LC1_Origin and Formation of Elements in the Universe PDF

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Km 7 Central Park Blvd, Talomo, 8016 Davao City, Philippines Tel No. +63 (82) 221.2411 local 8608 E-Mail: [email protected] * www.addu.edu.ph In Consortium with Ateneo de Zamboanga University and Xavier University SENIOR HIGH SCHOOL - CHEMISTRY AND PHYSICAL SCIENCE Subject: Physical Science Year level: Grade 12 Semester: First Lecture Note #: 1 Topic: Origin and Formation of Elements in the Universe Introduction When the universe began with a big bang, it started out with no elements at all. Many of the elements that make up Earth and the people on it had to be created in the nuclear furnaces inside stars and were only released once the star reached the end of its life. In fact, only light elements, like hydrogen and helium, were created at the start of the universe. The given picture below shows the timeline of the expansion of the universe and the evolution of the chemical elements in the universe right after the big bang. The light elements were created on the far left of this diagram at the beginning of the universe and became neutral atoms at around 380,000 years after the big bang. (Oakes,2011). Credit: NASA/WMAP Science Team In the Beginning What we're calling the beginning is the universe when it had a temperature of 100,000,000,000 K. The universe had already existed for a very small fraction of a second and was dominated by radiation. There was some matter present, but it was infinitesimal compared to the amount of radiation. The radiation was in the form of photons, neutrinos (and antineutrinos). The matter present was in the form AdDU-SHS | Physical Science | 1 of electrons, positrons, and a very small concentration of protons and neutrons (about 1 part per billion). As a result of the extremely high temperature and density, all these items acted like particles. This means that they were constantly colliding into each other, just like a sample of tightly packed marbles in a container. In the early universe there were no physical "walls" to contain these objects, but there were so many collisions happening so quickly that the collisions themselves acted like the walls of the universe. However, these walls were not static. With the collisions, the size of the universe was increasing. This expansion caused the density of energy to decrease as it was spread out over a larger volume. This resulted in a decrease in the temperature of the universe. This process continues to happen today. These collisions had three major results. The first was that the universe reached a condition called thermal equilibrium. The other two consequences of these collisions involve interactions between particles as they collided. The first interaction to be considered was the constant annihilation and re-creation of electrons and positrons. One of the most famous scientific discoveries of this century is the equivalence of matter and energy. The basic concept is that under the proper conditions, energy can be turned into matter, or vice versa. This is not something common to our experience because of the conditions in which we now live (it's too cold and there's not enough pressure). But in the early universe, with its high temperature and density, this was common. Photons were converted into electrons and positrons. (Known as PAIR PRODUCTION) They could not be converted into heavier particles (protons and neutrons) because they didn't have enough energy. These electrons and positrons would eventually collide with their respective anti-particle, and then be changed back into radiation. (Referred to as ANNIHILATION) The second interaction was the conversion between protons and neutrons. These heavier atomic particles were already present In the Beginning. They were continually changing back and forth by means of the following two reactions: Up to this time (just over three minutes past the Beginning) there had been no nucleosynthesis. This was a result of the high energy density. In order to form atomic nuclei, the nucleons (the scientific word for protons and neutrons) must be able to collide and stick together. In the early universe the key reaction was the collision of a proton and a neutron to form a deuterium nucleus (an isotope of AdDU-SHS | Physical Science | 2 hydrogen). Collisions between protons and neutrons had been happening continuously since the Beginning, but their energies were too high to allow them to stick together to form deuterium nuclei. This prevented further nuclear reactions leading to heavier nuclei. This type of situation where an intermediate product is the weak link in the overall synthesis is sometimes called a "bottleneck." This concept also applies in nucleosynthesis