present understanding of the equation of state is inadequate for determining the eigenfrequencies of solar models to the accuracy of the observations. The consequent revision of opacity calculations has already resolved several longstanding problems concerning a variety of pulsating stars. Moreover, we have a plausible estimate of the quadrupole moment J 2 of the sun’s gravitational potential, which is substantially more reliable than any other, and which, in conjunction with radar ranging measurements, is consistent with General Relativity. Currently available observations are limited by our failure to observe the sun continuously; in particular, we are thus inhibited from making firm deductions about the structure of the thermonuclear energy generating core.
The new generation of helioseismic instruments will surely add substantially to our diagnostic information. The instruments include ground-based networks of non-imaging spectrometers, such as IRIS and BISON which are already operational, the future ground-based network of imaging spectrometers of the GONG, and the suite of helioseismic instruments to be flown on the spacecraft SOHO. From these we hope to learn about the physical conditions under which the nuclear reactions are taking place, and thereby, in conjunction with the next generation of neutrino detectors, make a significant contribution to neutrino physics. We shall also determine the angular velocity throughout the solar interior, permitting us to calculate J 2 to much higher accuracy, and, of course, learn more about the thermodynamic properties of dense plasmas. Our hope is that we shall also learn about large-scale convective flows, asphericity in the interior structure, and how all these phenomena vary with the solar cycle.
Astroseismology is not yet a reality. Of course, it will not be possible to determine the interior structures of other stars in anything like as much detail as is possible for the sun. However, theoretical calculations have shown that in principle one can infer the structure of stellar cores, and one can determine the locations and possibly even the nature of near discontinuities in the stratification, such as occur at the boundaries of convection zones. Less ambitiously, one can obtain some information about the gross rotational shear, and one can calibrate stars in open clusters, and thereby provide important tests of the theory of stellar evolution. It is quite impractical for the observations required for such calibrations to be carried out from the ground, but it is relatively straightforward to do so from space.
KeywordsSound Speed Convection Zone Solar Model Solar Interior Solar Convection Zone
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