Journey of the Universe weaves together the discoveries of the evolutionary sciences together with humanities such as history, philosophy, art, and religion. This course draws on the Journey of the Universe Conversations, a series of 20 interviews with scientists and environmentalists. The first 10 interviews are with scientists and historians who deepen our understanding of the evolutionary process of universe, Earth, and humans. The second 10 interviews are with environmentalists, teachers, and artists who explore the connections between the universe story and the practices for a flourishing Earth community.
Galaxies Forming (Todd Duncan and Joel Primack)
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From the course by Yale University
Journey Conversations: Weaving Knowledge and Action
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Yale University
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Course 2 of 4 in the Specialization Journey of the Universe: A Story for Our Times
From the lesson
Beginning of the Universe: Galaxies, Stars, and the Solar System
A star is required for the production of carbon and oxygen and a galaxy is required for the production of stars. So the Milky Way galaxy as a whole is required for our hands. Understanding our cosmological nature is the first step into a new era.
Meet the Instructors
Mary Evelyn TuckerSenior Lecturer and Senior Research Scholar
Yale School of Forestry and Environmental Studies
John GrimSenior Lecturer and Senior Research Scholar
Yale School of Forestry and Environmental Studies
[MUSIC]
Morning on a Greek island is like the first day of creation.
And wandering around here you feel like the first person.
Inevitably humans would ask, what gave birth to all of this beauty?
What was the fall of creativity that brough this forth?
[MUSIC]
Consider galaxies.
What brought the galaxies forth?
You know, even a century ago this is hard to imagine.
We didn't know if there were two galaxies in the entire universe.
That was a main focus of attention among scientists.
Now we know there are a hundred billion, maybe even a trillion, galaxies.
What is the creativity that brought forth a trillion galaxies?
[MUSIC]
Let's consider our own Milky Way galaxy.
[SOUND] It's a galaxy with huge spiral arcs.
Now when we first began to discover galaxies,
we thought maybe these spiral arcs were composed of physical matter.
Actually, it's much more interesting.
The arcs are actually waves passing through the galaxy called density waves.
And as they passed through clouds of hydrogen and helium, they ignite stars.
[MUSIC]
That's the way the picture of the Milky Way Galaxy, not so much as a thing, but
rather as a activity.
It's an ongoing activity of bringing forth stars.
We live in the midst of this intense creativity.
[MUSIC]
Todd, welcome, it's so good to see you.
>> Thank you, It's good to be here.
>> As a cosmologist and as a person who's teaching science for
the larger public, I'm delighted to have you here and to share your knowledge.
We'll begin with the big scale.
Shall we? >> Okay.
>> Let's begin with the story of galaxies and the fact that 100 years ago
we were thinking the Milky Way was the only galaxy in the universe.
>> Mm-hm.
>> What happened to expand our knowledge of the size and
scale of the universe and galaxies?
>> Yeah, so that's probably one of the most amazing stories of human history,
actually, is the fact that we've gone from thinking that all the little
fuzzy spiral things that you could see and
telescopes spiral nebulae, which basically means spiral cloud fairly nearby,
just clouds like other kinds of clouds you'd see in the night sky.
And really the key piece that
broke through that was being able to have telescopes and techniques to
be able to measure distances to those objects by actually finding stars in them.
And this was pioneered by lots of people but Henrietta Levitt
is one of the astronomers who play the key role and being able to make that happen.
And Edwin Hubble and around the early 1900s, people
started being able to measure distances to galaxies like our Andromeda galaxy and
found out the modern distance is about 2.5 million lightyears away.
And so that was just the beginning sort of a waiting out into the surf,
a little bit, of being able to realize that now we can actually measure distances
to galaxies that are over ten billion light years away,
which means that it takes light going at 300,000 kilometers a second
ten billion years to get from that object to us.
It's really hard to wrap your mind around how much that expands,
how big our universe is.
You know if you stand and look out on a field or you look out at the ocean and
you sort of comprehend how big the earth is and
then you try to picture how much bigger.
You know it takes like way less than second to go around the earth and
to be able to talk about distances where it takes.
10 billion years it's just
hard to comprehend how much that changes your perspective.
