Earlier I said there are about 100 natural elements.
If asked to divide them into 2 groups, how would you do it?
The traditional answer is "metals and non-metals."
It's better to group elements as light or heavy if you think about their origin.
Light elements like "H" and "He" as well as trace amounts of "Li" (atomic no. 3) were formed at the beginning of the universe,
while heavier elements starting with carbon were formed in stars.
It took decades to learn that some elements were created in the Big Bang while others were created in stars.
The origin of elements was initially a thorny problem.
Elements were considered to have either been made in stars
or in the early universe. Opinion went back and forth.
To learn more about the origin of elements, let's return to Penzias' lecture.
He says,
"The full scientific understanding of the origin of the elements requires a description of their build-up from their common component parts (e.g., protons and neutrons) under conditions known to exist, or to have existed, in some accessible place..."
What is necessary for a "full scientific understanding" of rhe origin of elements?
We need to know the common components of atoms,
for example, protons and neutrons.
Also we have to know the process whereby protons and neutrons form atoms
as well as the conditions conducive to formation of different elements,
conditions "known to exist" somewhere
like inside the core of the sun,
or conditions known "to have existed" in the early universe.
We need to find a place with these conditions.
We cannot just say that such conditions probably were available somewhere unimaginable.
We should be able to identify some "accessible place" where conditions were right for the formation of elements.
In order for this to make sense.
A key process in the early universe is fusion of light nuclear particles into heavier nuclei.
For this to happen, the temperature of the universe has to decrease.
Protons and neutrons had already been formed less than 1 second after the Big Bang.
For these to fuse into helium, a proton and neutron must first join to become deuterium.
"p" and "n" stand for proton and neutron, respectively.
Their combination forms deuterium, and when another neutron is added, tritium is formed.
Tritium has only one proton.
A single proton is hydrogen, whether or not it is combined with a neutron.
If another proton is added to tritium, helium is formed.
For fusion to occur, these particles must lose some energy.
At extremely high temperatures, the energetic particles will separate before they collide and fuse.
That's why temperature has to drop.
The universe must expand, so that it will cool down.
This is the nuclear reaction forming helium in the early universe.
We need to understand how similar processes led to the formation of heavier elements.
"Once plausible build-up processes were identified and the conditions they required were determined, the search for appropriate sites for the nuclear reactions followed..."
Several decades ago, nuclear physicists were busy searching for "build-up processes" for heavier elements.
Once the processes were "identified", they needed to find the requisite conditions.
After the conditions, what do you think came next?
They searched the universe for places with these conditions,
i.e. appropriate sites where nuclear reactions could occur.
These efforts were made over several decades.
"Although this search was begun in earnest in the nineteen thirties,"
The 30's are referred to because fusion could not happen without the neutron,
which was discovered in 1932.
Before then, it was hard to comprehend fusion.
Research into the origin of elements began in the 30's and, "...it was toward the end of the 60's that the full outlines of a satisfactory theoretical framework emerged."
By the late 60's a theoretical understanding of the origin of all elements emerged.
The work begun in the 30's was recognized in 1967 by the awarding of the Nobel Prize in physics to Hans Bethe.
Around 1938 he revealed the process by which 'H' became 'He', and 'He' became 'C' through fusion in the sun.
This was a highly significant discovery that was remarkable for its time.
About 30 years later, in 1967, Bethe received the Nobel Prize for this work.
Although research into elemental origins started in the 30's it waffled about until a proper framework was established in the 60's.
The 60's were significant because in 1965 Penzias & Wilson discovered Cosmic Microwave Background Radiation (CMBR). By the end of the decade, the Big Bang theory was accepted,
which led to the conclusion that hydrogen and helium were formed in the Big Bang universe.
"In the broad outlines of the relevant scientific thought during this period one can discern an ebb and flow between two views..."
"This period" is the 30's to 60's.
During a period of 30 to 40 years, a broad picture about nuclear fusion and elemental origins was put forth.
There was an "ebb and flow" between two ideas.
"ebb" refers to tide going out while "flow" refers to tide coming in.
Majority opinion shifted between two ideas about elemental origin.
Penzias explains these points one by one:
"In the first...the elements were thought to have been made in the stars of our galaxy"
"...and thrust back out into space..."
When stars explode elements are ejected into space,
"...to provide the raw material for, among other things, new suns, planets and the rock beneath our feet."
