February 26, 2019 • 39 mins
What's at the center of the Milky Way? How do we know? Can we explain it?
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Episode Transcript
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Speaker 1 (00:08):
You know how the center of things are always the
most important things, you know, like in the cities, the
center of the city is the most important parts where
all the action happens, like the eye of the storm.
There's always something very special and significant about the center
of something big. Yeah, exactly, And so the center plays
a big role in our sort of mental organization. When
I walk around the city, I'm always doing it with
(00:30):
respect to the center. I had like an image in
my mind, where are the skyscrapers? Where am I going?
Everything is oriented relative to the center, right, Yeah, And
it's kind of interesting to think about the center of
the galaxy, right, Like, it's the center of the galaxy
where all the action is in our neighborhood. The center
of the galaxy is a hot, nasty place, and uh,
(00:51):
it's really interesting to think about what might be there.
So there could be a super cool, awesome galactic party
in the center that we have no idea to. Somebody
could be having the best dance party ever and we
weren't invited. It's happening downtown at the galaxy. I am more.
(01:23):
I'm a cartoonist, and I'm Daniel. I'm a particle physicist,
and we are the authors of the book We Have
No Idea and the host of this podcast Daniel and
Jorge explained the Universe. I hope we're the hosts because
there's nobody else here recording with us, so it's not
us who's who's doing it. I'm just here to chat
about science. Who's hosting this thing? After all? Where are
the adults? How did they let to uh have they
(01:46):
let to goof offs host host a podcast? Like basically,
I've been asking that question myself for quite a while.
Um My wife and I talked about that all the time.
We like look at each other and we're like, we're
running a household. When are the adults going to come
in and tell us what to do? We're actually taking
all these decisions when I don't know at some point,
well from irresponsible adulthood into attempt at responsible adulthood. But anyway, well,
(02:12):
I think we're in Daniel, I think we are the
people on this podcast. Well we better get hosting. Then,
welcome everybody to Daniel L. Jorge explain the Universe. Where
we try to take the universe and explain it to
you and make sure that it actually makes sense. Take deep,
big philosophical questions that everybody wants to know the answers
to and chat about them and make sure everybody goes
away with a deeper understanding of what's going on around
(02:34):
your planet, in your solar system, and in your galaxy. Yeah,
and today on the podcast, we are going to talk
about what is at the center of the galaxy exactly.
We're going straight to the heart of the matter. Yeah,
we're going into the aisle storm. That's right, what matters
in the galaxy? What is at deceething Maelstrom, That is
(02:58):
the craziest, densest place in the galaxy. Yeah, what happens
when everything swirls around and what's at the center? Are
you imagining the galaxy is like one giant toilet you
can just throw your trash at it all ends up
in the center. Well, it looks like a swirling toilet,
doesn't it. Yeah. I think of it more like a
like a dancing star spinning its way through the universe.
(03:18):
But you can go with your beautiful toilet analogy if
that helps you. You know, spinning star, toilet humor. You know,
it's all it's all poetry in the grand scheme of
the universe. Tell me why you are comic and not
a poety again, or tell me why I'm not a physicist.
I think it's pretty clear you can cross poet off
the list. Also, um, I think what you were saying
(03:40):
before it was really interesting about how the center is
the most important place, right, And I wonder if that's
for physical reasons or like social and mental reasons. I mean, physically,
the center is always the densest because everything gets attracted there, right,
gravity will pull stuff together and make you have a
dense core. But I wonder if that's why we think
of the center is as important, or if there's some
(04:01):
other reason. Well, it's it's technically the spot that is
closest to everything else, right, Like no other spot is
closest to everything else in the center of something. Right,
m m, that's true, you know, good geometric argument. Yeah, yeah,
I mean that's why cities build around a downtown, right,
because everyone wants to stay within a certain distance of
the downtown. But in the terms, in terms of like
(04:24):
social planning, we've had these interesting cycles in cities, right
where like the center is the most important, and then
people spread out to the suburbs and then the center
can sometimes die. Right. We have this like urban decay,
and then of course folks come in and rebuild condos,
and then we have urban renewal. But we have, you know,
these patterns in our cities where the center is really
important is where everything is happening. And that's sort of
(04:45):
you know, like in l a where it's just like
a bunch of newspapers blowing around empty streets, and then
people come back and move back in. Yeah. So that's
the topic of today's podcast is what is at the
center of our galaxy the Milky Way? What's going on
in the center of our our home, in our center,
the center of our galactic city. That's right the center
of mass of this beautiful bunch of hundreds of billions
(05:08):
of stars that we call the Milky Way. And so,
as usual, before we dove into this topic, we went
around town. We asked people, We said, do you know
what at the center of the galaxy? So those of
you listening think about it. Think about the picture of
the galaxy you may have seen in on TV or
online as this big swirling swirl of of of our
called our Milky Way, And what do you think is
(05:29):
lying there in the center of that swirl? Is it
turtles all the way down is bananas all the way
it's the turtles eating bananas. But anyways, here's here's what
people had to say. Black hole. Not a huge one,
but it's not tiny either. It's a decently sized one,
probably from maybe a red giant like the stars. Okay, cool,
(05:50):
probably black hole some some stars some like, but like,
that doesn't have to be some special thing. I guess this.
It's like a supermassive black hole. Not the biggest, but
it's super massive black hole. I think it's a great swarmhole,
big wormhole. Yes, probably nothing, but you know, it's just
(06:13):
a relatively dense cluster of stars, so probably black hole.
Isn't that a black hole? I thought that was the answer.
But alright, alright, so not a lot of consistent answers here,
know the center of the galaxy. PR team has some
work to do in advertising the real estate opportunities down there. Yeah,
(06:34):
nobody seems to know what's there. I mean, people had
some pretty confident guesses at least. Yeah. I fell into
a couple of categories. Some people are like, there's a
big black hole. Other people are like, well, the galaxy
is just stars, so more stars, right, and then you
have your exotic answer. If somebody who thought that there
was a wormhole the center of the galaxy and b
I wish that were true, that would be awesome. Yeah,
you mean like a toilet, like like you get to
(06:56):
the center and you you flush somewhere else. Do you
have something you need to get rid of? Horror, I
feel like this is on your mind. You need to
like something in your house, you've got, like some conjuvan
you're looking to get rid of. No. I think it'd
be awesome to have wormhole the center of the galaxy
from the sort of transportation point of view, Like, wouldn't
that be the perfect place for a hub, right, if
you wanted to get to the next galaxy over, just
(07:17):
go to the center and there's there's a wormhole that
connects you to other centers of other galaxies, and that's
how you can get to other galaxies. Right, that would
be pretty awesome. I mean, if I was designing the universe,
definitely would would go that way, like the downtown train
station or bus stations exactly. That's right, that's where you
can go to get to another galaxy or you know,
score some illegal stuff. Um, But the real answer is
(07:43):
that any of these answers could be true, right, Like,
there could be a warmhole we don't know. There could
be a black hole that we sort of don't know, right. Um,
the real answer is that all these answers are mostly true. Yes, Um,
there definitely are a lot of stars there. Um, we
have very strong evidence now that there is a huge
black hole at the center of the galaxy. Um, we
don't know that there's a wormhole. But you know, we
(08:04):
don't know that much about wormholes. We just recorded an
episode about them, and one hypothesis is that some black
holes really are the openings to wormholes. So it could
certainly be that the black hole the center of our
galaxy is actually a wormhole that could take you to
other galaxies. You know, I think I'd probably bet it's
more likely about your toilet theory of the universe, but
it's certainly a possibility. Yeah, he flushing down my ideas
(08:25):
down the toilet, and I'm putting them where they before.
But let me let's take a step back. So just
to give our listeners a little recap, So we are
on the planet Earth. The planet Earth is going around
the Sun in our solar system, and our solar system
is actually one of the many billions of stars in
(08:48):
the Milky Way galaxy. That's right. That's very helpful if
somebody's sending you a letter from another galaxy. I hope
everybody was running that down. They're like, do this podcast
apply to me? If you're listening into this podcast and
you're not on Earth, then send us a note. We'd
love to hear from you. We'd love to hear what
you think of our bad humor. That's right, interstellar podcast jokes.
(09:11):
We'll see if they We'll see if humor translates from
here to other galaxies. Yeah, that's roughly the the where
we are in the galaxy. I think it's also helpful
people know, like how big is the galaxy? You know,
it's just like, you know, a few solar systems? Is this? Uh,
you know most of the universe? Um, how what's the
scale of this thing we're talking about? Yeah, I looked
(09:31):
it up in these notes you sent me. And the galaxy.
I love the way you do research. By the way,
it gets lazier and lazier as we go forward here. Um,
somebody put these numbers in front of me. I will
not vouch for them, but I will read them to
you as if they were true. A physicists send these
numbers to me. So I'm pretty sure. You know you've
(09:53):
got an email from somebody who's claiming to be a physicist.
