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6. Nucleosynthesis
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Astronomia: Explorando oTempo e Espaço
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Star Birth and Death
Stars are the crucibles of heavy element creation, and the chaotic regions of their birth are being understood though long wavelength observations. Gravity is the ultimate victor in the life story of any star, leaving behind the exotic end states of white dwarfs, neutron stars, and black holes.
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Chris ImpeyDistinguished Professor
Astronomy
Let's talk about cosmic chemistry.
The business of stars is nucleus synthesis, the fusion of different atomic
nuclei under conditions of enormous temperature and
pressure to produce new elements.
Essentially, everything in the material world, except for hydrogen and helium,
which pri, primordial, and the Big Bang, were created in this way.
[SOUND] Welcome back to the star cook.
[APPLAUSE].
>> Thank you.
Thank you.
Welcome back.
I'm Michael and I'm here with my very special guest, Mr. Bill Nye!
>> Thank you.
Thank you everyone.
Michael, it's so good to be back on the show and you are looking fabulous!
>> Thank you.
You too.
You too.
>> [APPLAUSE] Oh thank you, Michael.
>> So tell me, Bill, what are we going to do today?
>> We have here the ingredients to make virtually anything.
>> Ooh. [APPLAUSE]
>> Oh, these are different kinds of atoms.
>> That's right.
These are all the different elements,
all the different kinds of atoms in the universe.
Now atoms come from stars.
Now.
Take for example our planet.
Our planet has a lot of iron and silicon, along with many other elements.
And our star has a lot of hydrogen and helium.
[APPLAUSE] Now, Michael.
Oh, that's our hydrogen.
>> [LAUGH] >> Almost lost it there.
[APPLAUSE] It's okay everybody, it's okay.
[LAUGH] Now Michaela, you and I are 65% water.
>> Right.
>> Now, what's water?
>> H2O! >> [APPLAUSE] That's right, H2O.
I have some water here, which I've prepared earlier.
>> [LAUGH] >> Of course.
>> [LAUGH] And the water is two parts H.
And one part O.
H2O!
Water!
>> So with this here, we can make virtually anything.
>> Well, actually no, because in order to make all these atoms,
you need the force of exploding stars and billions of years.
But brownies, on the other hand, [LAUGH] only take about [LAUGH] 35 minutes.
>> And even brownies are made of atoms.
>> That's right.
>> Oh, Bill this looks fabulous.
People, it looks fabulous doesn't it?
[APPLAUSE] >> Uh-huh, oh thank you, thank you.
Here Michaela, you please have the first one.
>> Oh, thank you.
Oh, delicious.
Mm.
We'll be right back.
>> Thank you.
>> Now people say, you know, why make observations of the stars?
Why look into space?
Are you kidding me?
It gives you a perspective of where you are in the universe,
of what we do all day.
What are you trying to make some kind of joke?
I mean we are made.
Off the same material [SOUND] as stars.
Stars, [SOUND] you, me,
the [SOUND] camera, the [SOUND] television [SOUND] you're [SOUND] watching,
everything made of the same stuff that [SOUND] stars are made of.
So you gotta, you have to [SOUND] [SOUND] study stars in order [SOUND] to understand
what we're [SOUND] made of, for crying out loud.
[SOUND].
>> Whoa.
[MUSIC]
Oh.
>> Low mass stars, like the Sun, and a little more massive than the Sun,
can go one step beyond helium to make carbon.
This is a critical step in the fusion process,
because of course biology would not be possible without carbon.
And it requires two extraordinary tricks,
coincidences of nuclear physics, if you like.
You might imagine, as with fusion of helium from hydrogen,
that you could do this in two simple steps.
By taking a helium nucleus, fusing it with another one to make beryllium, and
then adding a third to make a carbon nucleus.
But it doesn't work this way because beryllium, for
reasons of nuclear physics, has an incredibly short decay time.
It's radioactive and disintegrates into trillionth of a second or less.
That means that inside a star, when two fo, helium nuclei fuse,.
The result, the beryllium nucleus disappears almost as quickly as it's
assembled, disappearing like sand through your fingers.
And the star cannot make carbon that way.
The way the star makes carbon is by a much rarer triple collision
which instantly creates carbon from three helium nuclei arriving in the same place
at the same time.