>> It's staggering, isn't it?
And the fact that Andromeda we can see
with the naked eye even on a clear night and so on.
But tell us about galaxies, are there different kinds of galaxies and
then how are they formed?
>> The general type like our own is actually a barred spiral galaxy.
So it's just like the name sounds it's a spiral shape with
kind of a bar shape in the middle.
Then there are just pure spiral galaxies is another class.
And then there are irregular galaxies, which as the name implies,
they're just sort of funny looking shaped things, little blobs.
And then there are elliptical galaxies, which are sort of football shaped and
the basic formation process for galaxies is, I think, a really fun example of
how simple principles can produce complex, interesting, varied systems.
Basically they're all driven by gravity.
So, the way I like to think it, the whole evolution of the universe
from a cosmologist point of view is going from at one end, from simple and
smooth at the early parts of the universe, about 14 billion years ago,
to clumpy and complex, which is what we have now.
So, a lot of cosmology is understanding how we got from simple and
smooth to clumpy and complex, which of course,
we're very grateful for because we're clumpy and complex.
>> Complex, exactly.
>> So, the basic understanding of the process is that at the early stage,
the universe was very, very hot and dense and smooth,
dominated by light radiation.
And so everything was very evened out, so
you could picture it just like a, maybe a lot like if you were inside the sun.
Just glowing, hot, everything smooth.
The universe is expanding, which is a piece that
has only been recently discovered and is important to this.
But there's a competition going on that as it's expanding if you get one little
region that for whatever reason gets a slight extra bit of material there.
There is a really simple law of gravity that says that
every bit of material pulls on every other bit of material and
so, if you get a little bit of extra in one region, then it will tend to grow.
Imagine that you have a particle sitting here.
And if all around it were other particles evenly distributed,
then they'll all pull equally and it won't go anywhere.
But if you get a little bit of a clump somewhere,
now there's a little extra pull in one direction so the clumps tend to grow.
It's like you're you know stirring gravy or something, and
you get a little clump one place and it sort of tends to grow into a bigger clump.
So the galaxies formed really tiny quantum fluctuations in the very,
very early universe that were just tiny seeds that then grew
over time into these clumps that as they gathered more and
more material they became the galaxies that we see.
And if you look at pictures of galaxies in the Hubble website or
something like that, the reason they look like sort of a saucer, they're flattened,
is because if they're spinning even just a little bit, which they always kind of will
be as these particles are kind of gathering together under gravity.
If they're spinning a little bit, then probably everybody has seen the phenomenon
when an ice skater pulls his or her arms in, if you're spinning a little bit and
you pull in tighter, you spin faster, and this is a law of
physics called the conservation of angular momentum that makes that happen.
When that happens, as the galaxy shrinks It starts to spin faster and
faster because of that.
And when something's spinning fast like that,
it also tends to flatten out if it's able to.
So think about spinning pizza dough or something on your fingers right.
It flattens as it spins.
You have this matter coming together.
It starts spinning which means it flattens out.
And the process you end up getting these disks that we see as galaxies today.
And one of the other things that happens that's maybe not so obvious,
is it turns out that they form spiral arms.
It's similar to a traffic jam on a road on Earth.
>> Right.
>> And so, the stars are now orbiting the galaxy.
And if you look at the spiral arms,
you might think that it's like a rigid structure.
Like if you cut a spiral out of paper or cardboard, and
spun it around like a pin wheel or something.
But in fact, the spiral structure that you see pictures of,
it's not always the same stars.
So, the stars are moving through the spiral pattern.
And so, it really is just a pattern of kind of a wave of
density that's cycling through the galaxy.
And just like in a traffic jam where there are different cars
that participate at different times, right?
So, you can be watching from a helicopter and you can see there's a traffic jam,
that people are stuck here.
But cars are slowing down and then moving out the other side.
And then a new car comes in, slows down when the traffic jam is, and
moves out the other side.
So, the cars are moving through, maybe not as quickly as they would like to be, but
they are moving through.
But the pattern stays in tact.
>> And what about our Milky Way galaxy?
Give us a feel for it's shape and- >> So,
our Milky Way galaxy is called a barred spiral.