of heavier elements. Once the bottleneck is overcome, the remaining reactions can be completed. In the early universe, once the deuterium bottleneck was cleared, the newly formed deuterium could undergo further nuclear reactions to form Helium. This could happen by means of two different reaction pathways described as follows. Pathway #1 The deuterium nucleus collides with a proton to form He-3, then a neutron to form He- 4. Pathway #2 The deuterium collides first with a neutron to form H-3 (more commonly called tritium), then with a proton to form He-4. AdDU-SHS | Physical Science | 3  He nuclei were the heaviest to form. This was the result of the energy density being too low to allow heavier nuclei to collide with enough energy to stick. At the time that nucleosynthesis began, the relative abundance of protons to neutrons was 13% neutrons and 87% protons. When nucleosynthesis began, all the neutrons present were incorporated into He nuclei. When all the neutrons were used up, the remaining protons remained as hydrogen nuclei. So, when this first wave of nucleosynthesis was completed, the universe consisted of roughly 25% He and 75% H (by weight). Cosmic Origin of the Elements in the Unvierse Below is a table of the various processes in the universe that create elements. Also listed are the combined abundances (relative to hydrogen) of that particular groups of elements. Process Predominant elements Combined Abundance Big Bang H, He 1.1 x 1012 Small Stars C, N, selected elements 4.5 x 108 Between Nb and Bi Large Stars O and various elements 9.7 x 108 up through Zr Supernovae F and elements above Ti 3.4 x 107 above Fe Cosmic Rays Li, Be, B 6.4 x 102 The elements have their ultimate origins in cosmic events. Further, different elements come from a variety of different events. So, the elements that make up life itself reflect a variety of events that take place in the Universe. The hydrogen found in water and hydrocarbons was formed in the moments after the Big Bang. Carbon, the basis for all terrestrial life, was formed in small stars. Elements of lower abundance in living organisms but essential to our biology, such as calcium and iron, were formed in large stars. Heavier elements important to our environment, such as gold, were formed in the explosive power of supernovae. And light elements used in our technology were formed via cosmic rays. The solar nebula, from which our solar system was formed, was seeded with these elements, and they were present at the Earth’s formation. Our very existence is connected to these elements, and to their cosmic origin. (Lochner,2005) As the Universe continued to expand and cool, the atoms formed in the Big Bang coalesced into large clouds of gas. These clouds were the only matter in the Universe for millions of years before AdDU-SHS | Physical Science | 4 the planets and stars formed. Then, about 200 million years after the Big Bang, the first stars began to shine, and the creation of new elements began. With the birth of stars, new sources of light and energy emerged all over the Universe. They burned hydrogen to create helium. Helium was used to create carbon. Neon, oxygen, silicon, and iron were also created during the lives of stars. However, once these stars started running out of fuel is when things really got interesting. It’s in the massive explosions that resulted from certain stars running out of fuel that all the elements of the periodic table were created. Without the death of stars, our world would not exist today. (Khan Academy) Small stars Stars less than about eight times the mass of our Sun are considered medium and small size stars. The production of elements in stars in this range is similar, and these stars share a similar fate. They begin by fusing hydrogen into helium in their cores. This process continues for billions of years, until there is no longer enough hydrogen in the star’s core to fuse more helium. Without the energy from fusion, there is nothing to counteract the force of gravity, and the star begins to collapse inward. This causes an increase in temperature and pressure. Due to this collapse, the hydrogen in the star’s middle layers becomes hot enough to fuse. The hydrogen begins to fuse into helium in a “shell” around the star’s core. The heat from this reaction “puffs up” the star’s outer layers, making the star expand far beyond its previous size. This expansion cools the outer layers, turning them red. At this point the star is a red giant. Motion of the gas between these shells and the core dredges up carbon from the core. The helium shell is also replenished as the result of fusion in the hydrogen shell. This occasionally leads to explosive fusion in the helium shell. During these events, the outermost