You don't even know if it's from that person. That's right,
haven't senior diploma? Daniel? I should really check before putting
myself out. You know I never had a defense. You
never had a thesis defense? What do you mean? No,
no thesis defense? Wow, you were indefensible. The best thesis
(10:14):
defense is a good offense. You know, that's my favorite
X case. So you went into your committee's offices and
you just tackled them. At at UC Berkeley, where I
did my grad school, they do not have a thesis
defense required, and in the physics department there is no defense.
So you just turn in your thesis and then the
margins lady checks that the margins are correct, you know
(10:36):
that the number of pages is right and the everything's
in the right place, and then she gives you a
lollipop that says congratulations, and that's it. You have a PhD. Wow.
So it happens offline in the when you're writing it. Yeah, Well,
you turn it into this window list room in the
basement of the library, and she checks the margins personally,
and then when she decides that your margins are acceptable,
(10:57):
then you have a PhD. Okay, So the fact that
you have a lollipop makes you qualified to talk about
the galaxy and the universe. Yeah, problem is, I don't
know where that lollipop is anymore. So you didn't frame it.
It's not hanging in your office. Has a framed That
would be weird. So the galaxy is our galaxy. The
(11:22):
milk wa is about a hundred thousand light years. Why,
that's right. If you shine the light in one then
it would take a hundred thousand years for somebody on
the other side to see this light turn on. That's right,
which is really big, right, but it's tiny compared to
the distances between galaxies, which is much much, much larger. Right,
So you can think of, you know, the stars in
(11:44):
our galaxy as being pretty far apart. It takes light
years to get to another star. Um the galaxy itself
you can think of as a cluster, and then it's
super far to get to the next one. Right. The
thing I love about the structure of the universe says
that has all these hierarchies. You know, we think the
Sun is really far away, Well it's really close compared
to the next star. While you think like all these
stars are far apart from each other, Well they're really
(12:05):
close to each other compared to the next galaxy. Over rights,
it's really fun to think about all these scales. But yeah,
the galaxy is about a hundred light years a hundred
thousand light years across, and we are about twenty five
thousand light years from the center. So we're about a
quarter no wait, halfway out from the center to the
edge of it. That's right. And I don't think that
that's a coincidence because that you can't have life everywhere
(12:28):
in the galaxy. You cannot know, you can't have life
too close to the center, right. It's sort of the
opposite of a city, where things get more exciting as
you get too closer at the nightlife gets better and better. Um,
and a galaxy nightlife is pretty hard to come by
in the center of the galaxy because there's so much radiation.
It's like basically it's deadly. It's like it is like
the center of the storm. It's like the most the
(12:50):
craziest part of the galaxy. It's the center of the
nuclear storm, and there's a whole bunch of stuff going
on in the center of the galaxy we don't understand.
What we do know is that there's a huge amount
of deadly, really Asian coming from there. And if we
were much closer to the center of the galaxy, then
life could not have formed the way we know. It
would have to be like super radiation hard life or
something something like that. And you also don't want to
(13:12):
be too far out away from the center of the
galaxy because well, you need enough stuff right to to
form stars and to form planets, and you want to
have big planets going around your Solar system because that
can help you protect yourself against asteroids and stuff like that.
Like a lot of people think that Jupiter has helped
life form on Earth by acting as sort of like
a linebacker, pulling in asteroids and meteors and comments that
(13:34):
come into the Solar System that might have smashed into
Earth and protecting us. So the further out you get
from the center of the galaxy, the less stuff there is,
and so the fewer number of these big planets there are,
And so that's sort of like a goldilog zone. Right,
there's a goldilog zone around each star, but there's also
a goldlog zone around the center of the galaxy. Wow.
So it's lucky that our solar system is where it
(13:56):
is in the galaxy. It's convenient. It's convenient. Yeah. I mean,
if our star was somewhere else, we wouldn't have had life,
and we wouldn't be asking this question, okay. Or we'd
be a lot of thicker skin, maybe we look a
lot cooler. We'd be able to shoot leisure beams at
each other and all sorts of stuff. It's fun to imagine.
It's fun to imagine how life might have evolved into
(14:17):
totally different circumstances. Um. Yeah, So we're a little bit
away from the center. Like if the galaxy was like
a c D, we'd be we'd be halfway out in
the disc. Yeah, exactly. I think of it more like
a city, you know, Like if the center of the
galaxy was Manhattan, then we're out in Connecticut somewhere, right,
Like we're out in the burbs, okay. And I read
also that it takes about two hundred and fifty million
(14:41):
years for our solar system to go around the galaxy.
That's right. The galaxy itself is rotating, Like everything in
space seems to be spinning, and the galaxy itself is rotating,
and that's why you see these spiral arms that come
out of the galaxy and they're not straight, rather sort
of dragon behind, and that's because it's rotating. And it
takes two million years for the galaxy to rotate. You
(15:02):
could also sort of think of that as like a
galactic year, right, takes the Earth one year to go
around the Sun. We call that a year um and
so it takes the Sun two fifty million years to
go around the center of the galaxy. So that's one
galactic year. And you know, from that point of view,
the universe is not that old, right, But I mean
even from the from the age of the Earth, that's
(15:23):
not that much, right, because the Earth is several billion
years old, and so we've gone around the galaxy a
few times since the Earth was born, Yeah, only about
twenty times. Yeah, we've only done twenty times around the
galaxy since the Earth was born. Earth being four or
five billion years old, not that many times around. So
far we're pretty young as a galaxy. Yeah, So next
year the Earth will be able to drink legally. That's right.
(15:47):
All bets are off, and it's have no idea what
then it can go downtown to the center and really
get into the nightlife exactly. So that's been things really
start to go crazy. Yeah, and there's about a billion
stars in our galaxy, so we're nothing special. A hundred billion,
so a hundred thousand million stars and we're just one
(16:09):
of them. And we're just one of them. And the
other important thing to understand is that the stars in
the galaxy are not just spread out evenly. It's not
like you're spreading butter over a piece of bread and
you just you know, spread the stars evenly throughout the galaxy.
The galaxy really has a very strong gravitational pull and
so it sucks stars in towards the center. And it's
much much denser in the center of the galaxy than
it is out here in the suburbs. Yeah. I was
(16:31):
reading this also that as you get closer to the center,
the space between stars gets smaller and smaller. That's right.
If you measure the density of stars out here where
we are, then there's about point two stars per cubic PARSEK.
PARSEK is a unit of distance. So cubic parsk is
a unit of volume, right you. Um, I think there's
(16:53):
just over three light years in a parsak, So a
cubic parsk is like ten cubic light years, and um,
there's point two stars for ten cubic light years, so
that means, um, you know, you need like fifty cubic
light years on average to have a star in it.
Or then that's where we live, So we are in
any stars. We're kind of out in the boonies almost. Yeah,
(17:14):
we're out here, you know, where you can look through
the forest and maybe see another twinkle of your life
from your neighbor. But you don't have a house right
next to you. Right, the other stars are not that
close by. But if you go to the center of
the galaxy, this story is very different. Yeah. I read
that near the center of the galaxy, it's about fifty
times more dense in terms of stars. No, I think
it's fifty million times more dense. There's ten million stars
(17:37):
per cubic par sect. I was just off by six
orders of magnazums a little end there. Fifty million times
more stars per cubic volume than than us here. Now, Yeah, exactly.
Does that mean that if we're out there in the
middle of the galaxy and I look out into the
night sky, I would see fifteen million more stars and
(17:59):
I would see right out. It would be a lot brighter. Yeah,
And you know right now, the brightest kind of nights
that we have if you look up at the stars,
if you look at the sky, is when you have
a full moon, right, and that makes it pretty bright.
You can walk around, you can see stuff. If we
were near the center of the galaxy, then the all
the life from all those stars would be a two
hundred times brighter than the light from a full moon.
(18:21):
So it would be like it would be you'd rarely
have darkness, right, what would life be like? Um, if
it evolved on a planet that that rarely had true darkness,
it would be it's sort of daylight to hold all
the time. Yeah, exactly, exactly. All those stars would be
there at night. You would look up and the sky
would just be full of dots and maybe circles, because
it's maybe some of the suns might be close enough
(18:41):
that you would see them a circle. Possibly, that's right.
And you could have all sorts of different kinds of
experiences right, you could get like a sunburn at night.
You could call it a star burn, I guess, because
there'd be enough radiation from those stars to light up
your life. And even you know, toast your skin you
have to wear a hat all the time. That's right.
Sunscreen or star screen would be required. Even at nine
(19:02):
screen getting ready to go to sleep, kids, put on
your star screen. So it's it's a lot crazier, I mean,
And wouldn't it just be sort of chaos because everything
would be reacting gravitationally, You'd be pulled this way in
that way. Things would be pretty chaotic when they Yeah,
I think it is pretty crazy down there. You know,
the dynamics of a three body system are really hard
(19:22):
to understand because there's three things tugging and pulling on
each other. So if you get two, four, or five, six,
ten million things pulling on each other, it's a mess.
