Where do elements come from?

1920's

Astronomers studied the spectroscopic signals of light from the stars and came to the conclusion that most stars are actually made out of Hydrogen (~75%) and helium (~25%), and do not contain the types of heavier elements that we see here on Earth.

At around the same time, Edwin Hubble and Milton Humason (using a 100 inch diameter telescope at Mount Wilson, CA) studied the wavelengths of light from distant stars and came to the conclusion that the universe is expanding in all directions (due to "doppler shift" in characteristic spectra). In calculating the rate of expansion, and working backwards, they also concluded that:

• The universe is about 15 billion years old
• It appears to have started as some super-dense, super-hot bubble of space-time that contained all the energy and mass of the present universe. This subsequently expanded (the expansion was not an expansion of matter into empty space, but an expansion of space itself) and is known as the "Big Bang" theory of the Universe.

The early universe was so hot that no atoms existed - electrons would have far too much kinetic energy to remain with the nucleus of atoms. It is postulated to have been so hot, that even subatomic particles like electrons and protons would not have been able to form from constituent particles like quarks and gluons. Thus, in the early moments of the Big Bang, the Universe is postulated to have been composed of a type of soup of quarks and gluons.

• As the universe expanded and cooled, electrons and protons could form
• As the universe expanded and cooled further, electrons could be bound by protons and atoms of hydrogen could form
• Theories and models describe how simple atoms (hydrogen, deuterium, helium, lithium) can form in the early universe, but the problem is that the expansion and cooling of the universe prevents the atomic fusion necessary to form heavier elements. In other words, by the time things have cooled down to the point where helium atoms can form, there is not enough energy for any significant amount of hydrogen or helium atoms to fuse to form heavier elements.
• Modern calculations predict that not even a single atom of oxygen or nitrogen or any other heavier elements could have been produced in the big bang. Where did these elements come from?

Atomic fusion is the process of forcing two atoms together to the point that their nuclei fuse to form a new, heavier, element

• It takes a lot of energy to force two atoms together in this way due to electron-electron repulsion
• However, the arrangement of protons and neutrons in the resulting heavier element may be more energetically favorable than in the individual lighter elements. This may offset the energy cost of forcing the two atoms together. Thus, while it takes a lot of energy to achieve, nuclear fusion in some cases is, overall, energetically favorable.
• Fusion and changes within the nuclei can result in the conversion of a small amount of matter into energy. The amount of energy is substantial, due to Einstein's relationship between energy and matter: e = mc². As a frame of reference, the atomic blast in Hiroshima (equivalent to about 15,000 tons of TNT) was the result of about 1kg of mass being converted into energy (via a nuclear fission reaction).

Studies of nuclear fusion indicate a net energy gain for the fusion of the lighter elements, but only up to a point:

• Fusion of Hydrogen, Helium, Carbon, Oxygen, and up to Iron all occur with a net energy gain (i.e. the process goes downhill in energetic terms)
• Fusion of Nickel, Cobalt, Uranium, and other similar heavy elements actually requires more energy than it releases. Thermodynamically, the formation of the heavier elements would not be predicted to be spontaneous
• There was no known process by which the appropriate energy needed for fusion reactions involving the production of elements heavier than iron could be achieved

The distribution of hydrogen and helium in the universe

• The average distribution of hydrogen and helium atoms in space is actually only a few atoms per cubic meter
• However, the distribution of hydrogen and helium is not homogenous, and there are regions of higher or lower density. Over time, these regions of higher density can collapse and coalesce due to gravitational attraction
• Although the distribution of hydrogen atoms in space is quite sparse, the universe is huge, and subsequently the masses of collapsed clouds of hydrogen can constitute a tremendous amount of matter

The mass of a collapsing cloud of interstellar hydrogen can be so massive that the gravitational forces at the center are great enough to cause hydrogen atoms to undergo fusion. This initiation of fusion is sometimes called "ignition":

• This does two things:

1. It forms helium from hydrogen
2. It produces a tremendous amount of energy

• The energy released provides an outward force to the atoms in the cloud, this counteracts the inward force of gravity, and an equilibrium is achieved

We now have a reasonably stable object that emits a lot of energy and is fusing hydrogen to helium - this is a "Sun"

