What are Active Galactic Nuclei?

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  • 0:02 What Are Quasars?
  • 0:47 What Are Active…
  • 2:57 How Big Are Active…
  • 5:11 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson explores and defines active galaxies and active galactic nuclei. You'll learn how they relate to quasars and how big active galactic nuclei are thanks to the observations of the properties of light.

What Are Quasars?

We know that here on Earth, if you were to take a source of light, like a flashlight, and move it farther and farther away, it will get dimmer and dimmer. Knowing that, what would you tell me if I asked you why another source of light, the same distance away, appears to be much brighter than the dimmer one?

Logically, you'd probably say that it must be a more powerful and luminous flashlight that is emitting the light. And you'd be right. In the very distant reaches of space, we sometimes see very bright objects despite the fact that they are extremely far away. These objects, the extremely luminous centers of the farthest galaxies, are called quasars.

What Are Active Galactic Nuclei?

Quasars are small, yet very powerful, sources of energy. That is why they appear so bright to us despite their distance. That is why they drown out the light of stars in the galaxy at whose center they sit in. The energy they release also produces a ton of visible light, which accounts for what I just said about them.

A galaxy whose center emits a vast amount of energy is called an active galaxy. And the centers of active galaxies are called active galactic nuclei (AGNs). You can think of quasars as one type of active galactic nucleus.

Astronomers believe that in the centers of active galaxies are supermassive black holes. Matter swirls around these supermassive black holes in a disk, called an accretion disk, before plunging in. As this process occurs, a lot of energy is released.

If you watched the lesson on supermassive black holes, you're probably thinking, so what? Don't most galaxies contain supermassive black holes at their center? Why isn't our own galaxy releasing a ton of energy akin to active galactic nuclei?

Well, most galaxies do in fact contain supermassive black holes in their centers. But, the black holes at their centers need not be active. Instead, they can be dormant, like a dormant volcano isn't active. In such cases, these dormant supermassive black holes rarely swallow up anything that enters too close because they already did that long ago, and there's not much stuff nearby to swallow up anymore. That's the case with the Milky Way Galaxy today. Hence, active galaxies are normal galaxies that are simply going through a developmental phase many galaxies go through.

How Big Are Active Galactic Nuclei?

In the intro, I mentioned that a light that appears brighter is more powerful. You'd also expect it to be larger. However, the powerful active galactic nuclei are quite small in volume. The way astronomers have ascertained this is actually pretty cool.

Firstly, astronomers have observed that active galactic nuclei are variable. Meaning, their brightness changes over time. That's the first clue. The second clue is that they flicker very quickly. Let's put these clues together to find out why they occupy a pretty small volume of space as a result. Carefully study the diagram on your screen as we do this:

Diagram for example
diagram showing image 1 light year across emitting rays

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