How stars die

Radio astronomy has shown that in the universe there are many phenomena characterized by rapid and violent evolution. Supernovae, for example, are a phenomenon during which massive stars explode and are destroyed, throwing material out into space at speeds of up to 10,000 km/second. The star becomes exceptionally bright, billions of times brighter than the sun.
While the outer part explodes, the inner part (approximately 1.4 solar masses) implodes due to the effect of gravity. The final product of this collapse may be a neutron star.
The diameter of a neutron star is approximately 10 km, with a density of around10 14 g/cm³, more or less that of the atomic nucleus. An imaginary teaspoon of neutron star material on the earth would weigh one billion tons. Neutron stars are called pulsar because of their characteristic radio emission pulses.

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Fig. 1: The Crab Nebula which exploded in the year 1054. In visible light, radio and X. Inside there is a pulsar, visible in the X image. (Images are in false colours)

In 1993 a supernova (SN1993J) exploded in the galaxy M81. The phenomenon has been studied in great detail at all frequencies of the electromagnetic spectrum. High resolution radio observations (VLBI Interferometry), have allowed scientists to "see" the expansion and analyse its physical parameters.

Fig. 2: This image shows the galaxy M81 at radio wavelengths superimposed on the optical image. In the enlarged view, the supernova SN1993J. In false colours, the sequence of radio observations which show the expansion of the supernova.
(Credit: Cortesia di Bartel, Bietenholz, Rupen et al.)