Most of the galaxies we observe are normal galaxies. We inhabit a normal galaxy called the Milky Way. But what about active galaxies?

When you observe a normal galaxy most of the light comes from the stars in visible wavelengths and is evenly distributed throughout the galaxy. But if you look at some galaxies, you will see intense light coming from their nuclei. These are active galaxies.

An active galaxy emits up to thousands of times more energy than a normal galaxy. But most of this energy comes not in visible light but other wavelengths, from radio waves to gamma rays.

These types of galaxies represent a very small percentage of all galaxies we’ve detected.

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There are four types of active galaxies, including Seyfert galaxies, quasars, blazars, and radio galaxies.

  • Seyfert galaxies are active spiral galaxies. Its nucleus (center) has bright emission lines, including visible wavelengths.
    The nuclei of these galaxies change brightness every few weeks, so we know that the objects in the center must be relatively small (about the size of a solar system).
  • Quasars are the most energetic objects in the universe. They are distant star-sized energy sources located at the centers of galaxies that shoot jets that travel close to the speed of light. The extreme brightness of quasars can fluctuate over daylong periods. The word quasar is short for quasi-stellar radio objects.
  • Blazars are just like quasars, but the type we observe may depend more upon our viewing angle than structural differences. When the jets happen to point toward Earth, then it’s a blazar.
  • And finally, radio galaxies are galaxies that emit radio waves. They are elliptical and their nuclei emit jets of high-velocity gas above and below the galaxy.

So, what is happening in these galaxies to produce such an energetic output?

The activity of these galaxies is driven by a supermassive black hole at their center.

Most, if not all, galaxies have a supermassive black hole at their center. But in an active galaxy, its supermassive black hole is accreting material from the galaxy’s dense central region.

As the material from the accretion disk falls into the area around the black hole (the event horizon), it heats to millions of degrees Kelvin and is accelerated outward in the jets.

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