The Universe: More Than Just Galaxies#

You look up at the night sky, and it seems like a big, empty ocean with just a few islands of galaxies sprinkled around. But that’s not the whole story; it’s actually an illusion. See, only a small part of all the atoms out there are actually inside galaxies. Most of the rest are thought to be floating around in the space between galaxies, what we call the intergalactic medium.

Think of it like the roots of a giant tree. Gas spreads out from each galaxy, and gravity keeps pulling fresh matter into this busy, cosmic forest. This intergalactic medium holds the basic stuff for everything out there: hydrogen and helium. These elements are woven into huge sheets and long filaments that flow into galaxies, and that’s where they eventually form stars.

But if you look even closer, you’ll find something else is really calling the shots: Quasars. These are the single most powerful objects we know of in the entire universe.

What Are These Powerful Quasars?#

Compared to a whole galaxy, a quasar is tiny, like a grain of sand next to a huge river. They sit right in the centers of some galaxies. But don’t let their size fool you – they shine with the power of a trillion stars. They even blast out massive jets of matter that completely change the space around them. They’re so powerful, they can seriously mess up, or even “kill,” a galaxy.

So, what exactly are they, and how do they get to push the universe around like that?

The Incredible Power Source of Quasars#

Okay, so how could a young, early galaxy be so unbelievably bright and violent? That amount of light and radiation couldn’t possibly come from just stars, because there weren’t nearly enough of them yet. Plus, galaxies usually get bigger over time by merging, so the light from tiny early galaxies shouldn’t be way brighter than galaxies we see today.

There’s really only one way to make that much energy: by feeding supermassive black holes. We’re still not totally sure how these massive black holes formed, but it looks like pretty much every galaxy has one sitting in its middle.

Now, this sounds confusing. How can the brightest things in the universe be black holes, which suck up and trap everything that crosses their point of no return (the event horizon)? Well, the light you see from a quasar doesn’t actually come from inside the black hole. Instead, it comes from the space around it – specifically, a massive, spinning disk of gas called an “accretion disk.”

Quasars use the same basic stuff stars do to shine: Matter. It’s just that black holes are the absolute best engines in the universe for turning matter into energy. Here’s how it works:

• The energy released when matter falls into a black hole can be 60 times greater than the energy released by nuclear fusion happening in the center of a star.
• This is because the energy from a black hole comes from gravity, not nuclear reactions.
• As matter falls towards a black hole, it gets sped up to nearly the speed of light just before it crosses the event horizon. It’s buzzing with an incredible amount of kinetic energy.
• Of course, once it’s inside the black hole, that energy is gone to the outside world.
• You only get to see this energy if the matter falls in the right way. If it falls straight down, nothing much escapes.
• But when there’s a lot of matter, it spirals in incredibly fast towards the event horizon, forming that disk.
• All the crashing and rubbing between particles in the disk heats it up to hundreds of thousands of degrees.
• In a space not much bigger than our own solar system, the core of a galaxy can kick out many times more energy than all the stars in that galaxy put together.

So, basically, a quasar is just a supermassive black hole having a giant feast. And let me tell you, these black holes eat a lot. Typical quasars gobble up anywhere from one to a hundred Earth masses of gas every minute!

Thinking back 10 billion years ago, the universe was about a third of the size it is now. The intergalactic medium wasn’t as spread out, meaning those gas filaments around quasars could deliver a massive banquet right to them. This gorging made them spit out crazy amounts of light and radiation.

The very brightest quasars even power huge jets. They twist the magnetic fields of the surrounding matter into a tight cone shape. Like a particle accelerator, they shoot out enormous beams of matter, plowing through the gas around the galaxy (the circumgalactic medium). This forms giant plumes of matter that stretch out for hundreds of thousands of lightyears. It’s almost impossible to imagine the scale – a tiny spot in a galaxy carving out huge sections of the universe that are 100,000s of light years long.

But quasars can’t keep up this feeding frenzy forever. They only eat for perhaps a few million years. And in the end, their big feast ends up seriously hurting their galaxy.

How Quasars “Kill” Galaxies#

Alright, maybe “killing” is a bit strong. The galaxy is definitely still there after the quasar shuts down. But it won’t ever be quite the same.

Quasars, being some of the hottest and brightest things around, damage their galaxies mainly by heating them up too much, which stops new stars from forming. Hot gas just can’t form stars.

Now, that sounds weird, because stars are literally gas that collapsed and got super hot. But in a cloud of gas that’s already hot, the atoms are zipping around really fast. When they crash into each other, they hit hard, creating pressure that pushes back against gravity trying to squeeze them together. So, hot gas resists collapsing and can’t form stars easily. The best gas for making stars is already cold – it won’t fight back when it’s time to pull together into a star.

On top of just heating things up, quasars also push gas out of their galaxies. This not only cuts off the quasar’s food supply but also means the galaxy loses the raw materials it needs to make any new stars.

As sad as that might sound for the galaxy, this process might actually be a good thing for life. The alternative could be much more dangerous: having too many stars forming constantly. When lots of new, massive stars form, they eventually explode as supernovae, which could easily sterilize any planets nearby.

Of course, like most things in space (and life!), it’s more complicated. Think of it like the tricky balance in Earth’s environment – everything affects everything else. While hot stuff like quasars and supernovae usually push gas away, their shockwaves and jets can also sometimes compress gas, which can actually trigger new star formation, at least for a little while. And any gas that gets pushed out will eventually mix with gas coming back in and get recycled into the galaxy.

But generally speaking, if things didn’t quiet down a bit after the quasar phase, we probably wouldn’t be here today.

Did Our Milky Way Have a Quasar?#

This brings us to a final big question: Did our own Milky Way galaxy ever have a quasar in its past? It’s not clear if every single galaxy went through this really active quasar phase. But studying those distant quasars might give us hints about our Milky Way’s history.

Galaxies aren’t great at keeping records of their past. It’s like sand on a beach – the constant churning mixes away the clues. It’s possible the Milky Way was once a quasar. That might explain how our central supermassive black hole, Sagittarius A*, grew to be 4 million times the mass of the sun. But sadly, we just don’t know its ancient story for sure.

And while Sagittarius A* is pretty quiet now, it could potentially become a quasar again in the future. In a few billion years, the Milky Way is going to merge with the Andromeda galaxy. We’ve actually seen over a hundred examples of “double quasars” in galaxies that are smashing together. This happens because the collision brings in a huge amount of fresh gas, providing a feast for the black holes in the center of each galaxy. But this boosted activity wouldn’t last forever.

When galaxies merge, their supermassive black holes usually merge too, sinking down to the center of the new, bigger galaxy and kicking up dust and stars all over the place. We don’t know for sure if this will happen to us, but it would certainly be an unbelievable sight. Maybe some beings far in the future will get to see it and be totally awestruck.

Keep Exploring the Cosmos (and Everything Else!)#

But hey, you don’t have to wait that long to see incredible things. There are tons of fascinating stuff to explore right here on our own planet, right now – if you have the right knowledge to understand it.

To help you get that knowledge, we’ve put together a series of lessons designed to boost your scientific understanding. We made these with our friends over at Brilliant.org. These lessons dive deeper into topics we’ve covered in our popular videos, everything from things like rabies and how mammals use energy, to climate science.

There’s even a lesson all about black holes. You can really dig into the basic ideas behind how they form and how they act. Getting a better handle on black holes will also help you appreciate their huge role in powering those quasars we talked about.

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