So, it's a spiral that has two pretty distinct spiral arms,
from what we've been able to tell.
It's somewhat difficult to map it, because we're inside of it.
>> Right. >> So there's a big
challenge in astronomy of being able to do that.
And then, it's about 100,000 light years the long way across the saucer.
So, you think about for a minute and realize this is just
within our own milky way galaxy, if you could travel at the speed of light it,
would take you 100,000 years to get from one side to the other.
And on average, it's about 1,000 light-years thick,
it's thicker in the middle and thinner out at the edges.
And then, there's also a halo of what are called globular clusters,
that are groups of older stars that are in sort of a halo pattern, more circular and
more spherically shaped around the outside.
And then, part that we don't see that is almost certainly there,
probably 10 times bigger than the galaxy, visible part of the galaxy that we see,
there is pretty strong evidence that there's a dark matter halo that was
actually the matter that originally sort of formed the gravitational structure
that brought the visible part that of the galaxy in.
>> Yeah, so- >> If we want to talk about that,
it's a big mystery of what is this dark matter that makes up something
like 90% of the matter in the Universe.
[MUSIC]
[MUSIC]
>> Joel, did all galaxies emerge in the early universe, or
have they been forming throughout this 14 billion year history?
>> Galaxies start out small, and they agglomerate more stuff.
Gravity pulls things together.
So, as time goes on, galaxies grow larger and more complex.
And then, since the universe is also expanding rapidly,
the process of agglomeration slows down, and
the galaxies develop into a sort of a mature stage where they look
like the nearby galaxies, disc galaxies, and elliptical galaxies.
Big balls of stars elliptical galaxies.
Thin discs, often with spiral arms, and
a ball of stars in the center called the bulge, are disc galaxies.
Our own galaxy's a disc.
So, these big galaxies that we see around us,
emerged maybe after about four billion years.
If you go back before that, you just see small galaxies
that are much more irregular and have not assumed these nice, smooth shapes.
After about four billion years, about 10 billion years ago, we start to see
a lot of galaxies that look like nearby galaxies, but they're a lot brighter.
They're forming stars, and the young stars tend to be much more massive and brighter.
The massive stars continue to form, but they're much rarer as time goes on.
And massive stars are much brighter than low mass stars like our sun.
So, the nature of the stars changes.
And, of course, some things build up as galaxies age.
The amount of heavy elements that are produced in stars,
the elements that we're made of, oxygen, carbon, iron,
elements like that, those are produced by generations of stars.
And as those elements build up, the planets can form, planets like our Earth.
So, the early universe didn't have Earth-like planets because not enough
heavy elements had formed.
And then, only the centers of the galaxies that have lots of heavy elements,
had planets.
But those planets probably were rapidly destroyed by supernovae and
other violent events happening in the centers of galaxies.
So, organisms like us may very well only develop after many
billion years when planets form out in the suburbs of the galaxies,
out far from the center where we live.
The evolution of life is probably connected to the evolution of galaxies.
Galaxies are our cosmic home.
Of course, the Solar System and Earth are home in a much more local sense.
But all of this is part of a grand story that our observations are letting
us understand, and our theory is helping to interpret.
>> And we didn't even know if there was one galaxy, for example, in the sky.
And now, 100 billion galaxies probably.
But that sense of the knowledge that's emerging with scientists like you,
with your teams and so on, tell us as well about the shape and
the formation of galaxies.
Are they different?
Have they changed over time?
>> The first galaxies are these little itty-bitty things
that the first stars form in.
They're merging together at a rapid rate, and
that prevents them from assuming the regular forms that nearby galaxies have.
But it takes billions of years for a galaxy to assume a nice,
smooth, regular form.
For example, it takes about a quarter billion years for
the sun to go all the way around the center of the Milky Way.
And it takes a number of these revolutions to get a nice, smooth galaxy.
In the first few billion years, there isn't time for all of this to happen.
Besides, the galaxies are so busy merging together,
and when they merge together, if they're full of gas, huge number of stars form.
We call that a starburst.
Some of the gas fuels the gigantic black holes at the centers of these galaxies,
and that makes bright quasars.
