Latest / Talking Science / Galaxies – How the Milky Way Was Built

Transcript

Galaxies – How the Milky Way Was Built · Talking Science

0:00
I want you to picture yourself standing outside
0:02
on a truly perfectly dark night. Oh yeah, the
0:05
kind of dark where your eyes take like 20 minutes
0:08
just to adjust. Exactly. You know, you're far
0:11
away from city lights, far from the street lamps,
0:13
the headlights. Maybe you're out in the desert
0:15
or, I don't know, deep in the mountains. Just
0:17
zero light pollution. Right. And your eyes have
0:20
finally adjusted to the absolute pitch dark.
0:23
And as you look up, you see it. It's unmistakable
0:26
when you really see it. It is. It's this faint,
0:29
glowing, milky band of light stretching entirely
0:32
across the sky. Yeah. And, you know, for most
0:35
of human history, anyone who looked up saw that
0:38
exact same glowing river. But nobody had the
0:40
faintest clue what it actually was. Which is
0:42
wild to think about. Today, we are taking a deep
0:45
dive into an excerpted essay that explores the
0:48
deep science and, well, the history of the night
0:50
sky. Specifically focusing on our own cosmic
0:54
neighborhood. It's a really fascinating piece
0:56
of writing. It really is. Our mission today is
0:59
to understand exactly what you were looking at
1:03
when you gaze up at that glowing band, how it
1:06
got there, and what its violent past tells us
1:10
about our future. Because every ancient culture
1:12
had a story to explain that band of light, right?
1:16
I mean, the ancient Greeks looked up and imagined
1:18
it as a literal river of spilled milk. Which
1:20
is actually where we get the word galaxy. Right,
1:22
yeah. It traces all the way back to the Greek
1:24
word for milk. Yeah. But the modern scientific
1:26
reality detailed in this essay is, well, it's
1:29
infinitely more poetic. And deeply humbling.
1:32
Oh, absolutely. That faint band of light isn't
1:35
just some like celestial decoration painted on
1:38
the roof of the sky. It is our home. Yeah. You
1:40
are looking at the combined glow of hundreds
1:43
of billions of distant suns that make up our
1:45
own galaxy, the Milky Way. But you're seeing
1:48
it from a very specific, almost deceiving vantage
1:50
point, right? You are. You are looking at it
1:52
edge on from the inside. We are, you know, riding
1:54
along on one single star, looking out through
1:56
the dense flat disk of our own galactic home.
1:59
So the view is a river of... distant suns. Because
2:02
we are just floating right in the thickest part
2:05
of it. Exactly. We're embedded in it. Okay, let's
2:08
unpack this. Because to really wrap our minds
2:11
around what's happening inside that glowing river,
2:13
we need to establish the sheer scale of a galaxy.
2:17
Right, because galaxy is a word we throw around
2:19
a lot in sci -fi and pop culture. But holding
2:21
the actual physical concept in your head requires
2:24
some major mental gymnastics. It really does.
2:28
For anyone without a science background, a galaxy
2:31
at its most basic level is a massive, sprawling
2:35
collection of stars, enormous clouds of gas,
2:38
and cosmic dust. Just a giant collection of stuff.
2:41
Yeah, and all of this material is bound together
2:43
by the force of gravity, and it all orbits a
2:45
common center. In the case of the Milky Way,
2:47
we are talking about a structure containing somewhere
2:49
between 100 and 400 billion individual stars.
2:52
Which is just a staggering number. I mean, let's
2:55
ground those massive numbers in something tangible
2:57
for you listening. Imagine a giant, flat breakfast
3:01
pancake. Okay, a pancake. Yeah. Now, take a single
3:05
chocolate chip and bury it halfway between the
3:08
center of that pancake and the outer rim. I like
3:10
where this is going. That one chocolate chip
3:12
is our entire solar system, everything we know.
3:17
When you look up at the night sky and see that
3:19
milky band, you are standing on that chocolate
3:22
chip, looking out sideways through the batter.
