Seismology of the Stars

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Science  31 May 2002:
Vol. 296, Issue 5573, pp. 1571
DOI: 10.1126/science.296.5573.1571a

The Sun exhibits rapid and periodic variations in its luminosity. These oscillations are produced in the outer layers, where slight imbalances between gas pressure and gravitational force result in radial expansions and contractions. There are two primary modes: the low-radial-order pressure modes (p modes) and the higher-radial-order gravitational modes (g modes). The type and complexity of these modes relate to the diameter, mass, temperature, age, magnetic field strength, and environment around the star, making asteroseismology particularly valuable for understanding stellar dynamics and evolution.

Two types of pulsating stars, δ Scuti and γ Doradus, lie at the intersection of the main sequence and the instability curve (on which most pulsators live) on a color-versus-magnitude diagram of stellar evolution. Handler and Shobbrook have conducted a spectroscopic and photometric survey of 26 candidate pulsators in the southern skies, using telescopes in South Africa, Australia, and Chile. They found six bona fide γ Doradus stars (bringing the total number known to 20) and one δ Scuti. In a separate paper, Handler et al. describe one star, HD 209295, that showed both δ Scuti p-mode and γ Doradus g-mode oscillations, the first time this has been observed. The g-mode oscillations appear to be induced tidally by a secondary component that may be a neutron star or white dwarf. This binary system has an eccentric orbit, which is unexpected because a compact degenerate secondary tends to circularize the orbit rather quickly; HD 209295 may be a relatively young binary system that is on its way to becoming an x-ray binary. — LR

Mon. Not. R. Astron. Soc.333, 251; 262 (2002).

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