Education and Communications

Galaxies, Part 2: Crash Course Astronomy #39

Hey astronomers, Phil Plait here. In our last
episode, I talked about galaxies: vast collections of gas, dust, and upwards of hundreds of billions
of stars. We live in one, the Milky Way, a gigantic disk galaxy with sprawling spiral arms.
Other galaxies are elliptical, or irregular, or peculiar. But those are classifications based on shape.
We also classify galaxies on their behavior, and sometimes even on their location and mass!
To understand why, we have to take a step back, and look at the environments in which
galaxies sometimes find themselves. And if you thought galaxies were big and powerful,
well…I’m about to crush your brain again. In the 1960s, a peculiar object was found.
Called 3C273, through optical telescopes it looked like an unassuming blue star, but through
a radio telescope it was seen to be ablaze with light, a luminous powerhouse. Stars didn’t blast
out that much radio radiation, so this was baffling. The mystery deepened when spectra of 3C273
were taken. It wasn’t a star, it was an entire galaxy, and not just any galaxy, but
one very, very far away: Well over 2 billion light years. Far from being some dim thing,
3C273 revealed itself to be the most luminous object in the Universe ever seen at that time. It blasts
out over 4 trillion times the energy the Sun does. And yet it appears star-like, a mere dot in
the sky. Because of this, it was dubbed a “quasi-stellar radio source”, which is
pretty underwhelming for the most powerful energy source in the entire cosmos! Happily,
the name was shortened to quasar, which, you’ll admit, is way cooler. Once 3C273 became known, lots more such objects
were found. With the advent of X-ray observatories launched into space even more energetic point
sources were found, which is amazing. X-rays are a very high-energy flavor of light, and
it takes a lot of power to make them. Eventually, galaxies like these were even found to be blasting out
gamma rays, the very highest energy kind of light. Clearly, these were no regular galaxies. Astronomers
gave them the generic name “active galaxies,” and classified them into various subcategories
depending on how they emitted their light, and what kind of spectra they had. But what could power these immensely energetic
galaxies? It turns out, to create that kind of energy, you need to have an object with
a lot of gravity. And what kind of object has a lot of gravity? [evil chuckle] In the 1980s, astronomers were getting suspicious
that all large galaxies had very massive black holes in their cores. In fact, one of the
reasons the Hubble Space Telescope was built and launched was to explore this idea, and
characterize – that is, find out as much as it could about – these black holes. Over time, we’ve found this idea is absolutely
correct. Every big galaxy we see appears to have a huge black hole in its heart. Even
the smallest is a monster, with millions of times the Sun’s mass, and some tip the cosmic
scale at billions of solar masses. We now think that these supermassive black
holes form at the same time galaxies do. As the material coalesces to create a galaxy,
some falls to the center and feeds the black hole there; it grows as its host galaxy does. But I can hear you thinking, “Hey, Phil,
don’t black holes suck down everything, even light itself? How could they power active
galaxies, the brightest objects in the Universe?” Ah, you can’t escape from a black hole once
you fall all the way in. Just outside the black hole’s event horizon things can still
get out. If a black hole is sitting all by its lonesome
out in space, it’s, well, black. But if matter, like gas, dust, or even whole stars,
falls into the black hole, it can be shredded by the fierce gravity. This material forms
a flat disk called an accretion disk, the matter swirling madly at ferocious speeds
before falling in like water down a bathtub drain. Stuff closer to the black hole orbits faster
than stuff farther out. This means material in the disk rubs together, and heats up, just
like rubbing your hands on a cold day warms them up via friction. But around a black hole
the orbital speeds are near the speed of light. Try rubbing your hands together at a couple
of hundred thousand kilometers per second and see how much heat you make. So friction and other forces heat the material
falling in to millions of degrees, so hot that it blasts out light across the electromagnetic
spectrum. And that’s what powers active galaxies! The black hole is the energy source,
but the matter falling into it is the actual light bulb. Active galaxies are so bright
they can be seen clear across the Universe. Not only that, but some active galaxies have
jets: Magnetic fields coupled with the incredible rotation of the accretion disks can launch
twin beams of matter and energy directly away from the black hole, along the poles of the
disk. These beams pack a huge wallop, travelling for hundreds of thousands of light years.
