What are neutron stars?
Neutron stars, the targets of this research program, are extreme objects formed in supernova explosions.
Neutron stars typically have around 40 % more mass than the Sun, but are only about 20 kilometers in diameter: the only objects that are known to be more compact than this are black holes. This image shows a neutron star next to Hannover, home of the MPI for Gravitational Physics.
© NASA's Goddard Space Flight Centre
A pulsar is a compact neutron star that accelerates charged particles to relativistic speeds in its extremely strong magnetic field. This process produces gamma radiation (violet) far above the surface of the compact remains of the star, for example, while radio waves (green) are emitted over the magnetic poles in the form of a cone. The rotation moves the emission regions across the terrestrial line of sight, making the pulsar light up periodically in the sky.
© NASA/Fermi/Cruz de Wilde
Until now, the vast majority of neutron stars have been found via the pulsations that result from their beamed electromagnetic emission periodically sweeping past the Earth; for this reason they are often called pulsars.
While it is believed that the Milky Way contains about a hundred million neutron stars, fewer than 3000 have been detected so far. Either because they do not emit electromagnetically at all, or because their beamed radiation misses this Earth or is absorbed by interstellar matter.
Gravitational waves might well be the only way to unveil this invisible population of extreme objects.