ESA Science & Technology14-Jul-2005 14:01:05


Neutron Stars

When a massive star explodes, not all the material is ejected into space. Some of it collapses to form an extremely compact object: a neutron star or a black hole. A neutron star is a body of about the same mass as our Sun but with a radius of only 10 kilometres. Matter in a neutron star is so compressed that one drop of it would weigh millions of tonnes on Earth. Gravitation crushes protons and electrons together despite their electrostatic repulsion. A neutron star is therefore a huge atomic nucleus essentially constituted of neutrons.

A neutron star can sometimes possess an intense magnetic field, which can also accelerate particles and cause them to emit gamma rays.

Neutron stars can have a maximal mass of 2 to 3 times the mass of the Sun. Beyond this critical mass, the internal pressure against gravitational collapse is no longer sufficient and the star becomes a black hole. Nothing, not even light, can escape from a black hole.

Virtually all types of compact objects are significant sources of high-energy emission because the enormous gravitational field accelerates matter close by to extreme velocities. The fast particles emit gamma radiation. INTEGRAL will image the compact objects with unprecedented detail at high energies and the spectroscopic capabilities of the mission will provide the first detailed physical diagnostics of these systems at gamma-ray energies.

Many riddles remain to be solved. For example, on 27 August 1998 a powerful flash of gamma rays turned night into day in the Earth's outer atmosphere. The flash was caused by a distant neutron star with an extremely strong magnetic field: a magnetar. Are there really millions of magnetars in our Galaxy, as many scientists think? INTEGRAL will gather more data about these strange compact objects.

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