The galaxies are the fundamental building blocks of the Universe. Some of them are very simple in structure, containing only normal stars and showing no particular individual features. There are also galaxies that are almost entirety made of neutral gas. On the other hand, others are complex systems, built up from many separate components — stars, neutral and ionized gas, dust, molecular clouds, magnetic fields, cosmic rays .... The galaxies may form small groups or large clusters in space. At the centre of many galaxies, there is a compact nucleus that may sometimes be so bright that it overwhelms all the normal radiation of the galaxy.
KeywordsRadio Source Active Galactic Nucleus Rotation Curve Spiral Galaxy Surface Brightness
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