Astronomy & Astrophysics 101: Active Galactic Nucleus

Active Galaxy Markarian 509

This image from the NASA/ESA Hubble Space Telescope shows the galaxy Markarian 509. The bright object at the center of the galaxy, which appears like a star, is an active galactic nucleus. This is a bright celestial phenomenon caused by matter glowing as it falls into a supermassive black hole at the heart of the galaxy. Credit: NASA, ESA, J. Kriss (STScI) and J. de Plaa (SRON)

An active galactic nucleus, or AGN, is an extremely bright central region of a galaxy that is dominated by the light emitted by dust and gas as it falls into a black hole.

An active galactic nucleus (AGN) is a small region at the center of some galaxies that is far brighter than can be explained by the stellar population alone. The extremely luminous central region is emitting so much radiation that it can outshine the rest of the galaxy altogether. AGNs emit radiation across the entire electromagnetic spectrum, from radio waves to gamma rays. This radiation is produced by the action of a central supermassive black hole that is devouring material that gets too close to it. A galaxy hosting an AGN is called an ‘active galaxy’.


An active galactic nucleus, or AGN, is an extremely bright central region of a galaxy that is dominated by the light emitted by dust and gas as it falls into a black hole.

AGNs are the most luminous persistent sources of electromagnetic radiation in the Universe. This means they can be used to discover distant objects. Astronomers have also classified different types of AGN based on their observed characteristics. The most powerful AGNs are known as quasars, which give rise to extremely luminous galactic centers. A blazar is an AGN with a jet of light and energy that is pointed toward the Earth.

Over the years, Hubble’s instruments have observed various AGNs, including quasars. In 1996 Hubble’s 100,000th exposure was a quasar located 9 billion light-years from Earth. Hubble has also discovered the brightest quasar ever seen in the early Universe.

Word Bank Active Galactic Nucleus

Credit: ESA/Hubble, NASA, ESA, J. Kriss (STScI) and J. de Plaa (SRON)

In 2011 Hubble captured an image (as seen on the top of this page) of the AGN at the heart of the galaxy Markarian 509, 500 million light-years away. The AGN of this galaxy was chosen for study because it is known to vary in brightness, which indicates that the flow of matter is turbulent. Hubble’s studies of AGNs have also provided insight into how supermassive black holes interact with their host galaxies.

Some of Hubble’s most brilliant images of galaxies include those with luminous AGNs in the core, such as ESO 021-G004 and IC 4870.

Additional Hubble contributions and observations can be explored by learning more about Hubble’s work in the study of quasars.

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