宇宙之旅是由不断进步的科学发现和人文科学交织组成的，历史、哲学、艺术、宗教统统都含括在内。 这门课由一系列20个与科学家与环境学家关于宇宙之旅的访谈组成。前10个访谈是与科学家和历史学家的访谈，它们帮助我们深化对宇宙、地球、人类进化过程的理解。后10个访问是与环境学家、教师和艺术家的访谈，它们探索了宇宙奥义和人类社会之间的联系。
恒星的光芒（托德•邓肯及乔尔•普里马克）
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From the course by Yale University
宇宙之旅：对话 (Journey of the Universe: Weaving Knowledge and Action)
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From the lesson
宇宙的开端：星系，恒星和太阳系
要有碳和氧就要有恒星，而要有恒星就要有星系，所以想要有一双手先要有整个银河系。了解宇宙的本质是我们进入新时代的第一步。
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]
[SOUND] Come into this old church I want to show you something.
[MUSIC]
The ceiling as you can see is filled with stars.
The same is true of most of the churches on the island of.
The ancient Greeks, like Pythagoras, thought the stars were alive, even divine.
Throughout history every culture has been stunned ny
the presence of stars in the vastness of the night sky.
[MUSIC]
So deeply moved by the majesty emanating from the brilliance of the stars,
we've built our lives around them.
We've even organized the entire civilizations upon their beauty and order.
[MUSIC]
Here's the essence of the universe story.
The stars are our ancestors, out of them, everything comes forth.
[MUSIC]
Stars are dynamic entities, they're born, they develop, they even die.
Star birth occurs when gravity squeezes together a cloud of atoms so
tightly that nuclear fusion ignites in the center.
In the process, hydrogen fuses into helium.
This nuclear energy expands outwards and opposes gravity.
So stars represent an amazingly creative balance between the powers of
gravitational collapse and nuclear explosion.
[MUSIC]
And once a star's nuclear fuel is spent, there's nothing left to prevent
gravity from collapsing inward causing a death spiral to begin.
This super concentrated dot of matter, which we call a supernova,
explodes outward with the power of a hundred billion stars.
And as it expands, it creates all of the elements,
phosphorus, oxygen, carbon, gold.
These are spewed out into the Milky Way Galaxy and
then the whole process start again.
They drift as a cloud and then they collapse, give birth to a star, the Earth.
It's by this stupendous process that we can say, the stars are our ancestors.
[MUSIC]
It's just such an amazing discovery.
The carbon atoms of this beet, and of the lettuce, and of our brains,
our skin, all of it passed through an intense explosion of a star.
[MUSIC]
In pondering the source of the sun's power,
we can now reflect on something no earlier humans could know.
The sun is converting four million tons of its mass into energy every second.
[MUSIC]
All of life feeds on the roaring energy of the sun.
Our solar system then is a self-energizing room of creativity and
all of this had it start in a cloud of dust.,
[MUSIC]
>> It's extraordinary that this is still merging in our understanding, and even
this notion of the universe is expanding, relatively new for humans to absorb.
In the midst of this, stars are forming constantly, and
bursting constantly, enriching the universe as you've said.
Can you give us a feel for that process of star formation, and birth and death.
>> Well, stars form primarily
in big clouds of gas that we call giant molecular clouds.
Molecular, because when the gas gets dense enough and cool enough, hydrogen molecules
form and that turns out to be an essential part of the process of forming a star.
Surprisingly, before a star can form, and
of course become very hot with thermonuclear fusion.
The gas has to become extremely cold, far below 0 on the centigrade scale,
where ice freezes, or below 32 degrees fahrenheit.
Much, much colder than that, that's how it can become very dense.
And then as more and more gas piles on, the center becomes extremely dense,
and as it squeezes down, it also starts to become very hot.
When it becomes hot enough, the fusion starts and the star becomes a star.
It's not just a ball of gas anymore.
And the early stages of star formation and
evolution are quite interesting and rather violent.
There are winds, there are bipolar flows.
The star can eject a fair amount of material along its rotation axis.
We see all these things happening in nearby stars.
And in fact, one of the great things about our modern telescopes is that being able
to use infrared light that doesn't penetrate the atmosphere very well.