Eventually they slow down as they ram through the thin material between galaxies, but when
they do they puff up, looking like huge cotton swabs which glow in radio waves. Active galaxies can look pretty different
from each other, and we now think that’s due to our viewing angle on their accretion
disk. When we see it edge-on the thick dust in the disk blocks the intense highest-energy
light, but we do see lots of infrared as the radiation from the disk heats up clouds of
dust around it. If the accretion disk is tipped a bit to out line of sight we see more optical
and high energy light from it. And if the poles are aimed right at us, all that ridiculously
energetic X- and gamma ray light can be seen. The Milky Way has a supermassive black hole
in its heart too, with a mass of over 4 million times the Sun’s. That might sound huge,
but remember the galaxy has hundreds of billions of stars in it. The black hole is only a teeny
tiny fraction of the total mass of the Milky Way. Our black hole is quiescent, that is, not
actively feeding, so we’re not an active galaxy. Every now and again we’ll see a
flare from it as it swallows down a gas cloud or something like that, but nowhere near what’s
needed to switch it fully on. Happily, we appear to be safe from any tantrums it might
throw. But that may not always be the case. One way
to flip such a black hole from milquetoast to monster is through galactic collisions.
When two galaxies collide, a lot of gas can be dumped into their centers where it can
be gobbled down and heated up. We do see a lot of evidence that active galaxies are disturbed,
as if they recently collided. So, could that happen to us? Yes. Yes, it can. In fact, it will. But not
for a few billion more years. To understand that, we have to take a small
step back. Well, actually a huge step back: A few million light years, and take a look
at where galaxies live. Our Milky Way isn’t alone. It’s part of
a small knot of galaxies we call — in long, boring astronomical nomenclature tradition
— the Local Group. It consists of a few dozen galaxies, most of which are small and
dim; so faint that we’re still discovering them! Two galaxies completely overpower the
group: the Milky Way, and the Andromeda Galaxy. The Local Group is elongated, almost dumbbell
shaped, with the Milky Way on one side and Andromeda on the other. In the past, the Local Group probably had
lots more galaxies, but over the eons the two big galaxies ate them all, growing huge.
Andromeda is bigger than we are, and has more stars, but honestly we’re both pretty big
as galaxies go. And, someday, we’ll be bigger. The Andromeda galaxy is about 2.5 million
light years away — close enough that it can be seen by the naked eye on dark nights,
the most distant object easily seen without aid. Spectra taken of Andromeda reveal an
interesting fact: It’s headed right for us. Its spectrum is blue shifted, meaning it’s
approaching us, and it’s doing so at quite a clip: about 100 km/sec. That’s fast, but
2.5 million light years is a long way. The collision is inevitable, but it won’t happen
for several billion years. When it does, but galaxies will stretch out
due to galactic tides, forming long curving streamers of stars. They may pass by each
other during the first pass, but over the next few hundred million years they’ll slow,
fall back toward each other, and merge. They’ll then form one, much larger galaxy, probably
an elliptical, which astronomers have called “Milkomeda.” I know, that’s awful. But if you can come
up with a better name, let us know. Anyway, although this won’t happen for billions
of years, that’s still long before the Sun dies. The Earth may still be around when the
galaxies collide! It’s not clear what will happen to us; the Sun may continue to lazily
orbit the core of the new galaxy, or it may move farther in toward the center or farther
out in the galactic suburbs. And here’s another fun fact: Andromeda has
a gigantic black hole in its core, too, which has 40 million solar masses, ten times the
mass of ours. When the galaxies merge, the two monsters will probably go into orbit around
one another. Not only that, but any gas and dust left over from star formation during
the collision may fall toward the center of Milkomeda, where the two black holes will
gobble them down, and may turn the galaxy into an active one! Hopefully, any death rays
launched from that will miss Earth. But that won’t happen for like four billion
years anyway. I’m not too concerned over the fate of the Earth at that point. I feel that right now is a good time to give
you a heads-up: we’re about to take a very, very big step. Up to this point in the series
we’ve talked about some pretty big distances: millions or billions of kilometers to the
planets, trillions of kilometers to the stars, and then jumping to thousands of light years
– quadrillions or quintillion of kilometers! – when talking about the galaxy itself. But those distances are as nothing when you
start talking about intergalactic trips. We’re about to venture out into the greater Universe,
and things are about to get very large. When we step outside our Milky Way, we find
that a few galaxies have clumped together to form The Local Group. But as we look farther
out into the Universe we see that galaxies tend to clump together on larger scales as
well. Many are in small groups like ours, but sometimes they aggregate into much larger