3:25
towards the rim of the pancake. And you're seeing
3:27
the combined glow of all the other chocolate
3:29
chips stacked up along that flat plane. Right.
3:32
You're seeing the cross -section of the pancake.
3:33
The geometry of our situation maps perfectly
3:36
to that visual. And the essay expands on this
3:39
scale by looking at the space outside the pancake,
3:42
too. Like the rest of the breakfast table. Kind
3:44
of, yeah. In the universe at large, galaxies
3:47
are the fundamental building blocks. You can
3:49
think of them as Bustling, isolated cities scattered
3:52
across a vast, profoundly empty continent. So
3:55
the space between them is just nothing. The gulfs
3:58
of nothingness between these galactic cities
4:00
are so incredibly wide that the galaxies themselves
4:03
become the basic units of cosmic structure. And
4:06
within our own bustling city, our sun is completely
4:10
unremarkable. Very average. It's a very average
4:13
star just sitting out in the galactic suburbs.
4:17
The timeline of our orbit around the center is
4:19
what really grounds you, I think. Just as the
4:22
Earth orbits the sun, our entire solar system
4:24
is orbiting the center of the galaxy. Yeah, the
4:26
entire structure is in motion. It's not static.
4:29
And it takes our sun roughly 230 million years
4:33
to complete a single lap around the galactic
4:36
center. Wait, 230 million years just for one
4:39
lap? One single lap. That means the last time
4:41
our planet was in this exact physical spot in
4:44
space. Dinosaurs hadn't even appeared on Earth
4:47
yet. Not even the earliest dinosaur. That is
4:49
insane. Our planet has been through entire epochs,
4:52
mass extinctions, the complete evolution of humanity,
4:55
all while barely moving a fraction of an inch
4:58
along our cosmic racetrack. It really forces
5:00
a shift in how we perceive time, doesn't it?
5:02
It really does. And if we pull back from our
5:05
solar system and view this entire rotating structure
5:08
from the outside, the shape of it tells us a
5:10
lot about its history and its behavior. Right,
5:13
because not all galaxies look like pancakes.
5:15
No, not at all. Galaxies generally... fall into
5:17
a few broad categories. You have ellipticals
5:20
which are round, smooth, Almost featureless blobs.
5:24
Blobs in space. Basically. They are dominated
5:28
by older stars and have exhausted most of their
5:30
gas, meaning very few new stars are being born
5:32
there. They are the quiet, retired communities
5:35
of the cosmos. I love that. Retired communities.
5:38
And then there are irregular galaxies, which
5:41
lack any symmetrical shape at all. They are often
5:43
messy and chaotic, usually because their gravity
5:46
has been disrupted by a close encounter with
5:47
a neighboring galaxy. But the Milky Way falls
5:50
into the most photogenic category, right? We
5:53
are a spiral galaxy. We are indeed. We have that
5:56
flat rotating disc with a bright central bulge
5:58
of stars and these beautiful, graceful arms winding
6:02
outward like a massive pinwheel. It's the classic
6:04
image everyone thinks of. Right, but I have a
6:06
question about that. When we see pictures of
6:08
spiral arms, it looks like a solid wheel spinning.
6:11
Are those arms actual solid physical structures?
6:15
Like spokes on a bike. So glad you asked that.
6:18
Thinking of those spiral arms as solid structures
6:20
creates a huge physical paradox. Oh, so? Well,
6:23
if they were solid physical objects, the outer
6:27
edges of the arms would have to travel at impossibly
6:29
fast speeds to keep up with the inner edges.
6:32
Oh, because they have a much longer distance
6:34
to cover in the same amount of time. Exactly.
6:36
So if they aren't physical spokes, what creates
6:39
that distinct sweeping pinwheel shape? What's
6:42
fascinating here is the essay uses a brilliant
6:44
everyday analogy to explain this visual illusion.
