Extremely energetic subatomic particles (mostly protons, but also some heavier atomic nuclei) with energies >109 MeV. The record holder so far is a UHECR with an energy of 6 Ã— 1014 MeV! UHECRs are observed only in cosmic ray showers, because of their extremely low numbers. The source of UHECRs is a mystery, as is the mechanism that accelerates particles to these energies. They have sufficient energy to escape the typical magnetic field of a Galaxy with spiral arms. These are classified based on their appearance in optical light, into those in which the arms radiated from a central bulge (classic spirals, S), and those where the arms radiated from a central bar (barred spirals, SB). Both types of spiral galaxies have a central bulge, and most UHECRs are probably of extragalactic origin. Possible sources include active galactic nuclei, dormant quasars with associated supermassive black holes and Concentration of 106 to 1012 stars, dust and gas, that are gravitationally bound. Our galaxy contains ~2 × 1011 stars. There are four main types of galaxies: • Elliptical • Lenticular • Spiral • Irregular Click on Term to Read More mergers.