Asteroseismology
The study of stellar oscillations to probe internal structure.
Wikipedia / Wikimedia Commons
Asteroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the local speed of sound, which in turn depends on local temperature and chemical composition. Because the resulting oscillation modes are sensitive to different parts of the star, they inform astronomers about the internal structure of the star, which is otherwise not directly possible from overall properties like brightness and surface temperature.
- field
- Astrophysics
- known_for
- Studying stellar oscillations to infer internal structure
Lore & Background
Asteroseismology is closely related to helioseismology, the study of stellar pulsation specifically in the Sun. Though both are based on the same underlying physics, more and qualitatively different information is available for the Sun because its surface can be resolved. By linearly perturbing the equations defining the mechanical equilibrium of a star and assuming that the perturbations are adiabatic, one can derive a system of four differential equations whose solutions give the frequency and structure of a star's modes of oscillation. The stellar structure is usually assumed to be spherically symmetric, so the horizontal component of the oscillations is described by spherical harmonics, indexed by an angular degree and azimuthal order. In non-rotating stars, modes with the same angular degree must all have the same frequency because there is no preferred axis. The angular degree indicates the number of nodal lines on the stellar surface, so for large values of the angular degree, the opposing sectors roughly cancel out, making it difficult to detect light variations. As a consequence, modes can only be detected up to an angular degree of about 3 in intensity and about 4 if observed in radial velocity.
Reader's Guide
Excitation mechanisms include the kappa-mechanism, surface convection, convective blocking, and tidal excitation. Pulsations driven by the kappa-mechanism are coherent and have relatively large amplitudes; it drives the pulsations in many of the longest-known variable stars, including the Cepheid and RR Lyrae variables. In stars with surface convection zones, turbulent fluid motions near the surface simultaneously excite and damp oscillations across a broad range of frequency; this is the driving mechanism in all solar-like oscillators. Convective blocking is believed to drive pulsations in the Gamma Doradus variables. Tidal excitation occurs in eccentric binary systems known as heartbeat stars.
Did You Know?
- Asteroseismology is the study of oscillations in stars.
- The angular degree indicates the number of nodal lines on the stellar surface.
- Modes can only be detected up to an angular degree of about 3 in intensity and about 4 in radial velocity.
- Cepheid variables are core-helium burning stars with masses above about 5 solar masses.
- Solar-like oscillations also occur in evolved stars such as subgiants and red giants.
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