layers of the star are blown off, and a strong stellar wind develops. This ultimately leads to the formation of a planetary nebula. The nebula may contain up to 10% of the star’s mass. Both the nebula and the wind disperse into space some of the elements created by the star. While the star is an Asymptotic Giant, heavier elements can form in the helium burning shell. They are produced by a process called neutron capture. Neutron capture occurs when a free neutron collides with an atomic nucleus and sticks. If this makes the nucleus unstable, the neutron will decay into a proton and an electron, thus producing a different element with a new atomic number. In the helium fusion layer of Asymptotic Giants, this process takes place over thousands of years. The interaction of the helium with the carbon in this layer releases neutrons at just the right rate. These neutrons interact with heavy elements that have been present in the star since its birth. So over time, a single iron (Fe) nucleus might capture one of these neutrons, becoming Fe. A thousand years later, it 56 might 26 capture another. If the iron nucleus captures enough neutrons to become Fe, it would be 59 unstable. One neutron 26 would then decay into a proton and an electron, creating an atom of 27Co, which is higher than iron on the periodic table. During this Asymptotic Giant phase, conditions are right for small stars to contribute in this way to the abundance of selected elements from niobium to bismuth. After the Asymptotic Giant phase, the outer shell of the star is blown off and the star becomes white dwarf. A white dwarf is a very small, hot star, with a density so high that a teaspoon of its material would weigh a ton on Earth! If the white dwarf star is part of a binary star system (two stars orbiting around each other), gas from its companion star may be “pulled off” and fall onto the white dwarf. If AdDU-SHS | Physical Science | 5 matter accumulates rapidly on the white dwarf, the high temperature and intense gravity of the white dwarf cause the new gas to fuse in a sudden explosion called a nova. A nova explosion may temporarily make the white dwarf appear up to 10,000 times brighter. The fusion in a nova also creates new elements, dispersing more helium, carbon, oxygen, some nitrogen, and neon. Large Stars Stars larger than 8 times the mass of our Sun begin their lives the same way smaller stars do: by fusing hydrogen into helium. However, a large star burns hotter and faster, fusing all the hydrogen in its core to helium in less than 1 billion years. The star then becomes a red supergiant, similar to a red giant, only larger. Unlike red giants, these red super giants have enough mass to create greater gravitational pressure, and therefore higher core temperatures. They fuse helium into carbon, carbon and helium into oxygen, and two carbon atoms into magnesium. Through a combination of such processes, successively heavier elements, up to iron, are formed (see Table 1). Each successive process requires a higher temperature (up to 3.3 billion kelvins) and lasts for a shorter amount of time (as short as a few days). The structure of a red supergiant becomes like an onion (see Figure 3), with different elements being fused at different temperatures in layers around the core. Convection brings the elements near the star’s surface, where the strong stellar winds disperse them into space. Fuel Main Product Secondary Temperature Duration (years) Products (billion kelvins) H He N 0.03 1 x 107 He C, O Ne 0.2 1 x 106 C Ne, Mg Na 0.8 1 x 103 Ne O, Mg Al, P 1.5 0.1 O Si, S Cl, Ar, K, Ca 2.0 2 Si Fe Ti, V, Cr, Mn, 3.3 0.01 Co, Ni Table 1 – This table shows the nucleosynthesis reactions that occur in successive stages in large stars. The table summarizes the chief reactions and their products (including other elements that are produced in trace amounts), the temperature at which the reaction occurs, and how long it takes to use up the available input fuel. Another reason fusion does not go beyond iron is that the temperatures necessary become so high that the nuclei “melt” before they can fuse. That is, the thermal energy due to the high temperature breaks silicon nuclei into separate helium nuclei. These helium nuclei then combine with elements such as chlorine, argon, potassium, and calcium to make elements from titanium through iron. Large stars also produce elements heavier than iron via neutron capture. Because of higher temperatures in large stars, the neutrons are supplied from the interaction of helium with neon. This neutron capture process takes place over thousands of years. The abundances of selected elements from iron to zirconium can be attributed to this type of production in large stars. Again, convection and stellar