It's really hard to understand how those things operate. And
so things are moving and wiggling and bouncing, and it's
a pretty crazy place. It's like, you know this, it's
like a dance club in the center of Manhattan or
something for those who are of age clearly who've gone
(19:45):
around the galaxy twenty one times and are invited to
the dance party at the center. You know, it's pretty
crazy in there. Well, let's get it. Let's go deeper
and let's think about let's talk about what's actually at
the center of the gal see. But first let's take
a quick break. All right, let's talk about what's at
(20:16):
the actual center of the galaxy, Daniel, what's at the
center of the galaxy? What's the center of the galaxy?
Is a super massive black hole supermassive, not just massive supermassive.
And I don't and that's not something you should bandy
around when you're talking to your your mother in law
about how her dress makes her look right, But supermassive
is definitely appropriate when we're talking about this black hole
(20:38):
because it weighs as much as millions and millions of stars.
I like, I like how this is an actual technical
physics physics term super massive black hole. Yeah exactly. It
was between super massive or just f and heavy man,
and they went with supermassive. Yeah, but it's crazy. So
like if our son turned into a black hole, like
(20:59):
if you squish did an in turn to a black hole,
it would have the mass of one son basically, So
you're saying that these black holes at the black hole
at the center of the galaxy has millions of suns
in it. Kind of yeah, exactly, and it could have um,
you know, smaller suns and bigger sons, but it's slurped
them all together, and it's grown and it's huge. It's
an it's incredible, it's a it's this really massive blob.
(21:22):
And the fascinating thing is that our galaxy is not unique.
We have seen the black hole the center of our
galaxy that we'll talk in a minute about what that
means to see it, and we've also seen at the
center of other galaxies. In fact, it seems to be
more normal than abnormal to have a black hole at
the center of the galaxy. Right, most galaxies have a
super massive black hole in the middle, that's right. Yeah,
(21:43):
And you know that's what happens when things get dense.
You crowd enough stars together and eventually you're going to
get enough gravity to pull these things together to create
black holes, or one of them, you know, go supernova
and creates a black hole and then sucks in the
other ones and it just grows and grows and grows,
So it's not a surprise eyes, right, it's it's it's
sort of the best place to look for a black hole, Like,
where should you find one? You should find one where
(22:05):
there's the most mass, because that's what a black hole is,
a really high concentration of mass. Oh, that's kind of
what inevitably happens when you have that much stuff together
in the middle. Yeah, exactly. Squeeze enough stuff together and
you're gonna get a black hole. So is that what's
holding the galaxy together in a way? Yeah? Absolutely, I
mean the black hole the center is a tiny fraction
(22:26):
of the mass budget of the galaxy. Right, the mass
the galaxy has hundreds of billions of stars, and the
black hole the center, it weighs as much as millions
of stars. So we could do without it, right, if
you deleted it, it would change the way the galaxy rotated,
because the basic dynamics of a galaxy is that it's spinning, right,
although stars are orbiting around the center and they're getting
(22:48):
pulled in by all the gravity from all the stuff
that's closer to the center than they are, and that
black hole is a chunk of it. But it's it's
a it's a small fraction, so we could do without it.
But yeah, it is playing a role in keeping the
galaxy together, but it's not a huge role. It's not
like the anchor of the galaxy. It's just like it's
just helping the galaxy stay together. That's right. Because also,
(23:09):
remember most of the stuff in the galaxy that's providing
gravity is not stars or dust or gas or even
black holes. It's dark matter. Right. There's much more dark
matter in the galaxy than there is normal matter, five
times as much so. Also, in the center of the
galaxy is an enormous dense blob of dark matter that
we can't see at all. Wow. Well, we talk about
(23:31):
dark matter in another podcast episode. But let's let's focus
on this black hole. How do we know there's even
a black hole there? If you can't see black holes, Yeah,
you can't see them directly, which is really frustrating. Right,
And the reason you can't see them directly is because
they absorb light and they don't give off any light. Right.
It's easy to to get confused about this topic. People
(23:54):
think if something absorbs green light, for example, it makes
it green. Remember if something absorbed green light, then none
of the green light gets to your eye, and so
it doesn't look green. Something only looks green if it
reflects green light. Now, black hole absorbs all light. No
light can leave the black hole, which is why it
looks black. Right, So now you're looking for something black
(24:16):
with black space behind it. It's pretty tricky to see
black and black. It's camouflaged, that's right. It's perfectly camouflaged.
And it's not that large like it's it's not like
a huge thing, and so it's it's dense and it's significant,
but you can't really see it, right. It just be
like a little tiny dog black dot in the black background. Yeah, exactly.
It's like a black down in the black background, which
(24:36):
is really hard to spot. Um. And the best way
to see it is indirect and the best way to
indirectly study it is through its gravity, because that's really
its dominant feature. Right, It's a huge source of gravitational attraction.
So we can do is we can see it the
impact of the black hole on the stars around it.
We can look to see how they're moving. Oh, I see,
so that it kind of like you can tell there's
(24:57):
a son in our Solar system because all the planets
are going or on in a circle. You can tell
there's a black hole because of the way the stars
near the center are going in a circle. Exactly. Imagine
you couldn't see the Sun for some reason, you were
blind to the Sun, and you saw all these planets
orbiting the same location. You'd be like, what's going on?
There must be something there that's providing this gravitational force, right,
(25:18):
And you can calculate. You can tell exactly how much
it weighs and where it is just based on the
motion of the planets. So use exactly the same strategy
and then look at the stars that are near the
center of the galaxy and ask are they orbiting something?
Is there something there that they're moving around? And and
absolutely there are And it takes a long time because
these stars, you know, are moving pretty quickly, but this
(25:39):
stuff is far away, and so to watch a star
orbit the center of the galaxy takes years or decades.
And there's a couple of groups, one of the leading
ones is that U C. L A. And they've been
watching these stars near the center of the galaxy and
they are doing crazy stuff. They are swinging around a
crazy high velocities and changing directions in a way that
only makes sense if there's some enorm a source of
(26:00):
gravity right there at the center. Um. But we don't
see anything, right, So that's a pretty big clue. It's
like the observations on the math tells there should be
a black hole there, But then how do you separate
that out from dark matter or how do you separate
that out from just they're just a lot of regular
stars clumped together, right, Well, we don't see stars there, right.
(26:21):
I mean, if the stars are luminous, they're bright, and
so if there were stars there, we would see them.
So we don't see any stars in that location, right,
It's it's dark um. And how do we know it's
not dark matter? Well, there definitely is dark matter there also, um,
But we have a hard time studying dark matter because
it's not as localized, tends to be more spread out.
It's sort of a smoother blob as far as we know.
But that's a whole other area of discussion. Um. What
(26:44):
one way that we can tell it's probably a black
hole and not just something else is that we do
see some radiation from it that can only come from
a black hole. Yeah, that's consistent with coming from a
black hole, and somebody out there is probably thinking, hold
on his and it's a black hole. It doesn't admit
any radiation black except for hawking radiation. And we're not
talking about hawking radiation. We're not talking about the little
(27:04):
wiggles that come off the edge of the black hole
when particles decay near their edge. We can talk about
that in another podcast. Instead, we're talking about the radiation
that comes from the stuff around the black hole that's
not yet in it, but getting squeezed and pulled into
the black hole. This is something called an accretion disc.
It's like the stuff swirling around the edge of the
black hole that's not yet there. It's undergoing tremendous pressure.
(27:28):
It's being really pulled and squeezed. It's like the chaos
right before it falls down the toilet. That's right just
before it gets flushed. It does a few last circles
and the little bits that are about to go go
in get squeezed together, and that causes radiation. And that
has a very specific signature that you can say, Okay,
(27:49):
that's stuff that is about to fall into a black hole. Yeah,
And it's sort of like a flickering behavior. Um, it's
not constant. It's something that happens sometimes, and it's exactly
what you would expect from a black coal. And and
again you can't see this directly, right. It's really hard
to study the center of the galaxy because it's obscured
with huge clouds of gas and dust. And so these
stars we we want to image, and this radiation is
(28:12):
really difficult to see. We have to use all sorts
of techniques, some combination of radio waves and infrared and
X ray emissions and all sorts of stuff. And so
we we see this radiation. We also see X rays right,
X rays and other things. Yeah. Um, But each of
these are absorbed differently by the gas and the dust
that are between us and the center of the galaxy,
and and they're absorbed you know, in different wavelengths, etcetera.
(28:35):
And so we have to have really good maps of
that dust in order to account for how much that's
been absorbed, you know, how much of the signal are
we missing because we're looking through a big sandstorm essentially, um.
And that's why it's really important to have different ways
to see because infrared and radio and X ray these
are just different frequencies of light essentially, of radio of
electromagnetic waves that are going through the galaxy, and the
(28:58):
different frequencies are differently affected of the stuff that's between
us and them. So having different handles is really helpful
because you can tell um what's there and what's not there,
and it gives you a clearer picture. It's like having
um multiple waste idea. Somebody, right, you have their picture,
and you have to know their voice, and you have
their fingerprint or something like that, and you build up
(29:18):
this picture slowly because we only have fragments of each
of them. But if you had fragments of somebody's voice
and their picture and you know what they smelled like
or something that you could identify somebody even if you
couldn't see them clearly. Okay, So that's what's at the
center of the galaxy. It's just a lot. It's a
party in there, a lot of stars, ten million stars
per cubic parsect. So yeah, it's a pretty hot and
(29:39):
dense place. Yeah, and there's dark matter and there's super
massive there's a super massive black hole in there also.