6:48
I'm ready for it. Those spiral arms are actually
6:50
cosmic traffic jams. Traffic jams? Yeah. Imagine
6:54
driving on a highway where a slow -moving truck
6:56
is in the right lane. Cars naturally bunch up
6:59
behind the truck, creating a dense, slow -moving
7:02
cluster of traffic. Okay, I know that feeling
7:04
well. We all do. Eventually, individual cars
7:08
manage to pass the truck and speed back up, spreading
7:11
out again. The traffic jam itself? The dense
7:14
region of cars stays roughly in the same place
7:17
on the highway, even though individual cars are
7:19
constantly flowing into it and then back out
7:21
of it. Oh, wow. So the spiral arms are regions
7:23
of higher gravitational density. Precisely. As
7:26
clouds of gas and individual stars orbit the
7:29
center of the galaxy, they hit these dense regions
7:31
and slow down. They pile up. And when giant clouds
7:34
of space gas get compressed together in a cosmic
7:36
traffic jam, the gravity causes them to collapse
7:39
inward. And I assume the pressure and heat becomes
7:42
so intense that nuclear fusion ignites. You got
7:45
it. The compression triggers a massive wave of
7:48
star birth. The arms of a spiral galaxy are brightly
7:51
lit, not because they are solid structures, but
7:53
because they are active stellar nurseries. That
7:55
makes so much sense. And the essay mentions this
7:57
process completely dictates the colors we observe
8:00
in different parts of the galaxy. It does. Because
8:02
the spiral arms are full of brand new, incredibly
8:05
hot, massive stars, they glow with a brilliant
8:09
blue -white color. While the central bulge of
8:12
the galaxy glows in shades of yellow and red.
8:14
Right, because the center is packed tightly with
8:16
much older, cooler stars that have been burning
8:19
for billions of years. So you essentially have
8:21
a color -coded map of the galaxy's demographics
8:24
just by looking at the light. blue and white
8:26
in the spiral arms where the new stars are actively
8:29
igniting, and yellow and red in the center where
8:32
the older population resides. Yeah, you can read
8:34
the age of a galactic neighborhood purely through
8:37
its color temperature. Here's where it gets really
8:39
interesting, though. If this entire massive structure
8:43
is spinning at millions of miles an hour, there
8:45
has to be an anchor. Right. Gravity requires
8:48
mass. Exactly. For hundreds of billions of stars
8:52
to orbit a central point without just flying
8:54
off into the void of space, whatever sits at
8:57
the absolute core of that yellow -red central
8:59
bulge must be incredibly dense. What is actually
9:03
in the middle? At the very center of the Milky
9:05
Way and at the core of most large galaxies is
9:08
a supermassive black hole. A supermassive black
9:12
hole. Yes. Ours is named Sagittarius A star.
9:16
And to grasp the density we are talking about,
9:19
this single object weighs roughly 4 million times
9:22
as much as our sun. Four million suns. But all
9:26
of that mass is compressed into a physical region
9:28
no larger than our own solar system. See, but
9:31
this is the thing. A black hole is, by definition,
9:34
an object with gravity so intense that not even
9:38
light can achieve the escape velocity required
9:40
to break free from it. Right. They are totally
9:43
black. Which means it is completely invisible.
9:45
And the center of the galaxy is packed with billions
9:48
of stars and massive thick clouds of dust. Looking
9:51
toward the center should be like trying to see
9:53
a single unlit street lamp through a dense fog.
9:57
It's a logistical nightmare for astronomers.
9:59
So how did astronomers actually prove it was
10:01
there, let alone calculate its exact weight?
10:04
Well, the scientific process behind this actually
10:06
earned a Nobel Prize. Astronomers knew they couldn't
10:09
look for the black hole itself, and visible light
10:11
is indeed blocked by all that galactic dust you
10:13
mentioned. So what do they do? They use telescopes
10:16
equipped to see infrared light. Infrared light
10:19
has longer wavelengths, which allows it to slip
10:21
right through the dust grains and travel all
10:23
the way to Earth. Alas, Clover. By looking at
10:26
the infrared light, they focused on the individual
10:28
stars closest to the very center of the galaxy.
