galactic nucleus

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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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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When Galaxies Collide

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Meet the Whirlpool Galaxy, a spiral galaxy in the constellation Canes Venatici. This was actually the first galaxy to be classified as a spiral.

Specifically, the Whirlpool Galaxy is what we call a grand-design spiral: a galaxy with a very distinct pattern of two spiral arms. It was discovered by Charles Messier way back in 1773 and added to the Messier catalog as a “spiral nebula,” way back before astronomers knew what galaxies were.

But…hold on just a second.

Anybody notice there isn’t just one galaxy here?

The Whirlpool’s companion is known as NGC 5195–or, if you prefer, M51b. Together, the two galaxies are designated in the Messier catalog as M51.

We are, in fact, viewing them in the aftermath of a collision that drastically altered the shape of both galaxies. Both of these were likely once uniform disk galaxies with no spiral arms.

So, what happens when galaxies collide? And does this happen often?

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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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