The extremely luminous central region of some galaxies, powered by matter falling into a supermassive black hole. The intense radiation and jets produced can outshine the entire galaxy and influence star formation throughout the host galaxy.
The term was coined in the 1960s when astronomers realized that some galactic nuclei were far more active and luminous than normal stellar processes could explain. 'Active' distinguishes these from normal galactic nuclei, indicating ongoing energetic processes.
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