and black holes collide and merge and might be origin of the enigmatic cosmic gamma-ray bursts.
Explanations to some keywords:
A neutron star is an extremely dense star with about the mass of the Sun but a diameter of only 20 kilometers. Neutron stars are born as compact remnants of supernova explosions which occur when the iron core of a massive star collapses at the end of the star's life. Neutron star matter consists mostly of neutrons which are compressed to a density higher than that in atomic nuclei.
Gravitational waves are ripples of space-time which propagate at the speed of light and according to Einstein's theory of General Relativity are caused by moving masses or energy. Large masses, enormous energies and very high velocities have to be involved to produce gravitational wave signals which are strong enough to be possibly measured. Currently huge experimental facilities are being built in Europe, the US and Japan (GEO600, VIRGO, LIGO, TAMA) to detect the gravitational waves from supernova explosions and neutron star mergers even in distant galaxies. The picture on the left side shows LIGO. The size of the L-shaped facility is 4km on each side.
Developed by A. Einstein in 1915, the theory of General Relativity describes gravity by the curvature of space-time, which is caused by the presence of masses and energy. In curved space-time, particles as well as light propagate differently than in a flat space-time. Large deviations from Newtonian gravity occur in particular near compact astrophysical objects like neutron stars and black holes. Classical tests of the implications of General Relativity include the measurement of the deflection of light from background stars by the Sun, the redshift of photons travelling upward in the Earth's gravitational field, and the perihelion motion of Mercury's orbit.
A neutrino is an elementary particle whose existence was postulated by W. Pauli in 1930 to explain energy conservation in nuclear reactions. It took more than 30 years to experimentally confirm its existence. Neutrinos exist in three different "flavors'', do not have charge, probably have more than hundred thousand times smaller masses than electrons, and interact with matter extremely weakly. In astrophysical objects they are produced when particle or nuclear reactions take place at extreme densities and temperatures, for example in the early Universe, in stellar cores and supernova explosions.
More than to his gravitational radius compressed matter is forming a black hole. According to Einstein's theory of General Relativity, matter which collapses to a radius smaller than its gravitational radius or event horizon decouples from its surroundings, which means that nothing, not even light, can leave this region, called black hole, and reach the external world. The gravitational radius of the Sun is about three kilometers, but of course the Sun is a regular star and has a radius much larger than this. There are strong observational hints that very massive black holes might exist at the centers of most galaxies, and stellar mass black holes seem to be one of the components of a number of X-ray binaries observed in the Milky Way.
Cosmic gamma-ray bursts are extremely energetic outbursts of radiation at gamma-ray wavelengths which last between fractions of a second and many minutes. They were discovered first by US satellites in the late 60's. Their origin is still one of the biggest mysteries of modern astronomy. The COMPTON Gamma-Ray Observatory explores these bursts from the Earth orbit. It records one to two events per day. The discovery of afterglows in X-rays, optical, and radio emission, made possible by the Italian/Dutch BeppoSAX satellite, confirmed the hypothesis that the sources of these bursts are at cosmological distances, billions of light-years away from the Earth. To date it is not clear which kind of astrophysical events produces these most powerful explosions since the Big Bang, which are visible throughout the Universe. The mergers of binary neutron stars or neutron stars with black holes happen frequently enough and could provide sufficient energy.
Transcript of the movie text:
General Relativity predicts that binary neutron stars emit gravitational waves. Therefore their orbital separation shrinks continuously. After hundreds of millions of years, the distance of the two stars is comparable to their diameters. They are orbiting around each other at 20% of the speed of light. The final merging takes place in a thousandths of a second. This computer simulation shows how the stars heat up to a temperature of more than 100 billion degrees Celsius. Cool gas appears in red, while yellow, green, blue and white are hotter. The merger remnant collapses to a black hole shortly after the merging. This black hole swallows the surrounding gas within a fraction of a second. Before it disappears, the glowing gas radiates a huge amount of neutrinos. Once in a hundred thousand years in a galaxy like the Milky Way, a neutron star is going to collide with a companion black hole. In such a cosmic catastrophy, the neutron star is torn apart as it spirals in towards the black hole. The heated gas becomes a bright source of neutrinos. Reactions between these neutrinos give rise to a powerful burst of gamma rays. The animation shows an artist's conception of such a cataclysmic death of a neutron star. A brilliant outburst of gamma radiation can be seen across the whole Universe. Every day satellites discover one or two of these cosmic gamma-ray bursts. Afterglow emission, observed with huge telescopes for several days, has confirmed their origin from galaxies even billions of light-years away.
Two neutron stars - remnants from supernova explosions - orbit around each other sending out intense gravitational waves. They slowly approach each other on aspiral orbit until they collide and merge to form a dense massive object which will collapse to a black hole under its own gravity. However, a small part of the matter gathers in a doughnut-shaped gas cloud orbiting the central black hole. Internal friction heats up the matter to very high temperatures and helps to tap the gravitational binding energy of the gas cloud. This energy reservoir may serve as a source for a gamma-ray burst.