Nucleosynthesis: The Beginning of Elements The Formation of the Light Elements in the Big Bang Theory
Objectives In this lesson, you should be able to give evidence for and explain the formation of the light elements in the big bang theory. What elements elements were produced during during the big bang expansion? expansion?
Last Updated: 08.13.16
Learn about it! Big Bang Theory
The big bang theory The big theory is is a cosmologic cosmological al model stating that the universe started its expansion about 13.8 billion years ago. Pieces of evidence supporting this theory are (1) occurrencee of redshift, (2) background radiation, and (3) abundance of light elements. occurrenc Redshift In the 1910s, Vesto 1910s, Vesto Slipher Slipher and and Carl Wilhelm Wirtz measured Wirtz measured the wavelengths of light from spiral nebulae, which are interstellar clouds of dust and ionized gases. They discovered that the light from the nebulae increased in wavelength. They explained their discovery as a Doppler shift. The Doppler shiftor shift or Doppler effect explains effect explains that when an object gets closer to us, its light waves are compressed into shorter wavelengths (blueshifted, because blue light has the shortest wavelength in the visible region). On the other hand, when an object moves away from us, its light waves are stretched into longer wavelengths (redshifted, (redshifted, because red light has the longest wavelength in the visible region). Slipher and Wirtz then explained that the redshift or increase in wavelength was due to the increase in the distance between the Earth and the nebulae. They concluded that the redshift occurred due to the expansion of space. In 1929, Edwin Hubble used Hubble used the redshift of light from galaxies to calculate the velocities and distances distances of these these galaxies from the the Earth. He discovered discovered that they were were moving away from the Earth and from each other. His calculations supported the theory that the universe is expanding. Cosmic Microwave Background Radiation In 1965, Robert Wilson and Wilson and Arno Arno Penzias Penzias discovered a low, steady “hum” from their Holmdel Horn antenna (an antenna built to support NASA’s Project Echo). They concluded that the noise is Cosmic Microwave Background Radiation (CMBR), Radiation (CMBR), the remains of energy created after the big bang expansion. Abundance of of Light Elements Elements The observed abundance of light elements supports the big bang theory. The theory predicts that the universe is composed of 73% hydrogen and hydrogen and 25% helium by helium by mass.
The prediction correlated to the measured abundances of primordial material in unprocessed gas in some parts of the universe with no stars.
Learn about it! Formation of Light Elements
Big bang nucleosynthesis is the process of producing the light elements during the big bang expansion. In the beginning, the universe was very hot that matter was fully ionized and dissociated. Few seconds after the start of the big bang, the universe was filled with protons, neutrons, electrons, neutrinos, and positrons. After the first three minutes, the universe cooled down to a point where atomic nuclei can form. Protons and neutrons combined to form atomic nuclei such as deuterium.
However, the temperature of the universe was still much greater than the binding energy of deuterium. Binding energy is the energy required to break down a nucleus into its components. Therefore, deuterium easily decayed upon formation.
Learn about it! When the temperature cooled down below 1010 K, deuterium nuclei combined with other nuclei to form heavier ones. Helium-3 was formed from the fusion of two deuterium nuclei and a release of a neutron.
Tritium or hydrogen-3 was produced from the fusion of two deuterium nuclei and a release of a proton.
Helium-4 was also synthesized from deuterium and helium-3.
Helium-4 was produced from the fusion of deuterium and tritium.
Learn about it! For the first three minutes, a substantial amount of neutrons was converted into helium-4 nuclei, before their decay. Helium then combined to other nuclei to form heavier ones such as lithium-7 and beryllium-7.
Lithium-7 was synthesized from helium-4 and tritium.
Beryllium-7 was produced from helium-3 and helium-4.
Among the light elements formed, deuterium, helium-3, helium-4, and lithium-7 were stable. Beryllium-7 was unstable and decayed spontaneously to lithium-7.
What do you think? How are elements heavier than beryllium formed?
Key Points
Pieces of evidence that support the big bang theory are redshift, cosmic microwave background radiation, and abundance of light elements. Big bang nucleosynthesis is the process of light element formation.
The light elements that formed after the big bang were helium, deuterium, and trace amounts of lithium and beryllium. Deuterium, helium-3, helium-4, and lithium-7 were stable. On the other hand, beryllium-7 was unstable and decayed spontaneously to lithium-7.