That's right, and pretty soon we're hopeful that we'll get
an even better view of what's going on in the
center of the galaxy because we've had this project. It's
called the event horizon telescope. Event horizons the name for
the edge of the black hole beyond which nothing else
(29:59):
can come out of. And they're building this telescope. It's
essentially just tying together a bunch of different telescopes on Earth.
But if you do that, if you use data from
different telescopes at the same time, it makes like a
huge virtual telescope, one that's effectively the size of the
distance between the telescopes. And so they're making one that's
basically the size of the Earth. And they were supposed
(30:20):
to have essentially a picture of the black hole, the
event horizon of the black hole. Imagine that image from Interstellar,
you know, the picture of the black hole from Interstellar
has become sort of famous. That looks like a basically
a big black ball surrounded by kind of a halo. Yeah,
to me, it looks a bit like a pizza that
somebody sat on, a pizza, a pizza that somebody flushed
(30:44):
down the toilet, exactly. It's a big flushed pizza. Anyway,
we're going to see pretty soon what the center of
the galaxy actually looks like thanks to the new event
Horizon telescope. It was the data was supposed to come out.
The image is supposed to come out last year, but
they're still processing, so it could be any day that
we're gonna have this historic first image of the event
horizon of a black hole, of the black hole, and
(31:04):
you will see details like in the in the movie.
I don't know, I don't know what it's gonna look like.
I don't know how what kind of resolution they have, um,
but it's gonna be pretty exciting. All the all the
black hole ologists I know are very excited about. That
is an actual job description, absolutely, and I should mention
that in preparing for this podcast, I actually did some
research myself and I spoke to one of my esteemed colleagues,
(31:27):
Professor Aaron Barth, and he is a black holologist. He
studies supermassive black holes and he thinks about how do
you make them? And doesn't make sense? And why do
we have them? And do they work the way we expect.
There's a lot of open questions about the way black
holes work. This even though we know they're there, we
don't really understand why they're there and how they got
to be there. Black hole ologist. You can't get over that, right,
(31:49):
You're like, I wasn't awhere that was on the list.
I wouldn't have become a cartoonist. I would have gone
with something better, maybe like black holy Man. That would
have been more. I think that could be confused with
something else, or or a black holistic person. Doctor. Um. Okay, So,
(32:10):
so I heard there's a huge mystery surrounding black holes
at the center of galaxies, and we should totally get
into it. But first let's take a quick break, all right.
(32:30):
So there is a huge, supermassive black hole at the
center of the galaxy, and there's a huge mystery surrounding
these black holes, which is that nobody knows how they
came to be. They're inexplicable, that's right. They're too big, right, Um,
you know, and some people like big black holes and
they cannot lie. But we don't know. We don't know
(32:53):
how these black holes got to be so big. And
the reason is that it's you. You might think, well,
the black hole sits at the center of the galaxy.
Of course, it just a bunch of stars. It's just
been eating a lot. Yeah, but but you know, um,
it's not easy for black holes to suck in all
those stars because the stars are orbiting, right the same
reason that the Earth doesn't just like fall into the
Sun and make the Sun a tiny bit bigger. Those
(33:14):
stars are orbiting the black hole and their angular momentum
keeps them from falling in, and so according to our models,
the black holes should not be that big. They're like
a thousand times bigger than they should be if we understand,
you know, how they started and how they grew. So
the fact that they're so big is kind of awesome,
but also a big mystery. They're mysteriously big, like we
don't know where all the food came from. We don't
(33:36):
know how they got that big. Yeah, we don't know
what are they sucking in and where did it come
from and who's been feeding them? And people have crazy ideas,
you know, they think maybe there was a time in
the universe when galaxies were colliding a lot more than
they are now, and so it could be that, you know,
a bunch of galaxies crammed together, and what we have
at the center of our galaxy is basically the black
holes of a bunch of galaxies all merged together. Or
(33:58):
you know, maybe when that merger happens the black holes
just like goggles up a bunch of stars because they
come into its path. There's a lot of fun, really
fun but really crazy dramatic ideas. And the amazing thing
to me is that these these dramas are incredible, right,
black holes eating stars, But they also happened really slowly,
you know, like over millions and billions of years. You
(34:20):
see these galaxies colliding and merging and things getting sucked up.
You know, I love the idea of huge violence happening
really slowly. Well it's slowed for you, um, but you
know our lives would be slow to an ant right
or to a microbe. Yeah, exactly. And on the Microbe
Universe podcast they probably talk about that all the time,
Daniels slowly the disaster that is this podcast or it's happening.
(34:44):
Why is this podcast so long? Maybe the black hole
in our galaxy, the one at the center of the
supermassive one, came from two galaxies arashing into each other
and there are black holes joining into one. Or it
could be that we just don't understand the process that
(35:06):
starts a black hole, and then maybe they started off
much bigger and uh, and that explains why they are
larger than than we understood. There's just a lot of
basic questions, you know. And every time we do this
in science, we see something interesting, then we ask can
we explain it? Do our models predict exactly what we're
gonna see? It's not just like, oh, we figured there'd
be a black hole, Yeah there is. Now we come
up with like detailed quantitative models. Let's say how big
(35:28):
should it be? Or is our black hole unusual? And
let's look at all the other black holes and we
try to really understand the details of the process. This
is how we we we find problems, and this is
how we crack them. So it's really all in getting
into these details. I have a question. Can a black
hole sucking dark matter? Absolutely? Yeah? Nothing preventing that. Yeah, absolutely.
(35:49):
I mean dark matter feels gravity, right, and so it's
going to get pulled by by the black hole, and
it's probably there could be dark There could be black
holes made mostly of dark matter, right, dark holes or
whatever you want to call dark black holes. Now, dark
matter is definitely not immune from the gravitational poll of
a black hole. Well, it's amazing to think we have
(36:10):
this huge mystery in the center of our galaxy. A
huge hole in our knowledge about the universe is there
literally in figuratively, right, that's right. It's at the center
of everything we we live around. It's we've been orbiting
it for billions of years and still don't understand it.
And the list of mysteries of the galactic center is huge.
You know, we don't understand a lot of the radiation
that's coming from there. We think there might be weird
(36:32):
stuff happening that we don't understand, so strange stars being made,
all sorts of other kind of processes. You know. Another
big mystery is like, where do all the high energy
cosmic rays come from? These particles from space that have
so much energy we can't explain it. Yeah, we did
a podcast episode on that, right, Yeah. And one possibility
is maybe they're coming from centers of galaxies with these
(36:53):
enormous pressures and huge gravitational forces. So far, it doesn't
look like it because we don't have a whole lot
of examples of these energy particles, but we can't point
them all back to the centers of galaxies. But we
don't know, and so there's a there's a lot of
stuff to be discovered it's a really really rich source
of um astronomical mystery. Yeah, it really makes me think
how dynamics things still are. You know, things seem pretty
(37:16):
chill right now in our solar system, but we're really
part of this larger history of this crazy, giant, swirling
active disaster crash zone toilet. That was the alternate name
for the for the galaxy we considered instead of Milky
Way detective crash disaster toilet, but it didn't quite roll
off the tongue the same way. I didn't think it
(37:37):
was gonna sell t shirts, No, but you're absolutely right.
And remember, the galaxy is young. We've only been around
for twenty spins of the Galaxy UM, and so in
terms of galactic years, the whole universe isn't even not
much longer. It's like, you know, sixty or seventy galactic
years old. So things are just getting started. So in
that party is just getting started. The party, yeah, the
(37:58):
the downtown party, that's right. But it's interesting to think
that inside of that galactic center are clues about how
the galaxy formed and how this whole universe got put
together right, exactly, absolutely, and that's what makes it so exciting.
And you know, that's the process of science, Like let's
look around and see what we don't understand, and then
ask basic questions about it and try to figure it out,
(38:20):
and along the way we come up with better and
better and more accurate models of what's going on. And
that's how we figured out like the dark matter is
a thing, and that's how we discovered black holes and
all the A lot of the really great transformative discoveries
of the modern age have come from asking simple, basic
questions about stuff, not flushing them down the toilet right away,
(38:41):
and the podcast partner says, though, that's right, or asking
questions about how to flush things down the toilet, like
can I flush a black hole down the toilet? Yes?
Or no? I'll need the dark matter please, that's right.
That was such an obvious joke. Nice job. Well, thank
you for joining us. That's the mystery at the center
(39:03):
of the galaxy. That's right. So now you know what
is the center the hopping party at the center of
the galaxy, But we don't recommend you spend any time there.
Toys in two fifty million years when the Earth turns
twenty one. That's right. Thanks for listening. See you next. Time.
(39:28):
If you still have a question after listening to all
these explanations, please drop us a line. We'd love to
hear from you. You can find us at Facebook, Twitter,
and Instagram at Daniel and Jorge That's one Word, or
email us at Feedback at Daniel and Jorge dot com.
