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.
Active galactic nuclei are cosmic lighthouses that can be seen across the entire observable universe - they're so bright that a single AGN can outshine 100 billion stars! The 'engine' is surprisingly small, often no bigger than our solar system, yet it powers phenomena visible from billions of light-years away.
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