So, all of these events are happening, and
they probably peak about 4 billion years after the beginning.
In other words, almost 10 billion years ago.
That's high noon for star formation.
That's when a lot of the stars in our own galaxy formed.
And certainly,
the really massive galaxies formed most of their stars during that early epoch.
So, as we look back to the very early universe, which we're now just for
the first time being able to do with the latest instrument installed on
Hubble Space Telescope by the astronauts,
the Wide Field Camera 3, we can see what these early galaxies look like.
And they don't look anything like nearby galaxies.
They're irregular, they're clumpy, they are Too busy merging and forming
stars to develop their nice regular forms that the nearby galaxies have.
By, four billion years after the big bang,
we're getting these huge bust of star formation.
Then, galaxies slow down.
Basically galaxies go through a period somewhat like the growth spurts of
children and adolescents and they have a very troubled adolescence shall we say
with all kinds of worrisome, complicated events,
burst of star formation and black hole growth leading to
huge amounts of energy and light that can tear apart the centers of these galaxies.
But then, finally, in their middle age, form stars in a more or
less steady declining rate and become nice regular galaxies, like ours and
they enter a period of sort of pleasant old age.
Our galaxy will continue to form stars into the very distant future probably for
a trillion years, but at a slower and slower rate.
Incidentally in about five billion years our galaxy and
the other big galaxy nearby, the Andromeda galaxy, will merge.
The two galaxies are flying together now.
When they merge they're going to make a big elliptical galaxy.
And in many tens of billions of years the distant galaxies will be disappearing,
and our distant descendants if we have any will only see the nearby galaxies and
these very distant galaxies that are flying away very rapidly,
assuming that the dark energy is something like a cosmological constant.
So we're living now through a grand period
of let's say maturity of these galaxies before they enter their
sort of senescence where they've stopped forming stars very rapidly.
However, it turns out that the little stars,
the stars much smaller than the sun, will continue to shine for
trillions of years, and not only that, as time goes on, they get brighter,
and our galaxy will actually be brighter a trillion years from now than it is today.
>> So it's such a fascinating process, and
to think they have this lifespan that you've spoken about.
And when you say a disc shape, sometimes we also call it the spiral galaxy as well?
>> Sure, the the disks are basically about as thick as a CD or
a phonograph record compared to their size.
In other words they're really quite thin.
And when galaxies have a fair amount of gas in them as our galaxy does that you
can continue to form stars out of you get this spiral pattern in the disk,
spiral arms.
Once these galaxies run out of gas, there's still this galaxies the stars
continue to go around, but you don't get this spiral pattern anymore.
We have many, many images of galaxies like that.
We call them S0 or Linticular galaxies.
And of course, the most common kinds of big galaxies
are still spiral galaxies like our own Milky Way.
And practically all the stars are in the massive galaxies like the Milky Way.
But the vast majority of galaxies are little itty bitty things.
So, large number of them but each one just has millions or hundred millions
stars nothing like the hundred billions stars that a big galaxy like ours has.
>> And our location is on one of these spiral
arms of the- >> Yes.
>> Milky way galaxy which changes our whole sense of location, right?
>> Yes, we are actually out in the suburbs of the milky way.
The Milky Way has a big bulge in the center.
And the full size of the Milky Way is about 100,000 light years across.
So the radius is about 50,000 light years.
We're about 25,000 light years from the center.
As we look toward the center of the Milky Way, we see vast numbers of stars.
As we look in the opposite direction, there are very few stars.
Incidentally, the bright stars in the night sky
are especially concentrated around the Milky Way.
That's because, of course, we're looking in the direction where there's lots and
lots of stars.
In the southern hemisphere, the center of the Milky Way goes right overhead, and
you get a fabulous view,
especially if you go to one of the places that the great telescopes are located.
In the northern hemisphere, well we get a beautiful view of the universe and
of the nearby stars, but our view of the center of the Milky Way is
not nearly as nice because it never rises very high in the sky.
So if people have a chance to see the Milky Way from the southern hemisphere,
from a nice dark place take advantage of it.
It's a marvelous experience.
[MUSIC]
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