You know how the center of things are always the
most important things, you know, like in the cities, the
center of the city is the most important parts where
all the action happens, like the eye of the storm.
There's always something very special and significant about the center
of something big. Yeah, exactly, And so the center plays
a big role in our sort of mental organization. When
I walk around the city, I'm always doing it with
(00:30):
respect to the center. I had like an image in
my mind, where are the skyscrapers? Where am I going?
Everything is oriented relative to the center, right, Yeah, And
it's kind of interesting to think about the center of
the galaxy, right, Like, it's the center of the galaxy
where all the action is in our neighborhood. The center
of the galaxy is a hot, nasty place, and uh,
(00:51):
it's really interesting to think about what might be there.
So there could be a super cool, awesome galactic party
in the center that we have no idea to. Somebody
could be having the best dance party ever and we
weren't invited. It's happening downtown at the galaxy. I am more.
(01:23):
I'm a cartoonist, and I'm Daniel. I'm a particle physicist,
and we are the authors of the book We Have
No Idea and the host of this podcast Daniel and
Jorge explained the Universe. I hope we're the hosts because
there's nobody else here recording with us, so it's not
us who's who's doing it. I'm just here to chat
about science. Who's hosting this thing? After all? Where are
the adults? How did they let to uh have they
(01:46):
let to goof offs host host a podcast? Like basically,
I've been asking that question myself for quite a while.
Um My wife and I talked about that all the time.
We like look at each other and we're like, we're
running a household. When are the adults going to come
in and tell us what to do? We're actually taking
all these decisions when I don't know at some point,
well from irresponsible adulthood into attempt at responsible adulthood. But anyway, well,
(02:12):
I think we're in Daniel, I think we are the
people on this podcast. Well we better get hosting. Then,
welcome everybody to Daniel L. Jorge explain the Universe. Where
we try to take the universe and explain it to
you and make sure that it actually makes sense. Take deep,
big philosophical questions that everybody wants to know the answers
to and chat about them and make sure everybody goes
away with a deeper understanding of what's going on around
(02:34):
your planet, in your solar system, and in your galaxy. Yeah,
and today on the podcast, we are going to talk
about what is at the center of the galaxy exactly.
We're going straight to the heart of the matter. Yeah,
we're going into the aisle storm. That's right, what matters
in the galaxy? What is at deceething Maelstrom, That is
(02:58):
the craziest, densest place in the galaxy. Yeah, what happens
when everything swirls around and what's at the center? Are
you imagining the galaxy is like one giant toilet you
can just throw your trash at it all ends up
in the center. Well, it looks like a swirling toilet,
doesn't it. Yeah. I think of it more like a
like a dancing star spinning its way through the universe.
(03:18):
But you can go with your beautiful toilet analogy if
that helps you. You know, spinning star, toilet humor. You know,
it's all it's all poetry in the grand scheme of
the universe. Tell me why you are comic and not
a poety again, or tell me why I'm not a physicist.
I think it's pretty clear you can cross poet off
the list. Also, um, I think what you were saying
(03:40):
before it was really interesting about how the center is
the most important place, right, And I wonder if that's
for physical reasons or like social and mental reasons. I mean, physically,
the center is always the densest because everything gets attracted there, right,
gravity will pull stuff together and make you have a
dense core. But I wonder if that's why we think
of the center is as important, or if there's some
(04:01):
other reason. Well, it's it's technically the spot that is
closest to everything else, right, Like no other spot is
closest to everything else in the center of something. Right,
m m, that's true, you know, good geometric argument. Yeah, yeah,
I mean that's why cities build around a downtown, right,
because everyone wants to stay within a certain distance of
the downtown. But in the terms, in terms of like
(04:24):
social planning, we've had these interesting cycles in cities, right
where like the center is the most important, and then
people spread out to the suburbs and then the center
can sometimes die. Right. We have this like urban decay,
and then of course folks come in and rebuild condos,
and then we have urban renewal. But we have, you know,
these patterns in our cities where the center is really
important is where everything is happening. And that's sort of
(04:45):
you know, like in l a where it's just like
a bunch of newspapers blowing around empty streets, and then
people come back and move back in. Yeah. So that's
the topic of today's podcast is what is at the
center of our galaxy the Milky Way? What's going on
in the center of our our home, in our center,
the center of our galactic city. That's right the center
of mass of this beautiful bunch of hundreds of billions
(05:08):
of stars that we call the Milky Way. And so,
as usual, before we dove into this topic, we went
around town. We asked people, We said, do you know
what at the center of the galaxy? So those of
you listening think about it. Think about the picture of
the galaxy you may have seen in on TV or
online as this big swirling swirl of of of our
called our Milky Way, And what do you think is
(05:29):
lying there in the center of that swirl? Is it
turtles all the way down is bananas all the way
it's the turtles eating bananas. But anyways, here's here's what
people had to say. Black hole. Not a huge one,
but it's not tiny either. It's a decently sized one,
probably from maybe a red giant like the stars. Okay, cool,
(05:50):
probably black hole some some stars some like, but like,
that doesn't have to be some special thing. I guess this.
It's like a supermassive black hole. Not the biggest, but
it's super massive black hole. I think it's a great swarmhole,
big wormhole. Yes, probably nothing, but you know, it's just
(06:13):
a relatively dense cluster of stars, so probably black hole.
Isn't that a black hole? I thought that was the answer.
But alright, alright, so not a lot of consistent answers here,
know the center of the galaxy. PR team has some
work to do in advertising the real estate opportunities down there. Yeah,
(06:34):
nobody seems to know what's there. I mean, people had
some pretty confident guesses at least. Yeah. I fell into
a couple of categories. Some people are like, there's a
big black hole. Other people are like, well, the galaxy
is just stars, so more stars, right, and then you
have your exotic answer. If somebody who thought that there
was a wormhole the center of the galaxy and b
I wish that were true, that would be awesome. Yeah,
you mean like a toilet, like like you get to
(06:56):
the center and you you flush somewhere else. Do you
have something you need to get rid of? Horror, I
feel like this is on your mind. You need to
like something in your house, you've got, like some conjuvan
you're looking to get rid of. No. I think it'd
be awesome to have wormhole the center of the galaxy
from the sort of transportation point of view, Like, wouldn't
that be the perfect place for a hub, right, if
you wanted to get to the next galaxy over, just
(07:17):
go to the center and there's there's a wormhole that
connects you to other centers of other galaxies, and that's
how you can get to other galaxies. Right, that would
be pretty awesome. I mean, if I was designing the universe,
definitely would would go that way, like the downtown train
station or bus stations exactly. That's right, that's where you
can go to get to another galaxy or you know,
score some illegal stuff. Um, But the real answer is
(07:43):
that any of these answers could be true, right, Like,
there could be a warmhole we don't know. There could
be a black hole that we sort of don't know, right. Um,
the real answer is that all these answers are mostly true. Yes, Um,
there definitely are a lot of stars there. Um, we
have very strong evidence now that there is a huge
black hole at the center of the galaxy. Um, we
don't know that there's a wormhole. But you know, we
(08:04):
don't know that much about wormholes. We just recorded an
episode about them, and one hypothesis is that some black
holes really are the openings to wormholes. So it could
certainly be that the black hole the center of our
galaxy is actually a wormhole that could take you to
other galaxies. You know, I think I'd probably bet it's
more likely about your toilet theory of the universe, but
it's certainly a possibility. Yeah, he flushing down my ideas
(08:25):
down the toilet, and I'm putting them where they before.
But let me let's take a step back. So just
to give our listeners a little recap, So we are
on the planet Earth. The planet Earth is going around
the Sun in our solar system, and our solar system
is actually one of the many billions of stars in
(08:48):
the Milky Way galaxy. That's right. That's very helpful if
somebody's sending you a letter from another galaxy. I hope
everybody was running that down. They're like, do this podcast
apply to me? If you're listening into this podcast and
you're not on Earth, then send us a note. We'd
love to hear from you. We'd love to hear what
you think of our bad humor. That's right, interstellar podcast jokes.
(09:11):
We'll see if they We'll see if humor translates from
here to other galaxies. Yeah, that's roughly the the where
we are in the galaxy. I think it's also helpful
people know, like how big is the galaxy? You know,
it's just like, you know, a few solar systems? Is this? Uh,
you know most of the universe? Um, how what's the
scale of this thing we're talking about? Yeah, I looked
(09:31):
it up in these notes you sent me. And the galaxy.
I love the way you do research. By the way,
it gets lazier and lazier as we go forward here. Um,
somebody put these numbers in front of me. I will
not vouch for them, but I will read them to
you as if they were true. A physicists send these
numbers to me. So I'm pretty sure. You know you've
(09:53):
got an email from somebody who's claiming to be a physicist.
You don't even know if it's from that person. That's right,
haven't senior diploma? Daniel? I should really check before putting
myself out. You know I never had a defense. You
never had a thesis defense? What do you mean? No,
no thesis defense? Wow, you were indefensible. The best thesis
(10:14):
defense is a good offense. You know, that's my favorite
X case. So you went into your committee's offices and
you just tackled them. At at UC Berkeley, where I
did my grad school, they do not have a thesis
defense required, and in the physics department there is no defense.