Nucleosynthesis: The Beginning of Elements The Formation of Heavier Elements during Star Formation and Evolution
Objectives At the end of the lesson, you should be able to give evidence for and describe the formation of heavier elements during star formation and evolution. In the previous lesson, you have learned how the light elements – hydrogen, helium, lithium, and beryllium, were formed during the big bang nucleosynthesis. How were elements heavier than beryllium formed?
Learn about it! Elements heavier than beryllium are formed through stellar nucleosynthesis. Stellar nucleosynthesisis the process by which elements are formed within stars. The abundances of these elements change as the stars evolve. Evolution of Stars
The star formation theory proposes that stars form due to the collapse of the dense regions of a molecular cloud. As the cloud collapses, the fragments contract to form a stellar core called protostar. Due to strong gravitational force, the protostar contracts and its temperature increases. When the core temperature reaches about 10 million K, nuclear reactions begin. The reactions release positrons and neutrinos which increase pressure and stop the contraction. When the contraction stops, the gravitational equilibrium is reached, and the protostar has become a main sequence star. In the core of a main sequence star, hydrogen is fused into helium via the proton-proton chain. When most of the hydrogen in the core is fused into helium, fusion stops, and the pressure in the core decreases. Gravity squeezes the star to a point that helium and hydrogen burning occur. Helium is converted to carbon in the core while hydrogen is converted to helium in the shell surrounding the core. The star has become a red giant.
Learn about it! When the majority of the helium in the core has been converted to carbon, then the rate of fusion decreases. Gravity again squeezes the star. In a low-mass star (with mass less than twice the Sun’s mass), there is not enough mass for a carbon fusion to occur. The star’s fuel is depleted, and over time, the outer material of the star is blown off into space. The only thing that remains is the hot and inert carbon core. The star becomes a white dwarf .
However, the fate of a massive star is different. A massive star has enough mass such that temperature and pressure increase to a point where carbon fusion can occur. The star goes through a series of stages where heavier elements are fused in the core and in the shells around the core. The element oxygen is formed from carbon fusion; neon from oxygen fusion; magnesium from neon fusion: silicon from magnesium fusion; and iron from silicon fusion. The star becomes a multiple-shell red giant.
Learn about it! The fusion of elements continues until iron is formed by silicon fusion. Elements lighter than iron can be fused because when two of these elements combine, they produce a nucleus with a mass lower than the sum of their masses. The missing mass is released as energy. Therefore, the fusion of elements lighter than iron releases energy. However, this does not happen to iron nuclei. Rather than releasing energy, the fusion of two iron nuclei requires an input of energy. Therefore, elements lighter than and including iron can be produced in a massive star, but no elements heavier than iron are produced. When the core can no longer produce energy to resist gravity, the star is doomed. Gravity squeezes the core until the star explodes and releases a large amount of energy. The star explosion is called a supernova. Pieces of Evidence
The discovery of the interstellar medium of gas and dust during the early part of the 20th century provided a crucial piece of evidence to support the star formation theory. Other pieces of evidence come from the study of different stages of formation happening in different areas in space and piecing them together to form a clearer picture. Energy in the form of Infrared Radiation (IR) is detected from different stages of star formation. For instance, astronomers measure the IR released by a protostar and compare it to the IR from a nearby area with zero extinction. Extinction in astronomy means the absorption and scattering of electromagnetic radiation by gases and dust particles between an emitting astronomical object and an observer. The IR measurements are then used to approximate the energy, temperature, and pressure in the protostar.
Try it! Research about the nuclear binding energy and then, explain why the nuclear fusion reactions in massive stars stop in iron through the concept of binding energy.
What do you think? How are elements heavier than iron formed?
Key Points
Stellar nucleosynthesis is the process by which elements are formed within stars. The star formation theory proposes that stars form due to the collapse of the dense regions of a molecular cloud. A protostar is a stellar core formed when the fragments of a collapsed molecular cloud contract. A main sequence star is formed when gravitational equilibrium is reached during the hydrogen fusion in a protostar. A red giant is a star that has used up its hydrogen supply in the core and switched into the thermonuclear fusion of hydrogen in the shell surrounding the core. A massive star becomes a multiple-shell red giant when the elements oxygen, neon, magnesium, silicon, and iron are formed in its core together with carbon, helium, and hydrogen. A supernova is a star that blows apart and releases a large amount of energy. Evidence of star formation comes from studying IR emissions from the different stages of star evolution.
