The universe looks like a vast empty ocean
sprinkled with the rare islands of galaxies.. But this is an illusion. Just a small
fraction of all atoms are found in galaxies, while the rest is thought to be drifting
in between, in the intergalactic medium. Like the roots of some massive tree,
gas spreads out from each galaxy, gravity funneling fresh mass
into this dense, cosmic forest.
Here in the intergalactic medium, are the raw
materials of creation: hydrogen and helium, woven into sheets and filaments that flow into
galaxies where they eventually create stars. But if we look closely, we see who is actually in
charge: Quasars, the single most powerful objects in existence. As small as a grain of sand compared
to the amazon river, they reside in the centers of some galaxies, shining with the power of a
trillion stars, blasting out huge jets of matter, completely reshaping the cosmos around them.
They are so powerful that they can kill a galaxy. What are they, and how do they mold the
structure of the universe at their whim? Everywhere You Look, Weird Things in the Sky In the 1950s astronomers noticed mysterious
loud radio-waves coming from spots all over the sky. They were named “quasi-stellar
radio sources”, or “quasars” because they were dots like stars, but were seen in
radio waves rather than visible light. Everything about them was strange. Some flickered,
others emitted high energy X-rays in addition to radio waves, but all seemed to be tiny. They
also moved extremely fast, as much as over 30% the speed of light. The only explanation was
that they must have been so distant that their apparent speed was actually the expansion
of the universe moving them away from us. But these enormous distances meant that quasars
couldn’t just be stars, but the active cores of galaxies billions of lightyears away! And it
gets crazier. To appear so bright and loud, given these vast distances, they are thousands
of times brighter than the entire Milky Way. Monsters, exploding and screaming into the void
with a violence not thought possible before. As we mapped the sky, we discovered over a million
quasars. And they all seemed to be very far away. Looking into space, far away means very
long ago, because their light takes so long to reach us. Quasars were common
in the early universe, having peaked in number 10 billion years ago when galaxies,
and the universe itself was still very young. Let’s go back in time, just 3 billion years after
the big bang and see what was going on back then. The Incredible Power of Quasars How could an early baby galaxy be
so incredibly bright and violent? All that light and radiation couldn’t be
stars, as there weren’t nearly enough of them. And since galaxies tend
to grow with time by merging, the starlight from small galaxies shouldn’t
be far brighter than any galaxy today. There’s only one way to generate the vast
amounts of energy a quasar shines with: feeding supermassive black holes. We
still don’t know how exactly they formed, but it seems that every galaxy
has one in their center. But how can the brightest things
in the universe be black holes, which trap anything and everything
that crosses their event horizon? Well the light of a quasar is not coming
from inside these black holes. Rather, it comes from the space around them, a massive
orbiting disk of gas called an ‘accretion disk.’ Quasars use the same fuel as stars to shine:
Matter. It is just that black holes are the most efficient engines for converting
matter into energy in the universe. The energy released by matter falling
into a black hole can be 60 times greater than that released by nuclear
fusion in the core of a star. Because the energy released by a black hole comes
from gravity, not from nuclear reactions. Matter falling into a black hole speeds up to
almost the speed of light before it crosses the event horizon, buzzing with an incredible
amount of kinetic energy. Of course, once inside the black hole, it takes that energy with it. You
only get to witness this energy if you drop your matter in the right way. Fall straight down and
the outside universe gets nothing. But when you have a lot of matter, it spirals in incredibly
fast towards the event horizon forming a disk. Collisions between particles and friction heat
it up to hundreds of thousands of degrees. In a space not much bigger than our solar system, the core of a galaxy can release many times
more energy than all its stars combined. This is what a quasar is, a super
massive black hole having a feast. And these black holes eat a lot. Typical
quasars consume one to a hundred Earth masses of gas per minute! Ten billion years ago, the
universe was about a third of its current size, so the intergalactic medium was much less
spread out, meaning the filaments of gas around quasars could feed them a banquet, making
them vomit insane amounts of light and radiation. The brightest quasars power jets, tangling the
magnetic field of the matter around them into a narrow cone. Like a particle accelerator
they launch enormous beams of matter out, plowing through the circumgalactic medium,
forming plumes of matter that grow to hundreds of thousands of lightyears in size. It’s
almost unfathomable in scale. A tiny spot in a galaxy carving out patches of the
universe 100,000s of light years long. But quasars can’t eat for long,
maybe a few million years, because their feast ultimately kills their galaxy. How Quasars Kill Galaxies Okay, maybe “killing” is a bit of an exaggeration.
A galaxy is still there after its quasar turns off. But it will never be the same again. Quasars,
being among the hottest and brightest things in the universe, break their galaxies by heating
them up too much and stopping star formation. Hot gas cannot form stars. This sounds odd,
because Stars are gas that collapsed in on itself and then got really hot. But
in a cloud of gas that is already hot, atoms are moving quickly. When they collide,
they hit hard, exerting pressure that resists gravity’s squeeze – so hot gas can’t
form stars. Instead, the best gas for forming stars is already cold, and won’t put up
a fight when it’s time to collapse into a star. On top of that, quasars push gas out of
their galaxies. Not only does this starve the quasar, but its galaxy loses
the raw materials for new stars. As sad as this sounds, it might be a
good thing for life. The alternative can be far more dangerous: too many stars.
New stars forming is usually followed by massive stars exploding in supernovae,
so planets would be burned sterile. But of course it's more complicated. Like the
intricacies of our own planet’s biosphere, every piece of the galaxy is dependent on and
influencing every other part of the galactic environment. While hot things, like quasars and
supernovae, tend to push gas out of the galaxy, shockwaves and quasar jets can also compress
gas, making new stars at least for a short time. And gas that leaves will mix with gas coming
back in and recycle it back into the galaxy. But in general we can say that without
things becoming a bit more chill, we would not exist today. Which
brings us to our final question: Did the Milky Way Have a Quasar in the Past? It’s unclear if every galaxy went through a
quasar phase, but understanding distant quasars may provide clues to the history of the Milky
Way. Galaxies don’t do a good job of preserving their history. Like sand on a beach the endless
churning mixes away the clues to their past. It’s possible the Milky Way was once a quasar, which may have allowed our supermassive
black hole Sagittarius A star to have grown to 4 million times the mass of the sun.
But sadly we don’t know its ancient history. And as dormant as it is now, Sagittarius A star
could turn into a quasar in the future. In a few billion years the Milky Way will merge with
Andromeda. We’ve seen over a hundred ‘double quasars’ in galaxies smashing together, where
fresh gas is provided for the central black holes. But it won’t last for long. When galaxies
merge, so do their super massive black holes, sinking into the center of their new galaxy,
kicking up dust and stars in every direction. We don’t know whether this will happen, but
it would truly be an incredible sight. Maybe some beings in the far future are going to
witness it and be in awe of what they see. But you don’t have to wait that long. There
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