# relativistic astrophysics

## relativistic astrophysics

(rel-ă-tă-viss -tik) High-energy astrophysics, concerned with the extreme energies, velocities, densities, etc., associated with celestial objects such as white dwarfs, neutron stars, black holes, and active galaxies, and with the very early Universe. It involves the theories of special and general relativity, quantum mechanics, and the physics of elementary particles.

## Relativistic Astrophysics

the branch of astrophysics that studies astronomical phenomena and celestial bodies under conditions for which classical mechanics and Newton’s law of gravitation are inapplicable. These conditions include a speed close to that of light, extremely high values of pressure and energy density (reaching or exceeding the density of the rest mass multiplied by the square of the speed of light), and extremely high values of gravitational potential (close to the square of the speed of light). The special and general theories of relativity underlie relativistic astrophysics.

The first work whose contents addressed relativistic astrophysics appeared in 1916, when K. Schwarzschild theoretically investigated the gravitational field around a strongly compressed mass. He introduced the concept of the gravitational radius rg corresponding to a mass M: rg = 2GM/c2, where G is the gravitational constant and c is the speed of light (rg is equal to 3 km for the sun and to 1 cm for the earth). This concept played a major role in the subsequent development of relativistic astrophysics.

Superdense stars, whose mass is concentrated within a sphere of radius smaller than rg, have a number of extraordinary properties. Thus, a particle impinging on the star as it approaches the gravitational radius acquires a speed approaching that of light.

Relativistic time dilation becomes infinite near the gravitational radius. A distant observer having the necessary instruments would see that as t → ∞, a particle asymptotically approaches a sphere of radius equal to rg, but the observer would not be able to see how the particle would intersect the sphere. Energy cannot escape from within this sphere. Thus was laid the basis for the modern theory of “black holes.”

Ordinary stars of sufficiently large mass are transformed at the end of their evolution into neutron stars, in which the density of matter reaches 10l4-1015 g/cm3; this transformation was explained in the 1930’s and 1940’s by the American astronomers W. Baade and F. Zwicky, the Soviet physicist L. V. Landau, the American physicist R. Oppenheimer, and the Canadian physicist G. M. Volkoff. As a result, stars with a mass close to that of the sun are transformed into neutron stars with a radius of approximately 10 km and a gravitational potential reaching 0.3c2 at the surface. Later, the ways in which ordinary stars with a mass two to three times greater than that of the sun are transformed into black holes were also studied.

The rapid development of relativistic astrophysics in the 1960’s led to a purposeful search for possible manifestations of relativistic states of stars. It was found that stars in such a state can act as invisible satellites in binary systems where the second component is a normal star. Jets of gas captured from surrounding space and accelerated to a speed close to that of light can serve as a source of X-radiation on collision with the surface of a neutron star or on collision with another jet of gas. However, relativistic astrophysics gained wide recognition after the discovery in 1967 of pulsars, which are rapidly rotating neutron stars.

Sources of X-radiation within binary stars were discovered using instruments carried outside the earth’s atmosphere. Some of these sources have proved to be neutron stars with a strong magnetic field that emits directional fluxes of X-radiation. This radiation results from the flow of gas from the surface of a normal star in a binary system to the surface of a neutron star. In two cases it may be assumed with high probability that one of the components is a black hole, in whose gravitational field the gas flowing from the surface of the other component—the normal star—is heated and emits X rays. In an investigation of the process of the compression of a normal star into a neutron star, it was found that when the compression occurs, the magnetic field is amplified in inverse proportion to the surface area of the star, that is, by a factor of billions.

The theory of quasars has been worked out to a lesser degree. However, there is no doubt that the magnetic field, the internal motions of gas, and relativistic particles play a major role in these objects. A black hole also may be present at the center of a quasar. The study of cosmic rays and of the gamma radiation resulting from the interaction of protons and heavier nuclei in cosmic rays with interstellar matter is afforded a prominent place in relativistic astrophysics.

Explosions of supernovas accompanied by the formation of neutron stars and black holes and apparently leading to the ejection of high-speed particles, that is, cosmic rays, are also subjects of investigation in relativistic astrophysics. Relativistic astrophysics also studies gravitational waves.

Relativistic astrophysics closely borders on cosmology in its conclusions. Its problems are being studied intensively in the USSR, the USA, and Great Britain.

### REFERENCES

Zel’dovich, Ia. B., and I. D. Novikov. Reliativistskaia astrofizika. Moscow, 1967.
Zel’dovich, Ia. B., and I. D. Novikov. Teoriia tiagoteniia i evoliutsiia zvezd. Moscow, 1971.
Zel’dovich, Ia. B., and I. D. Novikov. Stroenie i evoliutsiia Vselennoi. Moscow, 1975.
Peebles, P. Fizicheskaia kosmologiia. Moscow, 1975. (Translated from English.) Ia. B. Zel’dovich
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