Nucleosynthesis: The Beginning of Elements The Nuclear Fusion Reactions in Stars
Objective At the end of this lesson, you should be able to explain how elements are formed in stars through nuclear fusion. What are the nuclear fusion reactions that happen in the stellar cores?
Last Updated: 08.13.16
Learn about it! Stellar nucleosynthesis is the process by which elements are formed in the cores and shells of the stars through nuclear fusion reactions. Nuclear fusion is a type of reaction that fuses lighter elements to form heavier ones. It requires very high temperatures and pressures. It is the reaction that fuels the stars since stars have very high temperatures and pressures in their cores. Hydrogen is the lightest element and the most abundant in space. Thus, the formation of heavier elements starts with hydrogen. Hydrogen burning is the stellar process that produces energy in the stars. There are two dominant hydrogen burning processes, the proton-proton chain and carbon-nitrogen-oxygen (CNO) cycle.
Learn about it! Proton-Proton Chain
The proton-proton chain is a series of thermonuclear reactions in the stars. It is the main source of energy radiated by the sun and other stars. It happens due to the large
kinetic energies of the protons. If the kinetic energies of the protons are high enough to overcome their electrostatic repulsion, then proton-proton chain proceeds. The sequence proceeds as follows: 1. The chain starts when two protons fuse. When the fused proton breaks, one proton is transmuted into a neutron. 2. The proton and neutron then pairs, forming an isotope of hydrogen called deuterium. 3. Another proton collides with a deuterium forming a helium-3 nucleus and a gamma ray. 4. Finally, two helium-3 nuclei collide, and a helium-4 is created with the release of two protons.
Learn about it! Carbon-Nitrogen-Oxygen (CNO) Cycle
For more massive and hotter stars, the carbon-nitrogen-oxygen cycle is the more favorable route in converting hydrogen to helium. The cycle proceeds as follows: 1. Carbon-12 captures a proton and gives off a gamma ray, producing an unstable nitrogen13. 2. Nitrogen-13 undergoes beta decay to form carbon-13. 3. Carbon-13 captures a proton and releases a gamma ray to become nitrogen-14. 4. Nitrogen-14 then captures another proton and releases a gamma ray to produce oxygen15. 5. Oxygen-15 undergoes beta decay and becomes nitrogen-15. 6. Finally, nitrogen-15 captures a proton and gives off helium (alpha particle) ending the cycle and returning to carbon-12.
Unlike the proton-proton chain, the CNO cycle is a catalytic process. Carbon-12 acts a catalyst for the cycle. It is used in the initial reaction and is regenerated in the final one.
Try it! Look at the periodic table of elements. Which elements are formed during the big bang nucleosynthesis? Which elements are formed during stellar nucleosynthesis?
What do you think? Are there elements found in the outer space that are not present in the earth?
Key Points
Nuclear fusion is a type of reaction that fuses lighter elements to form heavier ones. Hydrogen burning is the stellar process that produces energy in the stars. There are two dominant hydrogen burning processes, the proton-proton chain and carbon-nitrogen-oxygen (CNO) cycle.
Proton-proton chain is a sequence of thermonuclear reactions in the stars. It is the main source of energy radiated by the sun and other stars. Carbon-nitrogen-oxygen cycle is a catalytic cycle of gamma emission and beta decay that converts hydrogen into helium.
Nucleosynthesis: The Beginning of Elements How Elements Heavier than Iron are Formed
Objective At the end of the lesson, you should be able to describe how elements heavier than iron is formed. Nucleosynthesis is the process by which new nuclei are formed from pre-existing or seed nuclei. In the previous lessons, you have learned about the types of nucleosynthesis. The big bang nucleosynthesis produced hydrogen and helium, whereas the stellar nucleosynthesis produced elements up to iron in the core of the stars. If the stellar nucleosynthesis produced only elements up to iron, then what type of nucleosynthesis produced the elements heavier than iron?
Last Updated: 08.13.16
