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Nucleosynthesis in Stars: Our Bodies Made From Celestial Stardust

Hydrogen and helium were the only two elements produced in the first few minutes of the “Big Bang.” That is, when our universe was first formed, it was exclusive of all elements heavier than helium(i.e. elements like carbon, oxygen, sulfur and iron). How then were these elements, necessary for biological life formed?

Astrophysicists have determined that the heavier elements were formed from nuclear reactions going on in the interior of stars. Heavier elements are formed from lighter ones by a process called fusion. At sufficiently high temperatures ordinarily achievable only in the interior of hot stars four hydrogen atoms may fuse to form a helium atom. A helium atom is slightly less massive than the four hydrogen atoms from which it was formed. The difference in mass is emitted as energy which reaches us as heat and light of the sun.

When a star about the size of our sun exhausts its supply of hydrogen to feed into nuclear fusion reaction producing helium in its hot core the helium core begins to contract due to disturbance of hydrostatic equilibrium, which is the balance between gravitational force tending to compress the core and the force exerted by the pressure of hot gas which tends to make the star expand. The compression of the helium core of a star when its hydrogen is exhausted is due to the gravitational force of its mass which overwhelms internal pressure of hot gas when the reactions supplying internal heat cease.

But gravitational compression of the core in turn gradually builds up heat energy which finally causes the outer layer of the star to expand into what is called a red giant. The temperature at the core of a red giant may rise sufficiently from gravitational collapse to initiate helium fusion in the core of the star. When helium fuses into carbon, three helium atoms combine.

In massive stars, the mass in the outer layers may be sufficient to further compress the inner core after all helium has been used up to form carbon. The force of gravitational compression may generate enough heat to ignite the fusion reactions producing even heavier elements like oxygen, neon and silicon. A sufficiently heavy star may by gravitational collapse generate enough core heat to form all heavy atoms up to iron.

Nucleosynthesis must cease with the production of iron in all stars because the fusion reactions which produced atomic nuclei lighter than iron released energy. But the iron nuclei is so tightly bound that energy is required to fuse iron. Heavier atomic nuclei than iron are formed only in supernova explosions of massive stars. A supernova explosion occurs when the iron core of a massive star uses up its fuel and the mass of the outer layer of the star accumulates to beyond the Chandrasekhar limit of 1.4 times the mass of the sun. At below the Chandrasekhar limit the star is kept from collapse by the force associated with Pauli’s exclusion principle which states that two particles of the same kind cannot have the same position and momentum. A star core that is kept from further gravitational collapse only by Pauli’s exclusion principle is said to be a degenerate core. A degenerate core will, however, begin to collapse once the nuclear reactions at the outer layers of the core accrete the core to a mass over 1.4 times that of our sun. The collapse occurs very rapidly and halts only when the density of the core reaches the density of an atomic nucleus. The shock of the rebound of the sudden halt of rapid collapse of a degenerate core generates sufficient force to cause an explosive burst of the outer layers in what is called a supernova explosion.

In a supernova explosion, the massive star spews its heavy element rich material into surrounding space. This material in turn is the material from which new generations of stars are formed by gravitational collapse and explains why younger stars(population I stars) tend to be richer in heavy elements than older stars(population II stars).

Our sun is a population I star. Our solar system is therefore, relatively rich in heavier elements. The outer layers of the sun have been examined spectroscopically and shown to be relatively heavy element rich. Our earth, in turn, was made from heavy element materials derived from the sun. Thus contrary to Aristotle’s belief that heavenly bodies are of different nature from the earthly, our earth and life on it was made from stardust.

Astrophysicists have determined that the heavier elements were formed from nuclear reactions going on in the interior of stars. Heavier elements are formed from lighter ones by a process called fusion. At sufficiently high temperatures ordinarily achievable only in the interior of hot stars four hydrogen atoms may fuse to form a helium atom. A helium atom is slightly less massive than the four hydrogen atoms from which it was formed. The difference in mass is emitted as energy which reaches us as heat and light of the sun.

When a star about the size of our sun exhausts its supply of hydrogen to feed into nuclear fusion reaction producing helium in its hot core the helium core begins to contract due to disturbance of hydrostatic equilibrium, which is the balance between gravitational force tending to compress the core and the force exerted by the pressure of hot gas which tends to make the star expand. The compression of the helium core of a star when its hydrogen is exhausted is due to the gravitational force of its mass which overwhelms internal pressure of hot gas when the reactions supplying internal heat cease.

But gravitational compression of the core in turn gradually builds up heat energy which finally causes the outer layer of the star to expand into what is called a red giant. The temperature at the core of a red giant may rise sufficiently from gravitational collapse to initiate helium fusion in the core of the star. When helium fuses into carbon, three helium atoms combine.

In massive stars, the mass in the outer layers may be sufficient to further compress the inner core after all helium has been used up to form carbon. The force of gravitational compression may generate enough heat to ignite the fusion reactions producing even heavier elements like oxygen, neon and silicon. A sufficiently heavy star may by gravitational collapse generate enough core heat to form all heavy atoms up to iron.

Nucleosynthesis must cease with the production of iron in all stars because the fusion reactions which produced atomic nuclei lighter than iron released energy. But the iron nuclei is so tightly bound that energy is required to fuse iron. Heavier atomic nuclei than iron are formed only in supernova explosions of massive stars. A supernova explosion occurs when the iron core of a massive star uses up its fuel and the mass of the outer layer of the star accumulates to beyond the Chandrasekhar limit of 1.4 times the mass of the sun. At below the Chandrasekhar limit the star is kept from collapse by the force associated with Pauli’s exclusion principle which states that two particles of the same kind cannot have the same position and momentum. A star core that is kept from further gravitational collapse only by Pauli’s exclusion principle is said to be a degenerate core. A degenerate core will, however, begin to collapse once the nuclear reactions at the outer layers of the core accrete the core to a mass over 1.4 times that of our sun. The collapse occurs very rapidly and halts only when the density of the core reaches the density of an atomic nucleus. The shock of the rebound of the sudden halt of rapid collapse of a degenerate core generates sufficient force to cause an explosive burst of the outer layers in what is called a supernova explosion.

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