We can see end of the clouds where the stars are forming so
we can get good pictures of exactly how it happens.
Something that we're seeing lot of is the formation of the planetary system around
these early stars.
It turns out that is a very common event.
And of course another thing that we're discovering is we are able to track
planets around nearby stars.
We've discovered now over 400 planetary systems outside our own solar system.
And the rate of discovery is itself accelerating.
We're discovering more and more of them all the time.
The new satellite that the United States put up, a Kepler satellite,
has a big list that will soon be revealed.
Of planets that we're seeing around nearby stars.
And these are in many cases fairly small planets, not the great big Jupiter sized
planets, Neptune size or even smaller, maybe even some are Earth size.
So we're learning a lot about the process of star and planet formation.
Now whenever you form a bunch of stars,
the average star is a little less massive than the sun.
Lots of sun size stars form.
Maybe 10% of all the mass of stars formed are quite massive stars.
Stars that are so massive that at the end of their short lives,
they will become supernova.
These stars live only a few million years, they burn extremely brightly.
They're perhaps ten times as massive as the sun but
they're 10,000 times brighter than the sun.
So these starts will become supernovae, they'll have gigantic explosions where all
the heavy elements that they made in their centers are distributed to the universe.
And the very centers become either neutron stars or black holes.
>> So literally this notion, we're descendents of these elements, stardust.
>> Yes.
Unquestionably that's true.
We know for sure that we're made of star dust and
we even know a fair amount about the actual stars that must of
exploded to make the heavy elements that we are made of.
[MUSIC]
>> What about stars though?
They're part of this whole galaxy system.
And how did they emerge?
>> Yeah, so that's basically the same process, just on a different scale.
And that's one of the, I think,
the amazing things about knowing a little bit of physics.
Is that there are these simple laws,
like the law of gravity that apply on all these different scales and
you can see the same types of phenomenon repeated over and over again.
So just like the galaxy is formed on this big scale out of this clumping process,
on a smaller scale you can have a cloud of material that something,
maybe it's going through one of the density structures in the spiral arms or
maybe it's a supernova, an explosion of a dying start that
triggers the little bit of extra pressure and makes it a little bit denser.
Something starts it collapsing, and then the same kind of thing happens,
where you have material starts collapsing, little extra gravity here,
which brings in more material.
As it gets smaller, it starts spinning more just like the ice skater again.
That tends to flatten it out, which means that what you end up doing is you make not
just a star but you make a planetary system that we see.
And like in our own solar system,
around it that forms out of that disk of material that collects.
And that is on another scale,
gravity coalescing things in to planets with the bits of material there.
And then with a star, as it collapses you can think of it as a battle
between pressure which would like to hold it up and blow it apart maybe even,
and gravity just wants to crush it.
because gravity only works one way, it's always attractive.
So what happens is that as gravity continues to work on making the star
shrink, eventually it gets to a place where as you If you think about
compressing a gas like air in bicycle pump or something like that, it heats up.
So, if you get to a temperature of something like 10 million degrees, then
it gets hot enough where a process called nuclear fusion can start to happen, which
is tremendously important for us and our existence and the history of the universe.
And I think the way to understand that is that there's,
let's think of the simplest example which is if you have two protons.
Both have a plus charge, so they don't like to be pushed together.
They repel just like two bar magnets or
something if you try to push the same ends together, and you feel them push apart.
But if they're moving fast enough they can still get really close together, right?
So even if they want to repel, if they're moving fast enough and
they happen to be heading right toward each other, they'll get really close.
And then if they get close enough, which is really, really close,
it's about 10 to the power minus 14 meters.
So that's a decimal point, 13 zeroes and a 1.
When they get that close there is a force that takes over that's called the strong
nuclear force that's very, very, very strong.
Hence its very creative name of the strong force.
And so, if they can make it that close then they'll stick.
So, it's a lot like you can almost think of it like if you're riding a bicycle up
a steep hill.
And if the hill's really steep, you won't make it to the top if you don't have
a lot of speed, which corresponds to the temperature being really high.
And you might not know, unless you have enough speed to get there,
that there's actually a cliff at the end, and
that you keep finding that you roll part way up the hill and you go back down.