It turns out that this process would be vanishingly rare,
leading to essentially no carbon in the universe if it weren't for
a nuclear resonance state, first noticed by Willie Fowler 60 or more years ago.
This nuclear resonance state increases the probability that the triple fusion will
take place and means that we actually have carbon in the universe.
But the scarcity of the triple collision, nonetheless, means that carbon is hundreds
of times rarer than hydrogen and helium, the most abundant elements.
In general, as we move up the fusion chain, higher and higher temperatures and
pressures and densities are required to generate heavier and heavier elements.
To make hydrogen into helium requires overcoming the repulsion
of two hydrogen nuclei protons.
But, to make carbon from helium nuclei means we have to force
helium nuclei with double the electrical charge and therefore,
four times the repulsion to get close together.
Requiring correspondingly higher temperatures.
Carbon is a critical ingredient in this story.
And as we know, and in this video from Linda Williams tells us,
carbon is a girl's best friend.
[MUSIC]
>> If we consider the full evolutionary path of a low mass star like the sun, and
this of course includes the story of our sun, it goes through several phases.
The long main sequence phase ten billion years long for the sun.
Then the star reconfigures to a new state, starting a new energy source,
releasing a large envelope, and having a collapsing core.
This is the red giant phase.
In our sun when this happens, the outer envelope will expand past the orbit of
the earth, perhaps to Mars, engulfing the biosphere and frying it.
If life still exists on earth billions of years from now,
this will probably be the end point.
Thereafter, the sun goes through an unstable phase, ejecting
much of its mass into space as a planetary nebula, while the remaining core shrinks.
With no new fusion process possible, the core gradually cools as an amber.
Forever.
This final state for the Sun is a White Dwarf.
Since most stars are like the Sun or
even lower in mass, this is the fate of most stars.
So, most of the stellar remnants in the universe are expected to be White Dwarfs.
If we look at this picture.
Such as taken with the hubble space telescope of a planetary nebula.
We can see both parts of this fade.
Which awaits the sun.
The sloughing off of layers of gas out into space where they're heated up and
glow brightly and at the center, a cooling ember, the white dwarf.
Stars more massive the the Sun are able to ascend the fusion chain.
They are massive enough to create ever higher temperatures, pressures, and
densities.
Such that fusion can occur between more massive nuclei
with larger positive charges.
Fusion proceeds in stages.
For example, a helium nucleus can fuse with carbon to make oxygen.
Another helium nucleus can be added to make neon.
And yet another to make magnesium.
And these fusions can occur in different combinations as well.
Notice that because helium is the second most abundant element,
fusion can occur increasing the atomic number by one.
Such as when a hydrogen nucleus or a single proton is added but
also, by addition of two protons in a helium nucleus.
That's what gives the saw tooth pattern in the periodic table abundance curve.
It's caused by the fact that most of the fusion is of units of one or two and
the extra amount of additions of two means that even atomic numbers
are slightly more abundant than odd atomic numbers in the periodic table.
In the sun, at a low degree, and in other stars more massive than the sun,
at a higher degree.
There's a second fusion process that creates helium form hydrogen
called the CNO cycle.
In this case, carbon, nitrogen, and oxygen already present from pre-primordial as
the star formed, catalyze the reactions that lead hydrogen to helium.
This second energy process is more complicated, but,
it contributes significantly to the creation of helium in low mass stars.
The most advanced nuclear reactions to create the heaviest stable elements
require phenomenal temperatures.
For example co, oxygen could combine with carbon to make silicon.
And oxygen could combine with itself to make sulfur, and
two silicon nuclei combine to make iron.
This requires temperature of billions of Kelvins, perhaps five or
ten Kelvins to get up to the iron.
Only the very highest mass stars, which are intrinsically rare,
can create high enough core temperatures for the heaviest stable elements to form.
The result of this sequential evolution in a high mass star is an extraordinary
physical situation where the star has an onion skin structure.
It's not quite as pristine as this simple cartoon form
because there's some mixing between the layers.
But basically moving outwards we still have unfused hydrogen in the very center.
Followed by helium, followed by the heavier elements, carbon and so
on, up the periodic table,
until the very hottest center part of the sun contains an iron core.