In space, these materials are recycled, becoming new stars.
In this idea, all elements are created inside stars, ejected by supernovae, and form new stars
and planets like the Earth or Saturn.
These elements also form the ground and rock we walk on and the soil we use for farming.
Soil is vital.
All elements including hydrogen.
In the first view, all elements including hydrogen were made in stars.
Elemental formation in stars was considered before the Big Bang, because the latter was little-known in the 1930s.
"In the second view...
a hot soup of nuclear particles was supposed to have been cooked into the existing elements before the stars were formed..."
Before the stars were formed…
What does that mean?
There was a time when there were no stars in the universe.
Obviously, because stars need raw material.
The stars were formed from a "hot soup of nuclear particles"
such as protons and neutrons. Protons and neutrons are not the ultimate particles, but they were made of even more fundamental particles called quarks.
The "hot soup" of quarks was unimaginably hot, 100 billion degrees.
In comparison, the soup that we eat is at most 50 degrees Centigrade.
At about 10 billion degrees, this hot soup of particles was "cooked into" something good and useful,
namely, the elements that exist today, forming the basis for stars.
This second idea is totally different from the first.
Take a look at the history of the universe at a glance. In the beginning there was the Big Bang, a huge explosion.
At that time, various fundamental particles coexisted.
After the temperature dropped as time passed, quarks clumped together to form protons and neutrons. Within 3 minutes, "He" was formed, while much later other elements were made inside stars.
Initially much effort was spent to unify these two explanations of elemental origins.
Referring to the particle soup, Penzias said, "This pre-stellar state was generally associated with an early hot condensed stage of the expanding universe..."
"Expanding universe" appears for the first time.
The time before stars is called the "pre-stellar state." What was it like?
In general, this state was related to the early, expanding universe.
In its early stage, the universe was small and hot
because energy was concentrated in a tiny space.
The universe had a high density, so the distance between particles was small.
Elements were first formed in the hot soup of the early, condensed universe.
From this viewpoint, the early universe gave birth to hydrogen, carbon, oxygen, etc. up through uranium.
All elements were created in the early universe.
This is different from our current viewpoint.
Referring to these two viewpoints, Penzias says, "Historically...
...the first quantitative formulations of element build-up were attempted in the nineteen thirties..."
The "build-up" of heavy elements from lighter ones could not just be stated in words,
but had to be quantitatively described.
But "...they were found to require conditions then thought to be unavailable in stars..."
It might work for light elements, but not for heavy elements like uranium, with atomic number 92.
One half of uranium is a nucleus with 46 protons.
Two nuclei with 46 protons each will repel each other because they both have positive charges.
What is 46 times 46?
The repulsion of 46 x 46 is massively greater than 1 x 1 between two protons.
That's why fusing uranium is difficult.
Energy greater than this repulsive energy is needed.
In other words, the energy barrier must be overcome
in order to get to the next state.
Let me use the Himalayas for illustration.
It's not easy for hydrogen to overcome the energy barrier and become helium.
Assume that we went hiking in the Himalayas.
We came to the first mountain
and barely made it to the top. Now what?
At the top you see even higher mountains blocking your way. This is the situation for building up elements.
Helium was created with great difficulty. When helium attempts to fuse into carbon, the repulsive force is 2×2×2.
With heavier nuclei, it could be something like 30×30, and so on.
It is a pitiful situation
because huge energies are required. It is questionable whether stars have zones with such high temperatures.
After much deliberation, scientists concluded that it wasn't possible.
They tried to explain synthesis of all elements in stars, but realized that the energy barrier couldn't be overcome in stars.
What did they come up with next?
"As a consequence, attention turned in the 1940's to consideration of a pre-stellar state as the site of element formation."
In the 40's, scientists thought that stars just wouldn't do.
They began looking for a higher temperature state that existed before stars.
In the 40's, the Big Bang universe began to attract attention.
In the 30's, many doubted the Big Bang theory. Hubble's law was announced in 1929 as we will study in chapter 8.
During the 30's, scientists gradually accepted the Big Bang, and by the 40's many found it quite plausible.
So it was thought that at extremely high temperatures in the early universe, not in stars, elements could be formed.
However, "This effort was not successful in achieving its stated goal..." What was the "stated goal"?
Scientists' goal was not something abstract.
It was stated verbally and written down.
What was the goal in this case?
It was to explain the process of how about 100 elements were created in the early universe.