You roll a little bit higher when you go a little faster and you roll back down.
Then you go fast enough and you actually go over the edge of the cliff and
end up on the other side and you don't roll back down.
So it's kind of the same way with the strong nuclear force.
The electric force tries to push the protons apart, but
if they get close enough together, they fall of the cliff and they stick together.
And it turns out that there's energy produced by that process that's
released as light and heat.
So if a star gets compressed enough, at that point the star has this energy
coming out that provides pressure that can hold it up.
So there's a balance point.
If you look at any star, it's almost like it's a nuclear explosion
going on that's being contained by gravity.
So there's this delicate balance going on between those two forces.
One trying to blow it apart and one trying to pull it together.
And a star like our sun can live in that state for well,
something like 10 billion years staying roughly in balance and able to support
itself against gravity and shine due to this nuclear fusion that's going on.
>> And provided all the sources of life.
>> Which provides the source of life directly and it also other generations of
stars, this process, when you fuse two protons together and
then build on that, you go from hydrogen you can build helium.
Out of helium, you can build carbon and heavier and heavier elements.
So generation after generation of stars going through this process, in one way or
another, has made all of the elements that fill in our periodic table and
that make up, you know materials in your finger nails and your blood and your skin.
So the only reason we're able to be here,
the only reason the periodic table has anything basically other than hydrogen and
helium in it is because of the stars and that process.
>> Todd, give us this sense of the amazing emergence and
death of stars, and how the elements actually come out of this process.
>> Yeah if you really stop to think,
we should feel a lot of gratitude towards stars, I think, because for one thing,
they go through very much a life cycle just like we do.
They have a time that they're were born, they have an infancy, they have a early
youth, they have a middle age, and they have a death, essentially.
And the basic process is that stars collapse under gravity.
They reach a point where there's a balance between the gravitational
force trying to collapse them, the energy from nuclear fusion as they
turn on element into another helping to support it, so there's a balance.
And then, depending on the type of star,
depending on how much mass it has, at some point it runs out of fuel.
More massive stars run out of fuel more quickly, kind of like rockstars.
Sort of the same deal right?
I mean, they have shorter lives.
So, more massive stars run out of fuel more quickly.
Less massive stars can live a lot longer.
But eventually they run out of fuel.
And if they're a less massive star, what happens is that they create what's
called a planetary nebula, which are these beautiful images you see in Hubble and
elsewhere, of material that has just sort of quietly floated away from the star
while the middle part, the core of the star forms into a white dwarf star.
It's kind of the dying ember of a star.
And those elements are then available to make stars and
other planets and life like us.
If you're a more massive star, you end your life in a supernova explosion,
which more violently sprays all the rest of the elements in the periodic table
out into the medium between the stars.
And that material is than available to be recycled into
what's called the interstellar medium, the material between the stars.
That material can be recycled.
And so, when our sun came along hundreds or
thousands of generations of some kinds of stars had already done this processing.
So when our solar system formed, we formed out of this recycled material and
kind of the ecosystem of our galaxy.
And so, that material is now what makes up the carbon in your skin,
the gold in your jewelry, the iron in your blood.
All those things are literally star dust, either blown off violently or
blown off quietly, depending on the type of star.
But thanks to stars, we actually have all that material that enables life to exist.
>> It's an amazing lineage, isn't it?
>> It is. >> Our ancestry all of a sudden is
extended so vastly into the very elements of ourselves and life on Earth.
>> That's absolutely right.
If you think about tracing your ancestry back to your mother or
father and her mother and father, you can trace it all the way back through,
of course, all the different animals and organisms.
But you can completely trace it back to,
there was the carbon that is in your fingertip, lived in a star somewhere.
That's part of your lineage.
>> Which is why it makes sense in a way, doesn't it, to tell this as a story.
As our story.
>> It is our common cosmic story.
>> Yeah, give us a feel for how you might see that.
The significance of this lineage and story for humans.
>> The way I think of it, is we kind of crave wonder.
We crave a sense that what we're doing is part of a mystery,
part of something deep and meaningful.
Which is sometimes hard to see when you're studying for
a test or filling out income tax forms or whatever.
