Cosmic Origin of Elements PDF

Cosmic Origin of Elements 1. The Big Bang The existence of all matter is believed to have started with the birth of the universe. The most widely accepted explanation of the origin of the universe is the Big Bang theory.  1. The Big Bang The evidence of the big bang theory was...

Cosmic Origin of Elements 1. The Big Bang The existence of all matter is believed to have started with the birth of the universe. The most widely accepted explanation of the origin of the universe is the Big Bang theory.  1. The Big Bang The evidence of the big bang theory was first expressed in the early 1900s, when Edwin Hubble offered an explanation that the universe is expanding. He observed that many stars and galaxies shine with light shifted toward the red end of the visible spectrum (redshift), This developed into the Hubble law, which suggests that the size of the redshift is proportional to the distance and speed of the star moving away from Earth. With Hubble's data, cosmologists have traced the expansion back to a time when the entire universe was smaller than an atom.  1. The Big Bang The Big Bang theory postulates that approximately 14 billion years ago, sphere about one-centimeter diameter experienced a huge explosion, spreading its products as a fast-moving cloud of gas. The event was accompanied by an emission of a huge amount of light. > Within the first second after the explosion, subatomic particles such as protons, neutrons, and electrons were formed. > As the expanding universe cooled, the protons and neutrons started to fuse (combine) to form heavier nuclei of deuterium (an isotope of hydrogen with one neutron and one proton) and some into helium.  1. The Big Bang Isotopes are atoms that have the same number of protons but different numbers of neutrons.  1. The Big Bang Subsequent nuclear fusion reactions, in which two atomic nuclei join to form a new type of nuclei, resulted in the production of other light elements and their isotopes. Astronomers believe that a few minutes after the Big Bang, the universe was composed of approximately 75%, (by mass) hydrogen, 25% helium (primarily H), and trace amounts of lithium. The process through which these light elements formed are called the big bang nucleosynthesis as depicted in the reactions that follow.  1. The Big Bang Observe that two isotopes of helium (He-3 and He-4) have been formed from these reactions. H-3, also known as tritium, is also an isotope of hydrogen formed from big bang nucleosynthesis. 2. Stellar Formation and Evolution The universe continuously expanded for several years, and the cloud of hydrogen and helium gases condensed to form stars, including the sun. Over millions of years, the stars made of hydrogen became hotter and denser. During this stellar evolution, nuclear reactions continued, which produced elements heavier than lithium. The light elements combined to form atoms of carbon, neon, oxygen, silicon, and iron. The stars are described to have an "onion skin structure" as they evolve and produce new elements. 2. Stellar Formation and Evolution 2. Stellar Explosion A younger yellow star made up of hydrogen is fueled by the energy released from the fusion of hydrogen nuclei to form helium. The outer layer of the star is composed of burning hydrogen from the nuclear fusions which produce helium. Once enough helium-4 is produced, these nuclei become concentrated at the core of the star, thus making the temperature hotter at the core. Hydrogen fusion continues, but in a "shell" surrounding the helium core.  2. Stellar Explosion Hydrogen fusion continues, but in a "shell" surrounding the helium core.  When the core reaches the temperature enough for helium fusion to occur, helium burning begins. The outer temperature then becomes colder than the core, which causes the star to become red. From this fusion, beryllium-8 is formed. Another ielium-4 nucleus fuses with beryllium-8 to form carbon-12. These reactions happen in the helium fusion shell, beneath the hydrogen fusion shell.  When the core reaches the temperature enough for helium fusion to occur, helium burning begins. The outer temperature then becomes colder than the core, which causes the star to become red. From this fusion, beryllium-8 is formed. Another helium-4 nucleus fuses with beryllium-8 to form carbon- 12. These reactions happen in the helium fusion shell, beneath the hydrogen fusion shell. The carbon nuclei produced become more concentrated at the center, as helium was earlier. This produces a carbon core, that when it reaches a certain temperature to allow carbon fusion, it produces neon within the carbon fusion shell. Nuclear reactions occurring in this shell include the following:  Lastly, the fusion of silicon-28 produces radioactive nickel-56, which will then decay to iron. More nuclear fusions happen between different nuclei to form the other elements. However, the production of elements stops when iron is formed. Since, iron is the most stable nuclei, it is unable to undergo fusion. In all of the previous reactions, a great amount of energy is produced, enough to fuel more nuclear reactions; however, inorder to produce elements heavier than iron, energy input is necessary. At this point, the star has already exhausted its nuclear fuel. 2. Stellar Formation and Evolution  3. Stellar Explosion As the red giant star exhausted the nuclear fuel of light elements, its core started to collapse and eventually led to the explosion of the star. This violent explosion called supernova released huge amount of nuclear energy and produced, through neutron capture and radioactive decay, other elements heavier than iron. Neutron capture can be as fast as a fraction of a second or as slow as a few million years. This process- occurs as a seed nucleus captures neutrons, forming a heavier isotope of the element that can either be stable or radioactive. Stable isotopes can continue to capture neutrons and form other heavier isotopes of the seed nuclei. Unstable or radioactive isotopes, however, will undergo beta decay, producing an isotope of a new element. 3. Stellar Explosion For seed nuclei with relatively few neutrons (from iron to bismuth), neutron capture occurs so slowly that beta decay of the product isotope happens before it can capture another neutron. This is referred to as the slow process or s-process. An example of this process is the formation of copper (Cu) and zinc (Zn)nuclei (Ni) from nickel nucleus. 3. Stellar Explosion However, a series of neutron capture may occur very fast that the seed nucleus turns into a relatively heavier nucleus before beta decay takes place. Such process is referred to as rapid process or r-process. This process is exemplified in the formation of cobalt (Co) from iron (Fe). 3. Stellar Explosion Different isotopes and much heavier elements are formed during the neutron capture-and-decay process. All these elements, along with the fragments of the star during supernova, are released into the vast space and gradually condensed to form the different planets like Earth, new stars, and other heavenly bodies.