Supernova remnants and a bright crab

Supernovas are divided into two categories depending on the presence or absence of hydrogen lines in their light spectra. Those of the first type leave no compact residue. 

The electrons lose energy by radiating and so have a limited lifetime. Tired electrons must somehow be replaced. The continuous source of relativistic electrons is in fact the central pulsar. Indeed, at the centre of the expanding nebula is enthroned a rapidly spinning neutron star (turning at some 33 revolutions per second), as witnessed by the punctuated message we receive on Earth. This star is clearly an excellent electron accelerator. 

A second example of an explosion remnant is the fine lacework of the Cygnus loop, located 2500 light-years from Earth. In supersonic expansion, the gas produces shock waves that excite and ionise interstellar matter, causing it to glow. 

The study of galactic cosmic rays is perhaps more an exercise in taste than in visual appreciation. In fact we determine their composition without ever really seeing them. However, they constitute the only sample of matter in our possession that comes from outside the Solar System. The chemical and isotopic composition of this sample is measured using balloon- or satellite-borne particle detectors, since the Earth s atmosphere is fatal to them. When they slam into nuclei in the air, they fragment into tiny particles, thereby losing their original identity. 

Composition and energy distribution (energy spectrum) are the two clues that help us, after a long and painstaking enquiry, to understand the sources of these rapid nuclei and determine the mechanisms which first accelerated them. 

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