So you just turn in your thesis and then the
margins lady checks that the margins are correct, you know
(10:36):
that the number of pages is right and the everything's
in the right place, and then she gives you a
lollipop that says congratulations, and that's it. You have a PhD. Wow.
So it happens offline in the when you're writing it. Yeah, Well,
you turn it into this window list room in the
basement of the library, and she checks the margins personally,
and then when she decides that your margins are acceptable,
(10:57):
then you have a PhD. Okay, So the fact that
you have a lollipop makes you qualified to talk about
the galaxy and the universe. Yeah, problem is, I don't
know where that lollipop is anymore. So you didn't frame it.
It's not hanging in your office. Has a framed That
would be weird. So the galaxy is our galaxy. The
(11:22):
milk wa is about a hundred thousand light years. Why,
that's right. If you shine the light in one then
it would take a hundred thousand years for somebody on
the other side to see this light turn on. That's right,
which is really big, right, but it's tiny compared to
the distances between galaxies, which is much much, much larger. Right,
So you can think of, you know, the stars in
(11:44):
our galaxy as being pretty far apart. It takes light
years to get to another star. Um the galaxy itself
you can think of as a cluster, and then it's
super far to get to the next one. Right. The
thing I love about the structure of the universe says
that has all these hierarchies. You know, we think the
Sun is really far away, Well it's really close compared
to the next star. While you think like all these
stars are far apart from each other, Well they're really
(12:05):
close to each other compared to the next galaxy. Over rights,
it's really fun to think about all these scales. But yeah,
the galaxy is about a hundred light years a hundred
thousand light years across, and we are about twenty five
thousand light years from the center. So we're about a
quarter no wait, halfway out from the center to the
edge of it. That's right. And I don't think that
that's a coincidence because that you can't have life everywhere
(12:28):
in the galaxy. You cannot know, you can't have life
too close to the center, right. It's sort of the
opposite of a city, where things get more exciting as
you get too closer at the nightlife gets better and better. Um,
and a galaxy nightlife is pretty hard to come by
in the center of the galaxy because there's so much radiation.
It's like basically it's deadly. It's like it is like
the center of the storm. It's like the most the
(12:50):
craziest part of the galaxy. It's the center of the
nuclear storm, and there's a whole bunch of stuff going
on in the center of the galaxy we don't understand.
What we do know is that there's a huge amount
of deadly, really Asian coming from there. And if we
were much closer to the center of the galaxy, then
life could not have formed the way we know. It
would have to be like super radiation hard life or
something something like that. And you also don't want to
(13:12):
be too far out away from the center of the
galaxy because well, you need enough stuff right to to
form stars and to form planets, and you want to
have big planets going around your Solar system because that
can help you protect yourself against asteroids and stuff like that.
Like a lot of people think that Jupiter has helped
life form on Earth by acting as sort of like
a linebacker, pulling in asteroids and meteors and comments that
(13:34):
come into the Solar System that might have smashed into
Earth and protecting us. So the further out you get
from the center of the galaxy, the less stuff there is,
and so the fewer number of these big planets there are,
And so that's sort of like a goldilog zone. Right,
there's a goldilog zone around each star, but there's also
a goldlog zone around the center of the galaxy. Wow.
So it's lucky that our solar system is where it
(13:56):
is in the galaxy. It's convenient. It's convenient. Yeah. I mean,
if our star was somewhere else, we wouldn't have had life,
and we wouldn't be asking this question, okay. Or we'd
be a lot of thicker skin, maybe we look a
lot cooler. We'd be able to shoot leisure beams at
each other and all sorts of stuff. It's fun to imagine.
It's fun to imagine how life might have evolved into
(14:17):
totally different circumstances. Um. Yeah, So we're a little bit
away from the center. Like if the galaxy was like
a c D, we'd be we'd be halfway out in
the disc. Yeah, exactly. I think of it more like
a city, you know, Like if the center of the
galaxy was Manhattan, then we're out in Connecticut somewhere, right,
Like we're out in the burbs, okay. And I read
also that it takes about two hundred and fifty million
(14:41):
years for our solar system to go around the galaxy.
That's right. The galaxy itself is rotating, Like everything in
space seems to be spinning, and the galaxy itself is rotating,
and that's why you see these spiral arms that come
out of the galaxy and they're not straight, rather sort
of dragon behind, and that's because it's rotating. And it
takes two million years for the galaxy to rotate. You
(15:02):
could also sort of think of that as like a
galactic year, right, takes the Earth one year to go
around the Sun. We call that a year um and
so it takes the Sun two fifty million years to
go around the center of the galaxy. So that's one
galactic year. And you know, from that point of view,
the universe is not that old, right, But I mean
even from the from the age of the Earth, that's
(15:23):
not that much, right, because the Earth is several billion
years old, and so we've gone around the galaxy a
few times since the Earth was born, Yeah, only about
twenty times. Yeah, we've only done twenty times around the
galaxy since the Earth was born. Earth being four or
five billion years old, not that many times around. So
far we're pretty young as a galaxy. Yeah, So next
year the Earth will be able to drink legally. That's right.
(15:47):
All bets are off, and it's have no idea what
then it can go downtown to the center and really
get into the nightlife exactly. So that's been things really
start to go crazy. Yeah, and there's about a billion
stars in our galaxy, so we're nothing special. A hundred billion,
so a hundred thousand million stars and we're just one
(16:09):
of them. And we're just one of them. And the
other important thing to understand is that the stars in
the galaxy are not just spread out evenly. It's not
like you're spreading butter over a piece of bread and
you just you know, spread the stars evenly throughout the galaxy.
The galaxy really has a very strong gravitational pull and
so it sucks stars in towards the center. And it's
much much denser in the center of the galaxy than
it is out here in the suburbs. Yeah. I was
(16:31):
reading this also that as you get closer to the center,
the space between stars gets smaller and smaller. That's right.
If you measure the density of stars out here where
we are, then there's about point two stars per cubic PARSEK.
PARSEK is a unit of distance. So cubic parsk is
a unit of volume, right you. Um, I think there's
(16:53):
just over three light years in a parsak, So a
cubic parsk is like ten cubic light years, and um,
there's point two stars for ten cubic light years, so
that means, um, you know, you need like fifty cubic
light years on average to have a star in it.
Or then that's where we live, So we are in
any stars. We're kind of out in the boonies almost. Yeah,
(17:14):
we're out here, you know, where you can look through
the forest and maybe see another twinkle of your life
from your neighbor. But you don't have a house right
next to you. Right, the other stars are not that
close by. But if you go to the center of
the galaxy, this story is very different. Yeah. I read
that near the center of the galaxy, it's about fifty
times more dense in terms of stars. No, I think
it's fifty million times more dense. There's ten million stars
(17:37):
per cubic par sect. I was just off by six
orders of magnazums a little end there. Fifty million times
more stars per cubic volume than than us here. Now, Yeah, exactly.
Does that mean that if we're out there in the
middle of the galaxy and I look out into the
night sky, I would see fifteen million more stars and
(17:59):
I would see right out. It would be a lot brighter. Yeah,
And you know right now, the brightest kind of nights
that we have if you look up at the stars,
if you look at the sky, is when you have
a full moon, right, and that makes it pretty bright.
You can walk around, you can see stuff. If we
were near the center of the galaxy, then the all
the life from all those stars would be a two
hundred times brighter than the light from a full moon.
(18:21):
So it would be like it would be you'd rarely
have darkness, right, what would life be like? Um, if
it evolved on a planet that that rarely had true darkness,
it would be it's sort of daylight to hold all
the time. Yeah, exactly, exactly. All those stars would be
there at night. You would look up and the sky
would just be full of dots and maybe circles, because
it's maybe some of the suns might be close enough
(18:41):
that you would see them a circle. Possibly, that's right.
And you could have all sorts of different kinds of
experiences right, you could get like a sunburn at night.
You could call it a star burn, I guess, because
there'd be enough radiation from those stars to light up
your life. And even you know, toast your skin you
have to wear a hat all the time. That's right.
Sunscreen or star screen would be required. Even at nine
(19:02):
screen getting ready to go to sleep, kids, put on
your star screen. So it's it's a lot crazier, I mean,
And wouldn't it just be sort of chaos because everything
would be reacting gravitationally, You'd be pulled this way in
that way. Things would be pretty chaotic when they Yeah,
I think it is pretty crazy down there. You know,
the dynamics of a three body system are really hard
(19:22):
to understand because there's three things tugging and pulling on
each other. So if you get two, four, or five, six,
ten million things pulling on each other, it's a mess.
It's really hard to understand how those things operate. And
so things are moving and wiggling and bouncing, and it's
a pretty crazy place. It's like, you know this, it's
like a dance club in the center of Manhattan or
something for those who are of age clearly who've gone
(19:45):
around the galaxy twenty one times and are invited to
the dance party at the center. You know, it's pretty
crazy in there. Well, let's get it. Let's go deeper
and let's think about let's talk about what's actually at
the center of the gal see. But first let's take
a quick break. All right, let's talk about what's at
(20:16):
the actual center of the galaxy, Daniel, what's at the
center of the galaxy? What's the center of the galaxy?
