Neutron-star oscillation
Asteroseismology studies the internal structure of our Sun and other stars using oscillations. These can be studied by interpreting the temporal frequency spectrum acquired through observations. In the same way, the more extreme neutron stars might be studied and hopefully give us a better understanding of neutron-star interiors, and help in determining the equation of state for matter at nuclear densities. Scientists also hope to prove, or discard, the existence of so-called quark stars, or strange stars, through these studies. Fundamental information can be obtained of the General Relativity Theory by observing the gravitational radiation  from oscillating neutron stars. Types of oscillations. The modes of oscillations are divided into subgroups, each with different characteristic behavior. First they are divided into toroidal and spherical modes, with the latter further divided into radial and non-radial modes. Spherical modes are oscillations in the radial direction while toroidal modes oscillate horizontally, perpendicular to the radial direction. The radial modes can be considered as a special case of non-radial ones, preserving the shape of the star in the oscillations, while the non-radial do not. Generally, only the spherical modes are considered in studies of stars, as they  are the easiest to observe, but the toroidal modes might also be studied. In our Sun, only three types of modes have been found so far, namely p-, g- and f- modes. Helioseismology studies these modes with periods in the range of minutes, while for neutron stars the periods are much shorter, often seconds or even milliseconds. The extreme properties of neutron stars permit several others types of modes. More details on stellar pulsation modes and a comparison with the pulsation modes of black holes can be found in the Living Review by Kokkotas and Schmidt. Oscillation excitation. Generally, oscillations  are caused when a system is perturbed from its dynamical equilibrium, and the system, using a restoration force, tries to return to that equilibrium state. The oscillations in neutron stars are probably weak with small amplitudes, but exciting these oscillations might increase the amplitudes to observable levels. One of the general excitation mechanisms are eagerly awaited outbursts, comparable to how one creates a tone when hitting a bell. The hit adds energy to the system, which excites the amplitudes of the oscillations to greater magnitude, and so is more easily observed. Apart from such outbursts, flares as they are often  called, other mechanisms have been proposed to contribute to these excitations: Mode damping. The oscillations are damped through different processes in the neutron star which are not yet fully understood. The damping time is the time for the amplitude of a mode to decay to e. A wide variety of different mechanisms have been found, but the strength of their impact differs among the modes. Observations. So far, most data about neutron-star oscillations come from the blasts of four specific Soft Gamma Repeaters, SGR, especially the event of 27 December 2004 from SGR 1806-20. Because so few events  have been observed, little is known for sure about neutron stars and the physics of their oscillations. The outbursts which are vital for analyses only happen sporadically and are relatively brief. Given the limited knowledge, many of the equations surrounding the physics around these objects are parameterized to fit observed data, and where data is not to be found solar values are used instead. However, with more projects capable of observing these kinds of blasts with higher accuracy, and the hopeful development of w-mode studies, the future looks promising for better understanding one of the Universe's most exotic  objects. These oscilations can be observed through a gravitational wave observatories, like LISA. These kind of observations carry important information of the matter content of a neutron star, as well as fundamental information of the very neture of the spacetime itself.