It's hard to see that as, it feels mundane, right?
It's hard to see that as part of a magical, mysterious origin.
But if you can understand your lineage, then you can really see
every action you do is in a way a frozen creation.
If you can picture in your minds eye the thread that
connects whatever you're doing right at this moment to that lineage and
then ultimately back to the ultimate question of, how did it all begin?
Where did this all come from?
Then every action, the test that you're taking, the driver that you're angry
at that just cut you off in traffic, is part of that cosmic lineage.
And it has sort of a mystery and a wonder to it that I think changes how you look at
life and how you look at other people and how you look at other points of view.
In a way that you don't have if you're very narrow and
you don't see that lineage.
>> I think that's so true and so beautifully put.
And if we extend that lineage, what about the early universe?
And give us a feel for what was emerging, and
again, how we're part of that, even this expansion of the universe.
>> We also owe our existence to the fact that the universe is expanding, and
so we can think of tracing back that lineage as we did through the stars.
The first stars were thought to have formed probably 100 million or a few 100
million years after the beginning of our current universe as we know it.
So there was time before there were stars, and
we can't trace it back to the very beginning, but we can trace it back to
a time when all we know that it was just extremely hot and dense.
And if you had a suit that would protect you from the heat and radiation,
it would be like looking at this glowing uniform soup.
And something started that out and we don't know.
But everything was expanding, which drove everything else that happened.
Because if it hadn't been expanding it wouldn't have cooled down.
So, there's this expansion that allows things to cool.
And as things cool, it's very much like watching ice freeze for example, right?
You put a tray of water in our freezer and it's very smooth in uniform.
As it freezes, little imperfections, little irregularities crystallize out of
the smooth water, and in a way the history of the universe has been that sort of
crystallization of structure out of the that.
>> So in the early universe that you're describing, and
with this wonderful image of ice freezing and so on, what about the factor of time?
And how did that help shape this universe in it's early emergence?
>> One of the interesting things about the timeline is that in the very
early universe things happened very,
very quickly >> So for
example, there was thought to have been a time, a tiny tiny fraction of a second
on the order of ten to the minus 30 seconds, where there was this
rapid period of inflation where there universe expanded very very rapidly.
That created a lot of the seeds for
the density regions that we talked about that form galaxies and ultimately stars.
And then, I guess a really significant event about a few minutes after
the beginning, is when the basic hydrogen and helium elements formed.
So we talked about our cosmic lineage in the stars, elements beyond hydrogen and
helium mostly formed in the stars, hydrogen and
helium are formed imprinted a few minutes after the beginning of the universe.
Another key event, much later on now, about a little less
than 400,000 years after the beginning, was when for the first time it was cool
enough that electrons and protons could stick together.
Before that it was too hot.
So if an electron was stuck to a proton to make a hydrogen atom, it would
fly away because there was too much energy bouncing around and it would stay free.
The reason that was so
significant is that that was the point when the universe became transparent.
So at that point our ability to see across cosmic distances became available.
And so if you've ever seen pictures of the cosmic microwave background,
it's sort of this smooth early region, early phase,
kind of like, I've heard it described as baby pictures of the universe.
You could actually see that smooth light from that time.
Because that was the first time when the universe became transparent so
that light could leave and get to us.
And then, I guess one other thing about all that is that this expansion, that
we just take for granted as the backdrop, is that, if it had been expanding
a little bit faster, it would have, everything would have just blown apart.
And there wouldn't have been time for all these structures to form.
And if it was expanding a little bit slower, gravity would just recollapsed
everything right away and there wouldn't have been a chance for things to form.
There's a really interesting aspect of the universe that
had to be sort of the way it is in order for us to be there.
Gravity had to have a certain strength, expansion rate had to be a certain amount.
The strength of electric force had to be a certain amount.
The speed of light had to be a certain amount.
All these things had to be the way they are in order for
life as we know it to have any chance of ever coming into existence,
which is interesting to think about.
>> But I love, even metaphorically though,
this idea of a tuning of the music of the sphere, so to speak.
I mean, it's a symphony right?
That requires just these right conditions and chords and movement.
It's quite extraordinary, isn't it?
>> It is.
[MUSIC]
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