Is a super massive black hole supermassive, not just massive supermassive.
And I don't and that's not something you should bandy
around when you're talking to your your mother in law
about how her dress makes her look right, But supermassive
is definitely appropriate when we're talking about this black hole
(20:38):
because it weighs as much as millions and millions of stars.
I like, I like how this is an actual technical
physics physics term super massive black hole. Yeah exactly. It
was between super massive or just f and heavy man,
and they went with supermassive. Yeah, but it's crazy. So
like if our son turned into a black hole, like
(20:59):
if you squish did an in turn to a black hole,
it would have the mass of one son basically, So
you're saying that these black holes at the black hole
at the center of the galaxy has millions of suns
in it. Kind of yeah, exactly, and it could have um,
you know, smaller suns and bigger sons, but it's slurped
them all together, and it's grown and it's huge. It's
an it's incredible, it's a it's this really massive blob.
(21:22):
And the fascinating thing is that our galaxy is not unique.
We have seen the black hole the center of our
galaxy that we'll talk in a minute about what that
means to see it, and we've also seen at the
center of other galaxies. In fact, it seems to be
more normal than abnormal to have a black hole at
the center of the galaxy. Right, most galaxies have a
super massive black hole in the middle, that's right. Yeah,
(21:43):
And you know that's what happens when things get dense.
You crowd enough stars together and eventually you're going to
get enough gravity to pull these things together to create
black holes, or one of them, you know, go supernova
and creates a black hole and then sucks in the
other ones and it just grows and grows and grows,
So it's not a surprise eyes, right, it's it's it's
sort of the best place to look for a black hole, Like,
where should you find one? You should find one where
(22:05):
there's the most mass, because that's what a black hole is,
a really high concentration of mass. Oh, that's kind of
what inevitably happens when you have that much stuff together
in the middle. Yeah, exactly. Squeeze enough stuff together and
you're gonna get a black hole. So is that what's
holding the galaxy together in a way? Yeah? Absolutely, I
mean the black hole the center is a tiny fraction
(22:26):
of the mass budget of the galaxy. Right, the mass
the galaxy has hundreds of billions of stars, and the
black hole the center, it weighs as much as millions
of stars. So we could do without it, right, if
you deleted it, it would change the way the galaxy rotated,
because the basic dynamics of a galaxy is that it's spinning, right,
although stars are orbiting around the center and they're getting
(22:48):
pulled in by all the gravity from all the stuff
that's closer to the center than they are, and that
black hole is a chunk of it. But it's it's
a it's a small fraction, so we could do without it.
But yeah, it is playing a role in keeping the
galaxy together, but it's not a huge role. It's not
like the anchor of the galaxy. It's just like it's
just helping the galaxy stay together. That's right. Because also,
(23:09):
remember most of the stuff in the galaxy that's providing
gravity is not stars or dust or gas or even
black holes. It's dark matter. Right. There's much more dark
matter in the galaxy than there is normal matter, five
times as much so. Also, in the center of the
galaxy is an enormous dense blob of dark matter that
we can't see at all. Wow. Well, we talk about
(23:31):
dark matter in another podcast episode. But let's let's focus
on this black hole. How do we know there's even
a black hole there? If you can't see black holes, Yeah,
you can't see them directly, which is really frustrating. Right,
And the reason you can't see them directly is because
they absorb light and they don't give off any light. Right.
It's easy to to get confused about this topic. People
(23:54):
think if something absorbs green light, for example, it makes
it green. Remember if something absorbed green light, then none
of the green light gets to your eye, and so
it doesn't look green. Something only looks green if it
reflects green light. Now, black hole absorbs all light. No
light can leave the black hole, which is why it
looks black. Right, So now you're looking for something black
(24:16):
with black space behind it. It's pretty tricky to see
black and black. It's camouflaged, that's right. It's perfectly camouflaged.
And it's not that large like it's it's not like
a huge thing, and so it's it's dense and it's significant,
but you can't really see it, right. It just be
like a little tiny dog black dot in the black background. Yeah, exactly.
It's like a black down in the black background, which
(24:36):
is really hard to spot. Um. And the best way
to see it is indirect and the best way to
indirectly study it is through its gravity, because that's really
its dominant feature. Right, It's a huge source of gravitational attraction.
So we can do is we can see it the
impact of the black hole on the stars around it.
We can look to see how they're moving. Oh, I see,
so that it kind of like you can tell there's
(24:57):
a son in our Solar system because all the planets
are going or on in a circle. You can tell
there's a black hole because of the way the stars
near the center are going in a circle. Exactly. Imagine
you couldn't see the Sun for some reason, you were
blind to the Sun, and you saw all these planets
orbiting the same location. You'd be like, what's going on?
There must be something there that's providing this gravitational force, right,
(25:18):
And you can calculate. You can tell exactly how much
it weighs and where it is just based on the
motion of the planets. So use exactly the same strategy
and then look at the stars that are near the
center of the galaxy and ask are they orbiting something?
Is there something there that they're moving around? And and
absolutely there are And it takes a long time because
these stars, you know, are moving pretty quickly, but this
(25:39):
stuff is far away, and so to watch a star
orbit the center of the galaxy takes years or decades.
And there's a couple of groups, one of the leading
ones is that U C. L A. And they've been
watching these stars near the center of the galaxy and
they are doing crazy stuff. They are swinging around a
crazy high velocities and changing directions in a way that
only makes sense if there's some enorm a source of
(26:00):
gravity right there at the center. Um. But we don't
see anything, right, So that's a pretty big clue. It's
like the observations on the math tells there should be
a black hole there, But then how do you separate
that out from dark matter or how do you separate
that out from just they're just a lot of regular
stars clumped together, right, Well, we don't see stars there, right.
(26:21):
I mean, if the stars are luminous, they're bright, and
so if there were stars there, we would see them.
So we don't see any stars in that location, right,
It's it's dark um. And how do we know it's
not dark matter? Well, there definitely is dark matter there also, um,
But we have a hard time studying dark matter because
it's not as localized, tends to be more spread out.
It's sort of a smoother blob as far as we know.
But that's a whole other area of discussion. Um. What
(26:44):
one way that we can tell it's probably a black
hole and not just something else is that we do
see some radiation from it that can only come from
a black hole. Yeah, that's consistent with coming from a
black hole, and somebody out there is probably thinking, hold
on his and it's a black hole. It doesn't admit
any radiation black except for hawking radiation. And we're not
talking about hawking radiation. We're not talking about the little
(27:04):
wiggles that come off the edge of the black hole
when particles decay near their edge. We can talk about
that in another podcast. Instead, we're talking about the radiation
that comes from the stuff around the black hole that's
not yet in it, but getting squeezed and pulled into
the black hole. This is something called an accretion disc.
It's like the stuff swirling around the edge of the
black hole that's not yet there. It's undergoing tremendous pressure.
(27:28):
It's being really pulled and squeezed. It's like the chaos
right before it falls down the toilet. That's right just
before it gets flushed. It does a few last circles
and the little bits that are about to go go
in get squeezed together, and that causes radiation. And that
has a very specific signature that you can say, Okay,
(27:49):
that's stuff that is about to fall into a black hole. Yeah,
And it's sort of like a flickering behavior. Um, it's
not constant. It's something that happens sometimes, and it's exactly
what you would expect from a black coal. And and
again you can't see this directly, right. It's really hard
to study the center of the galaxy because it's obscured
with huge clouds of gas and dust. And so these
stars we we want to image, and this radiation is
(28:12):
really difficult to see. We have to use all sorts
of techniques, some combination of radio waves and infrared and
X ray emissions and all sorts of stuff. And so
we we see this radiation. We also see X rays right,
X rays and other things. Yeah. Um, But each of
these are absorbed differently by the gas and the dust
that are between us and the center of the galaxy,
and and they're absorbed you know, in different wavelengths, etcetera.
(28:35):
And so we have to have really good maps of
that dust in order to account for how much that's
been absorbed, you know, how much of the signal are
we missing because we're looking through a big sandstorm essentially, um.
And that's why it's really important to have different ways
to see because infrared and radio and X ray these
are just different frequencies of light essentially, of radio of
electromagnetic waves that are going through the galaxy, and the
(28:58):
different frequencies are differently affected of the stuff that's between
us and them. So having different handles is really helpful
because you can tell um what's there and what's not there,
and it gives you a clearer picture. It's like having
um multiple waste idea. Somebody, right, you have their picture,
and you have to know their voice, and you have
their fingerprint or something like that, and you build up
(29:18):
this picture slowly because we only have fragments of each
of them. But if you had fragments of somebody's voice
and their picture and you know what they smelled like
or something that you could identify somebody even if you
couldn't see them clearly. Okay, So that's what's at the
center of the galaxy. It's just a lot. It's a
party in there, a lot of stars, ten million stars
per cubic parsect. So yeah, it's a pretty hot and
(29:39):
dense place. Yeah, and there's dark matter and there's super
massive there's a super massive black hole in there also.