galaxy clusters. A typical galaxy cluster is a few tens of
millions light years across, and can contain thousand of galaxies. The nearest one to us
is the Virgo cluster, located about 50 million light years away in the direction of the constellation
Virgo. It has well over a thousand galaxies in it, maybe twice that much. It may have
as many as a quadrillion stars in it! Like star clusters, galaxies in galaxy clusters
are bound to the cluster by their own mutual gravity, and move through the cluster on long
orbits that can take billions of years to complete. Thousands of clusters are known, and they
contain every kind of galaxy imaginable. Spirals, ellipticals, irregulars, peculiars, active
galaxies… in many clusters, a huge elliptical galaxy sits right at the very center. This
is probably the result of collisions between smaller galaxies; when they smack into each
other their velocities through the cluster tend to cancel out (like two cars hitting
head-on and stopping), so they fall to the center. As more mass falls to the center,
the galaxy there grows huge. As mind-boggling as all this is, we’re not
done. Surveys of the sky have revealed that not only do galaxies clump together in clusters,
but clusters themselves fall into even bigger groups called superclusters! A supercluster
usually has several dozen clusters making it up, and are hundreds of millions of light
years across. Our Local Group is near the Virgo Cluster,
and both are part of the Virgo Supercluster. Recent observations indicate the Virgo Supercluster
is actually only an appendage of the even larger Laniakea Supercluster, which may have
100,000 galaxies in it stretching across 500 million light years. This new result is a
bit controversial — I mean, it’s hard to know exactly how big such a structure is,
especially when we’re inside it — but it gives you an idea of the vast sizes and
distances we’re talking about here. Superclusters themselves aren’t just randomly
distributed through the Universe either; they appear to fall along tremendously long interconnected
and intersecting filaments, making the Universe appear almost foamy on the biggest scales,
like a sponge. In between the filaments are vast regions relatively empty of galaxies,
called voids. This cosmic large scale structure — its
size, shape, distribution of matter, and more — holds clues to some of the biggest questions
we can ask: What is the Universe made of? How did it start? What is its eventual fate? These are questions we’ll get to in future
episodes very soon, and I promise you they’ll stretch your mind like nothing you’ve ever
encountered before. But before we wrap up, there’s one more
thing I want you to see. When you look at all these pictures of galaxies,
of clusters, of superclusters, a question pops up: How many galaxies are there? Can
we count them all? To help answer that question, back in the
1990s astronomers used the Hubble Space Telescope. They pointed it toward the emptiest part of
the sky they could: a spot with little or no stars, nebulae, or galaxies in it. They
then let it stare, simply collecting light from whatever it could see, letting light
accumulate until incredibly faint objects could be detected. And what did it find? Wonder. Pure, simple, wonder. Oh, yeah, and
thousands of galaxies. This is the Hubble Deep Field. Mind you, the
area of sky you see here is roughly the same as the apparent size of a grain of sand held
in your palm with your arm outstretched. And yet in that tiny section of sky there are
thousands of galaxies. Essentially everything you see in that image is a galaxy, a huge
collection of gas, dust, and billions of stars! The deep field was repeated in different parts
of the sky, and the result was always the same. Crowds of galaxies, jostling for position,
crammed together even in a tiny slice of the heavens. You can count all the galaxies in these deep
fields, and then use them to extrapolate to the entire sky, giving you the total number
of galaxies in the Universe. And what do you get? Well, give or take – a
hundred billion galaxies. A hundred. Billion. And each with billions
of stars. The Universe is mind-crushingly huge. And
yet here we are, a part of it, learning more about it all the time. It’s easy to think the Universe is too big
to comprehend, and makes us seem tiny and insignificant in comparison. To me, the opposite
is true: It’s our curiosity about this enormous cosmos that makes us significant. We yearn
to learn more, to seek out knowledge. That doesn’t make us small. It makes us vast. Today you learned that active galaxies pour
out lots of energy, due to their central supermassive black holes gobbling down matter. Galaxies
tend not to be loners, but instead exist in smaller groups and larger clusters. Our Milky
Way is part of the Local Group, and will one day collide with the Andromeda galaxy. Clusters
of galaxies also clump together to form superclusters, the largest structures in the Universe. In
total, there are hundreds of billions of galaxies in the Universe. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over to their YouTube channel to catch even more awesome
videos. This episode was written by me, Phil Plait. The script was edited by Blake de Pastino,
and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, edited
by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team is Thought Café.
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