This is an extraordinary form of iron in a high massive evolved star.
It's denser than water.
It has a temperature of billions of Kelvin and yet it's a gas, a plasma.
I said the heaviest stable elements because the fusion process only works to
get you up to iron.
In the binding energy per nucleon curve, there's a stability trough at iron.
And this means that in nature, you can get energy out of fusion and
going towards iron from lighter elements.
But you can get energy out of the fission of heavier nuclei moving towards iron.
Iron is the most stable element.
What this means in terms of a star's life is iron is the endpoint.
No more free energy is available by fusing beyond iron, so it creates a logjam.
Or a slag heap at the center of the star ending in iron.
Extra energy is required to go beyond iron.
Summarizing the stages of evolution of an extreme star like a 25 solar mass star,
we see that these different fusion stages involve a crescendo of time scales.
The carbon fusion of the massive star takes only 600 years.
The neon burning just a year.
Silicone forms in a day, and
the final collapsed state of the star takes less than a second.
In the blast wave that moves out at relativistic speeds from the collapsed
star, elements are synthesized all the way out to the transuranic
elements at the top of the periodic table.
In temperatures that instantaneously reach 10 billion Kelvin.
With this piece of the story added for
high mass stars producing heavy elements, we've explained most of
the characteristics of the periodic table chemical abundance.
The sawtooth pattern.
The fact that there's a third peak at iron, the most stable element.
And the subsequent falloff because of the difficulty of
creating even heavier elements requiring such high temperatures.
Summarizing cosmic chemistry, we have primordial hydrogen and
helium, the most abundant elements in the universe,
with everything else taken together just a trace element.
We have carbon, nitrogen, and
oxygen, the carbon produced in relatively no, low-mass stars.
And these are the life elements present at a few parts in a thousand
relative to hydrogen and helium.
We have the creation of the elements up to iron in massive stars,
in a flurry of activity at very high temperatures.
And then we have elements beyond iron produced in supernova explosions.
But also in a slow manner.
In practice, there are two methods for creating heavy elements called the R and
the S process.
The R process, or Rapid,
occurs in supernovae where the slow process is the gradual diffusion of
neutrons into an atomic nucleus to increase its atomic number by one.
And, this can occur in the envelopes and cores of massive stars.
The final features we can explain are the sawtooth pattern caused by the addition
of helium nuclei in the fusion as well as hydrogen nuclei, and
then the deep trough at the light elements caused by the unstable beryllium nucleus,
representing a bottleneck which leads to the low abundance of carbon
relative to hydrogen and helium.
Perhaps the final insight in cosmic chemistry
is that we have an erroneous way of thinking of stars.
It's inevitable to think of them as light bulbs in the sky.
But that's not actually what's going on.
Because we know that the efficiency of mass energy conversion is actually
quite low.
Less than a percent.
We should instead think of the stars as chemical factories.
It's as if you had a car factory somewhere in Michigan and you flew over it at night.
You might see light leaking out form the windows.
If you had an infrared camera, you'd see heat leaking out as well.
And you might imagine that the factory was really making heat and light.
Of course, it isn't.
The factory is making cars.
That's what it's built for.
Stars are making heavy elements by fusion, and
the light we see from them is simply a byproduct.
In that sense, stars are chemical factories, and
since they make the biogenic elements, the universe is built for life.
Since this process has been going on for over 13 billions years,
since the Big Bang and the first stars that formed,.
The universe has steadily been increasing in its abundance
of cosmic elements beyond hydrogen and helium and becoming more ripe for life.
Stars do naturally what alchemists have only dreamt of,
making heavy elements, including lead, and including gold.
Low-mass stars can make carbon, the biogenic element, an essential for life.
Higher mass stars make heavier elements and
these elements are produced into an onion skin layer where the highest densities and
highest temperatures can create the heaviest elements.
Up to iron, the most stable element.
Energy is needed to go beyond iron.
And often, this occurs in a supernova explosion.
The titanic blast wave resulting from the death of a massive star
can forge the heaviest elements up to uranium
instantaneously at temperatures of billions of degrees.
This process, cosmic nucleosynthesis, has been going on for 13 billion years,
since the first stars formed in the infant universe.
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