That's right, and pretty soon we're hopeful that we'll get
an even better view of what's going on in the
center of the galaxy because we've had this project. It's
called the event horizon telescope. Event horizons the name for
the edge of the black hole beyond which nothing else
(29:59):
can come out of. And they're building this telescope. It's
essentially just tying together a bunch of different telescopes on Earth.
But if you do that, if you use data from
different telescopes at the same time, it makes like a
huge virtual telescope, one that's effectively the size of the
distance between the telescopes. And so they're making one that's
basically the size of the Earth. And they were supposed
(30:20):
to have essentially a picture of the black hole, the
event horizon of the black hole. Imagine that image from Interstellar,
you know, the picture of the black hole from Interstellar
has become sort of famous. That looks like a basically
a big black ball surrounded by kind of a halo. Yeah,
to me, it looks a bit like a pizza that
somebody sat on, a pizza, a pizza that somebody flushed
(30:44):
down the toilet, exactly. It's a big flushed pizza. Anyway,
we're going to see pretty soon what the center of
the galaxy actually looks like thanks to the new event
Horizon telescope. It was the data was supposed to come out.
The image is supposed to come out last year, but
they're still processing, so it could be any day that
we're gonna have this historic first image of the event
horizon of a black hole, of the black hole, and
(31:04):
you will see details like in the in the movie.
I don't know, I don't know what it's gonna look like.
I don't know how what kind of resolution they have, um,
but it's gonna be pretty exciting. All the all the
black hole ologists I know are very excited about. That
is an actual job description, absolutely, and I should mention
that in preparing for this podcast, I actually did some
research myself and I spoke to one of my esteemed colleagues,
(31:27):
Professor Aaron Barth, and he is a black holologist. He
studies supermassive black holes and he thinks about how do
you make them? And doesn't make sense? And why do
we have them? And do they work the way we expect.
There's a lot of open questions about the way black
holes work. This even though we know they're there, we
don't really understand why they're there and how they got
to be there. Black hole ologist. You can't get over that, right,
(31:49):
You're like, I wasn't awhere that was on the list.
I wouldn't have become a cartoonist. I would have gone
with something better, maybe like black holy Man. That would
have been more. I think that could be confused with
something else, or or a black holistic person. Doctor. Um. Okay, So,
(32:10):
so I heard there's a huge mystery surrounding black holes
at the center of galaxies, and we should totally get
into it. But first let's take a quick break, all right.
(32:30):
So there is a huge, supermassive black hole at the
center of the galaxy, and there's a huge mystery surrounding
these black holes, which is that nobody knows how they
came to be. They're inexplicable, that's right. They're too big, right, Um,
you know, and some people like big black holes and
they cannot lie. But we don't know. We don't know
(32:53):
how these black holes got to be so big. And
the reason is that it's you. You might think, well,
the black hole sits at the center of the galaxy.
Of course, it just a bunch of stars. It's just
been eating a lot. Yeah, but but you know, um,
it's not easy for black holes to suck in all
those stars because the stars are orbiting, right the same
reason that the Earth doesn't just like fall into the
Sun and make the Sun a tiny bit bigger. Those
(33:14):
stars are orbiting the black hole and their angular momentum
keeps them from falling in, and so according to our models,
the black holes should not be that big. They're like
a thousand times bigger than they should be if we understand,
you know, how they started and how they grew. So
the fact that they're so big is kind of awesome,
but also a big mystery. They're mysteriously big, like we
don't know where all the food came from. We don't
(33:36):
know how they got that big. Yeah, we don't know
what are they sucking in and where did it come
from and who's been feeding them? And people have crazy ideas,
you know, they think maybe there was a time in
the universe when galaxies were colliding a lot more than
they are now, and so it could be that, you know,
a bunch of galaxies crammed together, and what we have
at the center of our galaxy is basically the black
holes of a bunch of galaxies all merged together. Or
(33:58):
you know, maybe when that merger happens the black holes
just like goggles up a bunch of stars because they
come into its path. There's a lot of fun, really
fun but really crazy dramatic ideas. And the amazing thing
to me is that these these dramas are incredible, right,
black holes eating stars, But they also happened really slowly,
you know, like over millions and billions of years. You
(34:20):
see these galaxies colliding and merging and things getting sucked up.
You know, I love the idea of huge violence happening
really slowly. Well it's slowed for you, um, but you
know our lives would be slow to an ant right
or to a microbe. Yeah, exactly. And on the Microbe
Universe podcast they probably talk about that all the time,
Daniels slowly the disaster that is this podcast or it's happening.
(34:44):
Why is this podcast so long? Maybe the black hole
in our galaxy, the one at the center of the
supermassive one, came from two galaxies arashing into each other
and there are black holes joining into one. Or it
could be that we just don't understand the process that
(35:06):
starts a black hole, and then maybe they started off
much bigger and uh, and that explains why they are
larger than than we understood. There's just a lot of
basic questions, you know. And every time we do this
in science, we see something interesting, then we ask can
we explain it? Do our models predict exactly what we're
gonna see? It's not just like, oh, we figured there'd
be a black hole, Yeah there is. Now we come
up with like detailed quantitative models. Let's say how big
(35:28):
should it be? Or is our black hole unusual? And
let's look at all the other black holes and we
try to really understand the details of the process. This
is how we we we find problems, and this is
how we crack them. So it's really all in getting
into these details. I have a question. Can a black
hole sucking dark matter? Absolutely? Yeah? Nothing preventing that. Yeah, absolutely.
(35:49):
I mean dark matter feels gravity, right, and so it's
going to get pulled by by the black hole, and
it's probably there could be dark There could be black
holes made mostly of dark matter, right, dark holes or
whatever you want to call dark black holes. Now, dark
matter is definitely not immune from the gravitational poll of
a black hole. Well, it's amazing to think we have
(36:10):
this huge mystery in the center of our galaxy. A
huge hole in our knowledge about the universe is there
literally in figuratively, right, that's right. It's at the center
of everything we we live around. It's we've been orbiting
it for billions of years and still don't understand it.
And the list of mysteries of the galactic center is huge.
You know, we don't understand a lot of the radiation
that's coming from there. We think there might be weird
(36:32):
stuff happening that we don't understand, so strange stars being made,
all sorts of other kind of processes. You know. Another
big mystery is like, where do all the high energy
cosmic rays come from? These particles from space that have
so much energy we can't explain it. Yeah, we did
a podcast episode on that, right, Yeah. And one possibility
is maybe they're coming from centers of galaxies with these
(36:53):
enormous pressures and huge gravitational forces. So far, it doesn't
look like it because we don't have a whole lot
of examples of these energy particles, but we can't point
them all back to the centers of galaxies. But we
don't know, and so there's a there's a lot of
stuff to be discovered it's a really really rich source
of um astronomical mystery. Yeah, it really makes me think
how dynamics things still are. You know, things seem pretty
(37:16):
chill right now in our solar system, but we're really
part of this larger history of this crazy, giant, swirling
active disaster crash zone toilet. That was the alternate name
for the for the galaxy we considered instead of Milky
Way detective crash disaster toilet, but it didn't quite roll
off the tongue the same way. I didn't think it
(37:37):
was gonna sell t shirts, No, but you're absolutely right.
And remember, the galaxy is young. We've only been around
for twenty spins of the Galaxy UM, and so in
terms of galactic years, the whole universe isn't even not
much longer. It's like, you know, sixty or seventy galactic
years old. So things are just getting started. So in
that party is just getting started. The party, yeah, the
(37:58):
the downtown party, that's right. But it's interesting to think
that inside of that galactic center are clues about how
the galaxy formed and how this whole universe got put
together right, exactly, absolutely, and that's what makes it so exciting.
And you know, that's the process of science, Like let's
look around and see what we don't understand, and then
ask basic questions about it and try to figure it out,
(38:20):
and along the way we come up with better and
better and more accurate models of what's going on. And
that's how we figured out like the dark matter is
a thing, and that's how we discovered black holes and
all the A lot of the really great transformative discoveries
of the modern age have come from asking simple, basic
questions about stuff, not flushing them down the toilet right away,
(38:41):
and the podcast partner says, though, that's right, or asking
questions about how to flush things down the toilet, like
can I flush a black hole down the toilet? Yes?
Or no? I'll need the dark matter please, that's right.
That was such an obvious joke. Nice job. Well, thank
you for joining us. That's the mystery at the center
(39:03):
of the galaxy. That's right. So now you know what
is the center the hopping party at the center of
the galaxy, But we don't recommend you spend any time there.
Toys in two fifty million years when the Earth turns
twenty one. That's right. Thanks for listening. See you next. Time.
(39:28):
If you still have a question after listening to all
these explanations, please drop us a line. We'd love to
hear from you. You can find us at Facebook, Twitter,
and Instagram at Daniel and Jorge That's one Word, or
email us at Feedback at Daniel and Jorge dot com.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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