supermassive black hole

Active Galaxies Demystified

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Active galaxies are, without a doubt, absolutely spectacular.

Though rare, they tell a critical part of the story of galaxies–and of the universe as a whole. And we’ve spent the last few posts exploring them in depth.

But I know they can also be a bit perplexing. These are some of the most powerful and violent objects in the universe; it comes with the territory!

So, let’s boil this down to the basics and tie it all together. And let’s try not to get caught in a supermassive black hole in the process 😉

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A Full Story of Galactic Evolution

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Over the course of the last few posts, we’ve explored different types of active galactic nuclei: Seyfert galaxies, double-lobed radio sources, and quasars.

At the heart of each of these galaxies lies a supermassive black hole, feeding off an energy feast of infalling material and producing titanic eruptions of energy.

Most galaxies, though, are not active. The majority of supermassive black holes–like the one sleeping at the heart of our own galaxy–are on “starvation diets,” living off minuscule streams of dust from cannibalized satellite galaxies.

We know what causes these supermassive black holes to erupt. But why are they so rare? What part do they play in galactic evolution?

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Why Galactic Nuclei Erupt

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Astronomers know that when galaxies’ nuclei become active–releasing tremendous floods of energy–supermassive black holes are the ultimate culprit.

We also know that supermassive black holes lurk at the hearts of most large galaxies.

But most galaxies are not active; only a small percentage are. Our home galaxy is host to a supermassive black hole, but it’s not active. Neither is the black hole within the nucleus of our nearest galactic neighbor, Andromeda.

So, what makes a supermassive black hole erupt?

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How Supermassive Black Holes Work

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We first took a peek at supermassive black holes back in our discussion of galaxies. But now that we’ve covered a few types of active galaxies, it’s time to take a deeper dive.

Okay, I guess we won’t really be diving into a black hole…sorry to disappoint!

(Honestly, though, you really wouldn’t want to. It would be very uncomfortable, to say the least…and, of course, it’s a one-way trip.)

Supermassive holes are thought to lie at the hearts of most galaxies, including our own. They seem to be key to galactic structure. Most of them–including our own–are quiet. But a few percent of the galaxies in the universe emit titanic amounts of energy from their nuclei, and supermassive black holes are the ultimate culprit.

But how?

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What the Heck is a Quasar?

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I’ll give you a hint: this is not an image of a quasar.

This image is from a first-season Star Trek: The Original Series episode, “The Galileo Seven”–the original footage, before it was remastered (and before astronomers had clear images of actual quasars).

According to the story, the crew of the USS Enterprise had standing orders to investigate any quasar they came across. A landing party spent the episode struggling to survive after they crashed on a primitive planetary body within a strange nebulosity that they called a “quasar.”

At times like these, Star Trek provides a fun glimpse into the past. TOS aired in the 1960s, just as very strange “quasi-stellar objects” were being discovered. Dubbed “quasars” for short, they were a new frontier of research and discovery.

The remastered footage uses an actual (if substantially edited) image of a quasar. But the “quasar” portrayed in the story…well…yeah, not even close.

So…what are quasars, really?

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Exploring Radio Lobes

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Meet galaxy NGC 5128, also known as Centaurus A.

Anyone notice something weird going on here?

If you’ve been following my recent posts on galaxies, you might notice that this does not look like a typical galaxy. It has a clear dust band, so it can’t be an elliptical; elliptical galaxies have no obvious dust or gas. But it doesn’t look much like a spiral, either: it has a bright, spherical cloud of stars you’d more expect from an elliptical!

And it’s definitely not an irregular galaxy. It definitely has a disk shape, and irregulars are not disk galaxies.

So what the heck is going on with this galaxy?

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What are Seyfert Galaxies?

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Meet NGC 1566, an intermediate spiral galaxy in the constellation Fornax.

To the human eye, this galaxy looks almost like any other spiral: It has a central nucleus and spiral arms, and it’s full of gas and dust. As an “intermediate” spiral, its nucleus has a bit of a barred structure, but not a strong one. All that’s pretty normal, as galaxies go.

But if we look a little closer, we see that this galaxy’s nucleus is unusually bright, especially for its small size.

As it turns out, NGC 1566 isn’t so normal after all…

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A Peek at Supermassive Black Holes

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There are few things in the universe quite as exciting as black holes.

They’re in all the movies–I even wrote a post a few years ago on what the movies get wrong about them! Though some movies, like Interstellar, do a pretty good job of capturing the relativistic effects.

There are also plenty of popular misconceptions floating about–probably thanks to those fictional portrayals!

Most of the time, when we geek out over black holes, we’re talking about stellar-mass black holes. That is, the remains of massive stars.

Wait a second…most of the time? What other kind of black hole could we be talking about?

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The Milky Way Demystified

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Alright, people…time to finish off our exploration of the Milky Way Galaxy, our home in the cosmos!

For the past nine weeks, we’ve covered everything from how our galaxy was “discovered” to how it may have formed. But there’s so much more to explore–and, starting next week, we’ll begin covering the vast universe of galaxies beyond our own!

But before we do that…I want to wrap up our discussion of our own galaxy with an overview to tie the last nine posts together.

(By the way, has anyone noticed I actually managed to chug out a post a week for the entire Milky Way “module”? I’m a bit impressed with myself for that!)

Anyway…on to the Milky Way!

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Exploring the Milky Way’s Nucleus

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Here is an edge-on illustration of our Milky Way Galaxy. (Keep in mind that the disk actually stretches quite a bit farther out from the budge than is apparent in this illustration. Proportionally, its full diameter makes its thickness less than that of a pizza crust.)

What if I asked you to imagine what that central bulge would look like to us–lifeforms living inside the galaxy? What would you imagine?

Perhaps you’d imagine looking inwards toward a glowing ball of light. Perhaps you’d imagine a region of our sky unusually thick with stars and interstellar clouds. Or perhaps you’d imagine something entirely different.

But…would you imagine this?

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