10:31
And they simply watched them, mapping their exact
10:34
positions year after year after year. Just tracking
10:37
their movement. Exactly. Because if you track
10:39
a star over a decade and watch it whip around
10:42
a completely empty looking patch of space at
10:45
a fraction of the speed of light, the math reveals
10:47
the invisible object. The laws of gravity are
10:50
stripped. Right. To force a massive star to turn
10:53
that sharply and move that fast in a tiny tight
10:55
circle, the invisible thing it is orbiting has
10:59
to possess an immense specific amount of mass.
11:02
Yeah, by measuring the speed of those stars and
11:04
the exact size of their orbits, astronomers could
11:07
calculate the exact gravitational pull required
11:10
to keep them on those paths. The mathematics
11:12
proved that an object weighing 4 million suns
11:15
had to be hiding in that tiny, dark space. That
11:18
invisible, immensely heavy anchor is Sagittarius
11:21
A star. It is. It is an eerie realization, you
11:24
know. You look up at this beautiful stately river
11:26
of milk in the sky and the whole thing is organized
11:30
around a literal gravitational monster lurking
11:34
in the dark. It is a bit spooky, but the essay
11:37
makes a point to reassure us. Sagittarius A star
11:40
is located tens of thousands of light years away
11:42
from our solar system. Solar safe. Very safe.
11:45
And for a supermassive black hole, it is actually
11:48
quite docile. It isn't actively consuming huge
11:51
amounts of material right now, which means it
11:54
isn't violently tearing stars apart or blasting
11:56
out dangerous radiation. It is simply sitting
11:59
there, quietly providing the gravitational anchor
12:02
for the inner galaxy. Which brings us to the
12:05
origins of this massive structure. We know what
12:07
the Milky Way looks like today and we know what
12:09
anchors it. But going from a completely empty
12:11
expanding universe to a spinning pinwheel of
12:14
400 billion stars, that requires a building process.
12:18
It does. We have to look back to the early universe
12:20
shortly after the Big Bang. Okay, setting the
12:22
time machine. The cosmos was a vast expanding
12:25
fog, mostly made of hydrogen and helium gas.
12:29
It was largely uniform, but there were microscopic
12:32
variations, places where the gas was just slightly
12:35
denser, slightly clumpier than others. And where
12:37
there is a clump of matter, there is a slightly
12:40
stronger gravitational pull. Exactly. That slightly
12:43
stronger gravity pulls in more neighboring gas,
12:46
making the clump bigger, which makes its gravity
12:48
even stronger, creating a snowball effect. That
12:51
is the basic mechanism of how structure forms.
12:53
However, there's a catch. There usually is. When
12:56
astrophysicists run the mathematical models using
12:58
only the ordinary matter, the hydrogen and helium,
13:01
we can actually observe the timeline doesn't
13:03
work. There simply wasn't enough gravity from
13:05
the ordinary gas to pull a massive galaxy together
13:08
as quickly as the Milky Way formed. Oh, I see.
13:11
The spinning gas would have flown apart. There
13:13
had to be a massive amount of unseen mass holding
13:16
it all together. This invisible ingredient is
13:19
dark matter. Right. Dark matter is the stuff
13:22
we can't see, can't touch, and doesn't interact
13:24
with light at all. But its gravitational pull
13:27
vastly outweighs all the ordinary matter in the
13:30
universe combined. Exactly. So is it basically
13:32
like the invisible steel rebar hidden inside
13:35
a concrete building, giving the visible universe
13:37
the structural integrity it needs to exist? That's
13:40
a great way to think of it. If we connect this
13:42
to the bigger picture, the essay uses a concept
13:46
of cosmic valleys to explain how dark matter
13:49
guided the creation of galaxies. Cosmic valleys.