• There are about 100 billion suns in our galaxy, and about 100 billion galaxies in the universe. This means there are about 1 x 10²² suns (i.e. stars) in the universe.
• This number is about 10 times the number of grains of sand in all the worlds beaches and deserts. This estimate for the number of stars also does not take into account light from stars that has not yet reached us (i.e. from the other side of the universe). The actual number of stars in the universe could be much larger.
• These suns are "factories" that produce helium from hydrogen

A few facts about our own sun:

• It appears to be a typical sun, more massive than the average, maybe in the middle of its lifetime
• Only one hydrogen atom out of every ten billion trillion collides with another with enough energy and the correct angle to result in a fusion event. Even in the heart of the sun it is an extremely rare event
• However, there is so much mass in the sun that 616 million tons of hydrogen are fusing each second, and 5 million tons of this mass converting directly into energy (e = mc² = 2 x 10²⁴ kJ of energy liberated by fusion reactions in the sun each second)
• The sun is so massive, that even after 4.5 billion years of "burning" hydrogen at this rate, only 4% of the suns mass has been converted to energy.

Initial fusion of hydrogen produces helium and enriches the center of the star with helium

• Helium can enter into fusion reactions with hydrogen and other subatomic particles to produce heavier elements
• These are rare events, but suns "live" for a long time (millions to billions of years)

What happens over time when the hydrogen begins to be used up in the center of the star?

• Energy released by fusion of hydrogen decreases and the star begins to collapse due to gravitational forces (which previously were at equilibrium with outward forces of energy released by hydrogen fusion)
• The helium-rich core collapses, resulting in energy release (due to gravitational forces), and the helium begins to undergo fusion (star starts "burning" helium). The release of energy from helium fusion has two consequences as seen before:

1. A new equilibrium is reached between inward gravitational forces and outward pressure due to released energy of nuclear fusion
2. Heavier elements are produced

• The conversion from hydrogen to helium fusion is not gentle
• Collapse can release latent gravitational energy. Also, it can allow outer hydrogen to fall inward, and this can be rapidly fused in the now more dense nucleus
• The outer part of the sun expands rapidly (forming a "Red Giant") and can eject a significant portion of the outer mass of the sun into the interstellar medium

Thus, a sun like our starts out primarily composed of hydrogen with some helium, but it can be thought of as a factory for the production and dissemination of more massive atoms:

• A star like our sun will fuse hydrogen to helium, and helium to heavier atoms
• Some Carbon, Oxygen and Nitrogen will be formed
• Trace amounts of elements as heavy as iron may be formed (but nothing more massive)
• Transitions in the fusion reactions can result in ejection of some of this material (but only ~15%, and it will be mostly hydrogen and helium with a sprinkling of C, N, O ) into neighboring space (these are called "planetary nebula")
• Suns with a mass between 1-4 times the mass of our own sun will behave like this

Stars with about 18 times the mass of our own sun behave a little differently...

• The mass is great enough to result in collapsed cores that can promote a significant amount of fusion of heavier elements up to and including iron:

• More massive stars, due to gravitational forces, fuse their atoms more rapidly (and therefore have shorter lifespans than less massive stars)
• Each successive type of fusion proceeds more rapidly (because it yields less energy and is therefore "used up" faster)
• H fusion - a few million years
• He fusion - a million years
• C fusion - 12,000 years
• Ne, O fusion - 10 years
• Si fusion - a few days!

For millions of years, fusion energy opposed the forces of gravity, preventing the giant star's collapse. However, once the star gets to fusing iron there is a real problem:

• Iron fusion does not result in a net release of energy
• Once the fusion process gets to iron, gravity wins and the core of the massive star collapses upon itself
• A star about 18 times more massive than our own will collapse in this process to a size about 10 kilometers in diameter (about the size of Manhattan) in a matter of seconds
• It collapses rapidly to where the atomic particles cannot be compressed any further.
• This is equivalent to an abrupt stop like hitting a brick wall
• This almost unimaginable collapse and abrupt stop results in a rebounding shock wave outward at about 15% the speed of light
• This shock wave also includes the release of latent gravitational energy during the collapse
• Energy is released in the form of gamma rays, which in turn produce subatomic particles(according to e = mc²); >10⁵⁷ neutrinos for example screaming outward at nearly the speed of light
• At the same time the shock wave is going outward, the outer "onion ring" layer of material is falling inward due to the initial collapse.

