Pulsars become black holes

Black holes & Co. / beginners tour part 1: neutron stars and pulsars

More massive stars, which combine between five and forty times as much mass as our sun, come to a dramatic end. If the nuclear fuel, from the fusion of which its glow is fed, is used up, a gigantic explosion occurs, a so-called supernova. On the one hand, the shell of the star is thrown into space, combined with an unimaginable glow.

On the other hand, the core regions of the star continue to collapse. Within a few seconds their density has increased so much that conventional atoms can no longer withstand the pressure. Their electrons and protons combine to form electrically neutral neutrons, and an unimaginably dense ball of nuclear matter is created, not much more than 20 kilometers in diameter, but with a greater mass than the sun: a neutron star. A pinhead full of neutron star matter has more than twice as much mass as the world's largest passenger ship, the Queen Mary II.

Just as the speed of rotation of a figure skater increases like a pirouette when he draws arms and legs, the contraction of the slowly rotating star interior can lead to the formation of a neutron star that rotates around its own axis a few hundred times per second. It is important that an interplay of rotation, magnetic field and surrounding elementary particles generally leads to a neutron star emitting two sharply focused beams of radio waves like a beacon. If the star is oriented so that one of its radio beams sweeps over the earth, a lighthouse effect occurs, as can be seen in the following animation:

 

The neutron star appears to radio astronomers as pulsar, as an object that emits highly regular radio pulses.

Due to their compactness, neutron stars are ideal cosmic laboratories for testing general relativity. The indirect detection of gravitational waves using a double neutron star has already been mentioned in the chapter on gravitational waves. The details of the relativistic influencing of light signals can also be studied with high accuracy using suitable neutron star systems.