13:52
Yeah. In the early universe, this invisible dark
13:55
matter clumped together and formed a massive
13:57
interconnected scaffolding across space. It created
14:01
dense gravitational regions. Okay, so if the
14:04
dark matter forms the underlying landscape, creating
14:06
deep gravitational valleys, then the ordinary
14:09
hydrogen and helium gas acts like... rainwater
14:12
that's spot on the ordinary gas naturally flowed
14:15
toward the strongest sources of gravity it fell
14:17
into these unseen dark matter clumps pooling
14:20
in the deep gravitational valleys like puddles
14:22
forming after a storm Exactly. It was deep within
14:25
these invisible dark matter wells that the hydrogen
14:28
gas got dense enough and hot enough to finally
14:31
ignite. That specific process is what burst the
14:35
very first stars. And those dense pools of gas
14:38
and stars became the first small galaxies. So
14:41
everything we can see, every glowing star, every
14:44
nebula, every spiral arm is entirely shaped by
14:47
a framework we cannot see. The visible universe
14:50
is just the glowing dust resting in the valleys
14:53
of invisible dark matter mountains. Beautifully
14:55
said. But wait, those early dark matter pools
14:58
only created small, early galaxies, right? The
15:01
Milky Way is a behemoth. Right, so how did it
15:03
get so big? Galaxies do not remain static. The
15:05
crucial mechanism for how giant galaxies like
15:08
the Milky Way attain their staggering size is
15:10
through merging. Merging. Like corporate mergers.
15:12
A little more violent than that. Over billions
15:14
of years, driven by the relentless pull of gravity,
15:17
smaller galaxies drift toward one another. They
15:20
collide, their gravitational fields tangle, and
15:22
they fold into each other. Large galaxies are
15:24
built by consuming smaller ones. Which is wild.
15:27
And we actually have physical proof of this violence
15:29
written into the sky. We do. The European Space
15:32
Agency launched a spacecraft called Gaia. Its
15:36
entire mission was to map the precise positions,
15:39
velocities, and chemical compositions of over
15:42
a billion individual stars within the Milky Way.
15:45
It's an incredible undertaking. Right. By measuring
15:48
a star's chemical signature like a unique barcode
15:51
of light, it emits based on what elements it
15:54
is made of. Astronomers can tell where that star
15:57
was born. And the Gaia data revealed something
15:59
amazing. It found streams of stars moving through
16:02
our galaxy in highly unusual coordinated groups.
16:06
Just moving together like a school of fish. Exactly.
16:08
These specific star streams share the exact same
16:11
chemical signatures, which are different from
16:13
the native stars of the Milky Way, and they share
16:16
the same orbital projectories. Wow. The data
16:18
acts as a fossil record, proving beyond a doubt
16:21
that these stars are the shredded remains of
16:23
smaller galaxies that the Milky Way swallowed
16:25
whole long ago. So what does this all mean? Is
16:28
our beautiful galaxy actually a Frankenstein
16:30
monster built out of shredded neighbors? I mean,
16:32
yes. The Milky Way is literally built from the
16:35
bodies of its neighbors and is still cannibalizing
16:38
small companion galaxies today, pulling them
16:41
apart with its gravity and adding their stars
16:44
to our outer halo. We are living in a predator
16:46
galaxy. We really are. But this raises an important
16:49
question about our own future. We are not the
16:52
only massive galaxy in our local cosmic neighborhood.
16:56
Oh boy. The Milky Way has a near twin, the Andromeda
16:59
Galaxy, located roughly two and a half million
17:02
light years away. And the two of us are caught
17:04
in each other's gravitational pull, actively
17:07
falling towards one another through space. Okay,
17:09
so in roughly four to five billion years, the
17:12
Milky Way and Andromeda are going to collide.
17:15
we will merge into one single, unimaginably massive
17:18
super galaxy. That's the current forecast, yes.