The outward shockwave, and neutrinos, have two consequences for the incoming layer of material:

• Fusion reactions occur, that produce heavier elements (copper, uranium, silver, mercury, lead, etc.) than were possible in normal stellar core reactions
• All of this incoming material, and newly synthesized heavier elements, are blasted out into space

The energy associated with this collapse is simply unimaginable

• The total energy released represents a significant fraction of the mass of the sun (e = mc²) - a significant amount of mass of the sun is converted into energy according to Einstein's equation. This is due to the consequences of the latent gravitational energy released during the collapse.
• For a brief moment, the energy released in the visible spectrum alone makes the resulting implosion/explosion brighter than the combined stars in an entire galaxy (one star shines brighter than the other 100 billion stars combined)

 Supernova in the MilkyWay galaxy (our galaxy)

• Tycho Brahe saw one in 1572
• Johannes Kepler saw one in 1604
• Haven't seen once in the Milky Way since. A very rare event.
• One was observed in 1987 in the Large Magellanic cloud (a small companion galaxy to the MilkyWay)

There is another type of supernova called type I

• Although it is brighter in the visible spectrum, it releases let energy than a type II supernova
• It releases iron and lighter elements, but not the heavier elements, into the surrounding interstellar space
• It involves a binary star system, when one star is "normal" and the other is a dense neutron star (probably the remnant of a collapsed core of a smaller sun)

The two stars revolve around each other, with material (hydrogen) streaming from the "normal" star to the neutron star (due to the dense gravitational field of the neutron star)

• A layer of Hydrogen and Helium builds up on the surface of the dense neutron star
• Once the layer builds up to about a couple of kilometers thick, it will suddenly all "ignite" in a fusion reaction due to the gravitational force of the neutron star
• This will produce some lighter elements (possibly even up to Fe, but no higher)
• These elements are subsequently blasted out into space from the energy of the ignition - which is also visible as a visible flash brighter than a type II supernova
• This explosion does not usually effect the binary star system, and so after a while the neutron star starts sucking more hydrogen from the nearby "normal" star.
• The process repeats itself once the critical mass of Hydrogen has accumulated on the surface

The early universe appears to have contained primarily hydrogen and helium, maybe a tiny bit of Lithium and no heavier elements

• Heavier elements have subsequently been synthesized inside of stars, in a nuclear chemistry known as "Nucelosynthesis"
• Since hydrogen atoms do not contain neutrons, the early universe also had far fewer neutrons than at present. Thus, nucleosynthesis also has has altered the number of subatomic particles in the universe
• Mass and energy freely interconvert according to Einstein's equation in the process of Nucleosynthesis

The vast majority of stars are only large enough to synthesize elements such as Carbon, Oxygen and Nitrogen.

• Living systems are composed primarily of these elements - all the carbon, oxygen and nitrogen in the molecules in your body were synthesized by some star somewhere in the distant past.
• DNA contains primarily C, H, O, N and P
• Proteins contain primarily C, H, O, N and S

Since we find heavy metals on Earth, it suggests that prior to formation of our solar system and sun, there was a previous very large sun nearby that ended life as a type II supernova

• The gold and silver jewelry you wear was made in a type II supernova

Very large stars, that can give rise to type II supernova, have very short lives due to gravitationally related rapid fusion reactions

• They don't exist in "old" star regions in space, but rather in areas of new star formation. The Orion nebula is an example of where the next supernova might come from.

Many scientists assumed that the expansion of the universe would slow due to the effects of gravitational attraction. However, recently (over the last decade) it has become clear that the expansion of the universe is not slowing. In fact, it is accelerating. The reason for this is an utter mystery. However, it has been confirmed that the universe is accelerating its expansion and there is no known physical force to explain it. Thus, physicists have concluded that they have discovered a previously unknown force of nature that is a repulsive force acting over great distances. Few people are actually aware of this monumental discovery. Physicists are working to try to discover the details of this new physical force.