winds help disperse these elements. Supernovae AdDU-SHS | Physical Science | 6  As red super giants, they fuse many elements, finally producing iron in their cores. Iron is the end of the line for fusion. Thus, when the core begins to fill with iron, the energy production decreases. With the drop-in energy, there is no longer enough energy to counteract the pull of gravity. The star begins to collapse. The collapse causes a rise in the core temperature to over 100 billion Kelvin and smashes iron’s electrons and protons together to form neutrons. Because of their smaller size and lack of electric charge, the neutrons can pack much closer together than atoms, and for about 1 second they fall very fast toward the center of the star. After the fall, they smash into each other and stop suddenly. This sudden stop causes the neutrons to violently recoil. As a result, an explosive shock wave travels out from the core. As it travels from the core, the shock wave heats the surrounding layers. In addition, neutrinos (elementary particles with very little mass) arise from the formation of the neutrons. The energy from the neutrinos causes the majority of the star’s mass to be blown off into space, in what is called a supernova. Astronomers refer to this as a Type II supernova. Supernovae often release enough energy that they shine brighter than an entire galaxy, for a brief amount of time. The explosion scatters elements made within the star far out into space. Supernovae are one of the important ways these elements are dispersed into the Universe. As it moves through the original onion layers of the star, the shock wave also modifies the composition of the layers (particularly the C, Ne, O, and Si layers), through explosive nucleosynthesis. This contributes to the production of elements from Si to Ni, with many elements below iron being made from the Si shell. The tremendous force of the supernova explosion also violently smashes material together in the outer layers of the star before it is driven off into space. Before it is expelled, this material is heated to incredible temperatures by the power of the supernova explosion and undergoes a rapid capture of neutrons. This rapid neutron capture transforms elements into heavy isotopes, which decay into heavy elements. In seconds or less, many new elements heavier than iron are created. Some of the elements produced through this process are the same as those made in the star, while others come solely from the supernova process. Among the elements made only from supernovae explosions are iodine, xenon, gold, platinum and most of the naturally occurring radioactive elements. Cosmic Rays From the “Binding Energy Per Nucleon” chart (Figure 4) we see that moving from 1 4 4 hydrogen (H) to helium (He) creates a more stable nucleus but moving from helium (He) to 7 lithium (Li) does not create a stable nucleus. In fact, the next element with a more stable 12 nucleus than helium is carbon (C). The nuclei between helium and carbon are much less stable, and thus they are rarely produced in stars. So, what is the origin of lithium, beryllium, and boron? As it turns out, some of the heavy elements, including lithium, beryllium and boron, are produced from cosmic ray interactions. Cosmic rays are high-energy particles traveling throughout our galaxy at close to the speed of light. They can consist of everything from tiny electrons to nuclei of any element in the periodic table. Scientists first observed evidence of high-speed particles entering our atmosphere in 1929, although their exact nature was unknown. They were initially dubbed “cosmic rays” due to their mysterious origin, since scientists believed these particles were high energy photons from outer space. AdDU-SHS | Physical Science | 7  When cosmic rays hit atoms, they produce new elements. During its journey across the galaxy, a cosmic ray may hit an atom of hydrogen or helium in interstellar space. Since the cosmic ray is traveling so fast, it will hit with great force, and part of its nucleus can be “chipped off.” For example, the nucleus of a carbon atom in the outer layers of a large star may be accelerated to near light speed when the star explodes as a supernova. The carbon nucleus (which we now call a cosmic ray) flies through space at a high speed. Eventually, it collides with a hydrogen atom in open space. The collision fragments the carbon nucleus, which creates two new particles: helium and lithium. This same process can happen for all elements. Since lithium, beryllium, and boron are small atoms, they are more likely to be formed in cosmic ray collisions. The Periodic Table of Elements coded according to the predominant processes which produce the elements. AdDU-SHS | Physical Science | 8