17:21
And I'm sure you listening might be panicking
17:23
right now, picturing our sun crashing head -on
17:25
into another star, or planets colliding like
17:28
billiard balls in some apocalyptic demolition
17:30
derby. It sounds like a violent, destructive
17:32
event. But this is where the sheer, mind -bending
17:36
scale of empty space comes into play again. Right,
17:39
because of how far apart things are. Exactly.
17:41
The distances between individual stars are so
17:44
staggeringly vast. compared to the physical size
17:48
of the stars themselves. If you shrunk the sun
17:51
down to the size of a ping pong ball, the nearest
17:54
star would be another ping pong ball hundreds
17:56
of miles away. Hundreds of miles. Yeah. Because
17:59
of that immense emptiness, when these two huge
18:01
galaxies collide, the statistical probability
18:04
of any two stars actually hitting each other
18:06
is practically zero. So they don't crash into
18:09
each other, they just slip past one another in
18:11
the dark. It is less like a car crash and more
18:14
like... Two massive swarms of bees flying directly
18:17
through one another. That's the perfect analogy.
18:19
The swarms mingle. Over millions of years, the
18:22
combined gravity of the two massive galaxies
18:24
will violently scramble their structure. So the
18:27
pinwheel is gone. Yes. Those beautiful spiral
18:29
arms will be pulled out into long, chaotic loops
18:32
and tails of gas and stars. Eventually, the chaos
18:35
will settle down, and the two galaxies will reshape
18:38
into something entirely new, most likely a giant,
18:41
featureless, elliptical galaxy. Why those retired
18:43
communities? Exactly. But our own sun and whatever
18:46
is left of our solar system will survive the
18:49
encounter unharmed. We will simply be flung into
18:52
a brand new orbit within the newly combined super
18:56
galaxy. That is just incredible. The next time
18:59
you are outside on a dark night and you let your
19:01
eyes adjust and you look up at that faint milky
19:04
band of light, remember the sheer weight of what
19:06
you were looking at. It's not just a pretty view.
19:09
No, it's a structure that was meticulously assembled
19:11
over more than 10 billion years. It started from
19:14
primordial hydrogen pooling into invisible dark
19:17
matter valleys. It grew into a behemoth by swallowing
19:21
its smaller neighbors. It spins endlessly around
19:24
a monstrous supermassive black hole. And it is
19:27
currently falling towards a future collision
19:29
that will erase its spiral shape forever. The
19:31
most profound realization from the essay, I think,
19:33
is the perspective it gives us. We are experiencing
19:36
this epic 10 billion year history from the inside
19:38
out. From our little chocolate chip. Right. We
19:41
are standing on one small planet orbiting one
19:44
perfectly average star among hundreds of billions
19:47
of others. And we are looking out into the dark
19:50
using eyes and scientific instruments that took
19:52
the entire history of the cosmos to develop.
19:56
We figured out how our galactic home was built
19:58
while trapped inside the walls. It is an absolute
20:01
triumph of human curiosity. But I want to leave
20:04
you with a final thought to mull over. Something
20:06
that stretches this timeline to its absolute
20:09
limit. Always expanding the horizon. We know
20:12
the universe is expanding and that expansion
20:13
is accelerating. While our local galaxies, like
20:17
Andromeda and the Milky Way, have enough gravity
20:19
to pull together and merge, the expansion of
20:22
space is driving all the other distant galaxies
20:24
away from us faster and faster. Right. Space
20:27
itself is stretching. Exactly. So billions of
20:30
years from now, after we have merged with Andromeda
20:32
into one giant super galaxy, that accelerating
20:35
expansion will eventually push every other galaxy
20:37
beyond our observable horizon. The light will
20:40
literally no longer be able to reach us. Yes.
20:43
Whoever... or whatever, is living on a planet
20:46
in our super galaxy in the deep future, will
20:49
look up into the night sky and see their own
20:51
stars. But beyond that, complete and utter blackness.
20:54
They will have absolutely no evidence that the
20:57
rest of the universe ever existed. They will
20:58
think their isolated, merged galaxy is the only
21:01
thing in the entire cosmos. Keep looking up while
21:04
we still can.