Side Trip – Nucleosynthesis

(Excerpted from Russell Kempton, his 1998 article Melrose (a), No “Ordinary” Chondrite, published in Meteorite!, Pallasite Press, February 1998, Vol. 4, No. 1.)

Somewhere around 15 billion years ago, the universe was a vast, expanding cloud of hydrogen.

Figure 4.

With a single proton and electron, the hydrogen atom is the simplest atom that can exist. If you take away either of the parts, you no longer have an atom at all.


Figure 5.

Within the cloud, gravitational “ripples” arose, clumps of hydrogen started to collapse under their own force of gravity forming spherical globes.


Figure 6. Color composite image of RCW120 reveals how an expanding bubble of ionized gas is causing the surrounding material to collapse into dense clumps. And this is where new stars form.

The collapse caused compression, producing the enormous amounts of heat needed for nuclear fusing – producing the very first stars.

Deep in the interiors of these first generation stars, protons and electrons collided forming deuterons, the nuclei of hydrogen isotopes. They, in turn, collided forming the element helium.



Figure 7.

As the stars aged and their cores heated up, a successive stream of heavier elements were synthesized: carbon, nitrogen, oxygen, neon, and others all the way up to iron (1 to 26).



Figure 8.

 

But Gold is not synthesized. It takes a different type of star to move past Iron.



Figure 9.

The largest members of this first generation of stars burned so furiously, consuming their hydrogen nuclear fuel, that they burned themselves out in around only 20 million years, finally exploding as supernovae, seeding the interstellar medium with their mix of heavier elements.



Figure 10. Eta Carinae - stellar mass being ejected.

Nature recycles everything, and the second generation of stars incorporated this mixture of elements during formation. The fusion process and life cycle of these now, lightly, metal-enriched stars is a little different.


Figure 11. The structure of a massive star with an iron-rich core, silicon, oxygen,  and carbon burning shells surrounding the core. There is no fusion reaction involving iron that produces energy (it has the strongest binding energy in its nucleus of any element, thus there is no way to add protons and get energy out).

At the end of their life, a massive star of this generation is layered similar to an onion, beginning with an outer layer of hydrogen encasing successively thinner layers of helium, carbon, oxygen, on to silicon, along with trace proportions of other elements.

At the center is an iron core of growing mass, as silicon burning in the layer immediately outside it creates additional iron. The core's density, at this point, is somewhere around 3000 to 12,000 tons per cubic centimeter and the pull of gravity is immense.

Unfortunately, Iron does not "burn" to create even heavier elements. So, the dynamic balance between the outward-directed pressure caused by heat release and the inward-directed gravitational pressure is lost....

And, gravity wins – the inner core implodes in less than a second, collapsing to an incredibly dense solid sphere generating temperatures of over a billion degrees Kelvin. The still-collapsing portion of the outer core collides with this surface and immediately rebounds outward at high speed.



Figure 12. Within a massive, evolved star here are the steps of its collapse:

a The onion-layered shells of elements undergo fusion, forming an iron core
b The iron core eventually starts to collapse. The inner part of the core is compressed into neutrons
c This causes infalling material to bounce
d This forms an outward-propagating shock front (red). The shock starts to stall
e But it is re-invigorated by a process that may include neutrino interaction. The surrounding material is blasted away
f What's left is only a degenerate remnant.

The shock wave generated in this collision does two things:

1. It rips the star apart not only at the physical level,
2. But also at the atomic level – Stripping the electrons out of their orbits (which requires massive amounts of energy) and creates high energy radiation and particles called neutrinos.

As the shock wave moves up through the layers, gases fuse forming more elements, all the way up to uranium.


Figure 13.

Finally, this supernova explodes, adding its metal-rich mix of elements to the interstellar medium as seeds for the construction of new solar systems!


Figure 14.

And Gold is sprayed among the stars to be added to the next mix of planets, planetoids, and, of course, asteroids and meteorites.

Side Trip – Nebular Hypothesis

Here's an illustration that shows the nebular hypothesis. It does not, however, show the first and second generation stars that came before ours. It starts with the early formation of our solar system, seeded with material from earlier stars. When we learn about other meteorites, this view will evolve:


Figure 15.

The Nebular hypothesis
A. solar nebula
B. contraction into rotating disk
C. Cooling causing condensing into tiny (dust sized) solid particles
D. Collisions between these form larger bodies
E. These accrete to form planets

Side Trip – Hydrogen Burning

The process a star takes to create new elements may take several forms, but commonly involves the fusion of light atomic nuclei to form heavier nuclei.

As a by-product of these reactions, energy is released (the life-sustaining heat and light we receive from the Sun), and various other particles are produced (gamma rays, electrons, neutrons, and even hydrogen or helium nuclei).

Hydrogen (H) may burn to produce helium (He, with two protons) but only at temperatures that exceed 10 million K (See Figure 15).

Helium burning can produce carbon (C, atomic weight 12), which can combine with other helium nuclei to produce oxygen (O, atomic weight 16).

Similarly these burned products, sometimes called "ashes," may react during carbon burning to produce oxygen, neon (Ne), sodium (Na), and magnesium (Mg); during neon burning to produce oxygen and magnesium; during oxygen burning, to produce the element magnesium through sulfur (S); and during silicon (Si) burning to produce elements up to iron (Fe, atomic weight 56).

Notice that this is the very process discussed in second generation stars and shown above.

This figure shows how Hydrogen is burned.

It's the proton-proton chain, the important energy-producing nuclear reaction that takes place inside stars like our Sun.

In this process two hydrogen nuclei (protons in red) combine to form a deuterium nucleus (D, hydrogen with an atomic weight of 2--red and gray) and a positron (electron with a positive charge--white).

Neutrons are gray; protons are red. Reaction with an additional hydrogen nucleus (red) produces a helium nucleus with a mass of three (3He).

The fusion of two such nuclei results in the production of stable helium (4He, which has two protons and two neutrons) and the ejection of two hydrogen nuclei (red), which can be consumed in other reactions of this sort.

Energy is released by these reactions as a small amount of matter is converted into energy in accordance with Einstein's equation,

E (energy) = m (mass converted to energy) c2 (the speed of light squared).


Figure 16.