Did it work or not?
Things didn't turn out so well.
Why not?
The early universe didn't last long enough, and as it quickly expanded the temperature plummeted.
And in an expanding universe, the separation between particles increased.
Particles had time to collide and fuse into "He", but this becomes harder for "C", "Fe", all the way up to "U".
Scientists realized that this was not the answer, either.
"...and in the 1950's interest again turned to element formation in stars..."
The discarded viewpoint from the 30's was again espoused.
What led to the revival of this viewpoint?
The following development in the 50's was responsible:
"By then..." (the 1950's) "the existence of a wide range of stellar conditions which had been excluded in earlier views had become accepted."
What's happening?
In the 1930's attempts to explain elemental origins in stars failed.
The problem concerned the feasibility of attaining tremendous stellar temperatures.
Temperature and density must be very high.
But scientists revisiting this question found that it might be possible
and began to reconsider the stars.
Fred Hoyle played an important role here.
Hoyle was a professor of astronomy at the University of Cambridge in England.
He was one of the greatest astronomers of his time, and he realized something important in the 50's. What is the historical significance of that era?
It was just after WW II.
During WW II, which ended In 1945, wartime research such as the atomic bomb project had proceeded in secret, but eventually information was revealed to the public.
Hoyle studied the wartime experiments from nuclear physics
and realized that he could explain the evolution of stars
through the secret of the nuclear bomb.
There is something called "implosion".
While an "explosion" is pressure directed outward,
an "implosion" is pressure directed inward.
In 1945, two atomic bombs were dropped on Japan.
One of the bombs required uranium to be at a very high temperature and incredible density in order to explode.
At the critical density, particles would collide and trigger an explosion.
When uranium is extracted from ore and purified, its density is low.
The uranium core was surrounded by ordinary bombs,
dozens of explosive charges like dynamite, designed to detonate simultaneously.
Triggering a synchronized explosion is no small feat.
What happens when all the charges explode at once?
Some explosive pressure will go outward, but it will also go inward.
The uranium's density will skyrocket due to the inward pressure.
Scientists succeeded in creating a nuclear bomb, didn't they?
They knew that it worked.
The fusion of hydrogen into helium in stars cannot increase the temperature high enough to make heavier elements.
At a certain stage of a star, fusion of hydrogen into helium can take place somewhat outside the core. I will explain this later.
When fusion occurs outside the core of a star, immense inward pressure is exerted, greatly raising the temperature and density at the core, enabling the creation of heavier elements.
Although it is a little hard to understand, this is a very important development.
Anyway this is why scientists revived the idea that all elements were created in stars.
They went back and forth.
Stars in the 30's, early universe in the 40's, and again stars in the 50's.
Did this theory win out?
With immense temperatures, "Fe" could be created in stellar cores, but what about uranium?
It would be impossible.
Stellar origin of all elements fell into doubt, but finally in the 1960's there was a synthesis of ideas:
"Finally the 1960’s saw a reawakened interest in the idea of a pre-stellar state at the same time that decisive observational support was given to the 'Big Bang' universe by the discovery of cosmic microwave background radiation and its identification as the relic radiation of the initial fireball."
The sentence is long, so let's break it up.
Cosmic Microwave Background Radiation (CMBR) is referred to.
Penzias received the Nobel Prize for this discovery.
It was discovered in 1965, and in the 60's interest in the Big Bang theory "reawakened".
The idea of the "pre-stellar state" from the 40's returned while "at the same time...
decisive observational support" came from observatio, not theory.
Additional evidence was found.
Evidence for what?
Observational data supported the Big Bang.
The data were provided "by the discovery of cosmic...radiation’.
CMBR was evidence of the Big Bang.
"Identification" of CMBR as leftovers of the Big Bang's "initial fireball" was very significant.
The early universe was like a fireball.
"Relic radiation" means leftover radiation
which still exists as a low energy background. This was discovered by Penzias and Wilson.
This discovery was consistent with the Big Bang theory, making it more credible. So was the early universe theory valid?
It was touch and go before the discovery of CMBR sealed the deal;
now we know what happened in the early universe, how light elements were formed, and how heavier elements were later created.
The conundrum of the origin of elements was now answered.
We discussed the outline using Penzias' lecture.
Now we will break it down and discuss how light elements were formed in the early Big Bang universe
and how heavier elements were formed in stars.
We will study these two topics separately.