Week 1 Dana Desonie, Ph.D. Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) To access a customizable version of this book, as well as other interactive content, visit www.ck12.org CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-source, collaborative, and web-based compilation model, CK-12 pioneers and promotes the creation and distribution of high-quality, adaptive online textbooks that can be mixed, modified and printed (i.e., the FlexBook® textbooks). Copyright © 2016 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®” and “FlexBook Platform®” (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (CC BY-NC 3.0) License (http://creativecommons.org/ licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/about/ terms-of-use. Printed: August 15, 2016 AUTHOR Dana Desonie, Ph.D. SOURCE http://www.haystack.mit.edu/ www.ck12.org Chapter 1. Week 1 C HAPTER 1 Week 1 C HAPTER O UTLINE 1.1 Formation of the Universe - The Big Bang Theory 1.2 Star Formation 1.3 Where did all the elements come from? 1.4 References 1 1.1. Formation of the Universe - The Big Bang Theory www.ck12.org 1.1 Formation of the Universe - The Big Bang Theory How did everything begin in the Big Bang? If the universe is expanding, the next logical thought is that in the past it had to have been smaller. A point even. The time when the universe began is the explosion known as the Big Bang. 2 www.ck12.org Chapter 1. Week 1 The Big Bang Theory FIGURE 1.1 Timeline of the Big Bang and the expansion of the Universe. The Big Bang theory is the most widely accepted cosmological explanation of how the universe formed. If we start at the present and go back into the past, the universe is contracting — getting smaller and smaller. What is the end result of a contracting universe? According to the Big Bang theory, the universe began about 13.7 billion years ago. Everything that is now in the universe was squeezed into a very small volume. Imagine all of the known universe in a single, hot, chaotic mass. An enormous explosion — a big bang — caused the universe to start expanding rapidly. All the matter and energy in the universe, and even space itself, came out of this explosion. What came before the Big Bang? There is no way for scientists to know since there is no remaining evidence. After the Big Bang In the first few moments after the Big Bang, the universe was unimaginably hot and dense. As the universe expanded, it became less dense and began to cool. After only a few seconds, protons, neutrons, and electrons could form. After a few minutes, those subatomic particles came together to create hydrogen. Energy in the universe was great enough to initiate nuclear fusion, and hydrogen nuclei were fused into helium nuclei. The first neutral atoms that included electrons did not form until about 380,000 years later. The matter in the early universe was not smoothly distributed across space. Dense clumps of matter held close together by gravity were spread around. Eventually, these clumps formed countless trillions of stars, billions of galaxies, and other structures that now form most of the visible mass of the universe. If you look at an image of galaxies at the far edge of what we can see, you are looking at great distances. But you are also looking across a different type of distance. What do those far away galaxies represent? Because it takes so long for light from so far away to reach us, you are also looking back in time (Figure 1.2). Background Radiation After the origin of the Big Bang hypothesis, many astronomers still thought the universe was static. Nearly all came around when an important line of evidence for the Big Bang was discovered in 1964. In a static universe, the space between objects should have no heat at all; the temperature should measure 0 K (Kelvin is an absolute temperature scale). But two researchers at Bell Laboratories used a microwave receiver to learn that the background radiation in the universe is not 0 K, but 3 K (Figure 1.3). This tiny amount of heat is left over from the Big Bang. Since nearly 3 1.1. Formation of the Universe - The Big Bang Theory www.ck12.org FIGURE 1.2 Images from very far away show what the universe was like not too long after the Big Bang. all astronomers now accept the Big Bang hypothesis, what is it usually referred to as? An explanation of the Big Bang: http://dvice.com/archives/2009/08/big-bang-animat.php . How we know about the early universe: http://www.youtube.com/watch?v=uihNu9Icaeo . "History of the Universe," part 2: http://www.youtube.com/watch?v=bK6_p5a-Hbo . "The Evidence for the Big Bang in 10 Little Minutes" provides a great deal of scientific evidence for the Big Bang: http://www.youtube.com/watch?v=uyCkADmNdNo (10:10). MEDIA Click image to the left or use the URL below. URL: https://www.ck12.org/flx/render/embeddedobject/1486 KQED: Nobel Laureate George Smoot and the Origin of the Universe George Smoot, a scientist at Lawrence Berkeley National Lab, shared the 2006 Nobel Prize in Physics for his work on the origin of the universe. Using background radiation detected by the Cosmic Background Explorer Satellite (COBE), Smoot was able to make a picture of the universe when it was 12 hours old. Learn more at: http://science.k qed.org/quest/video/nobel-laureate-george-smoot-and-the-origin-of-the-universe/ 4 www.ck12.org Chapter 1. Week 1 FIGURE 1.3 Background radiation in the universe was good evidence for the Big Bang theory. MEDIA Click image to the left or use the URL below. URL: https://www.ck12.org/flx/render/embeddedobject/60947 Summary • The Big Bang theory states that the universe began as a point and expanded outward. • No one can know what came before the Big Bang because there is no remaining evidence. • The tiny bit of background radiation in the universe is energy remaining from the Big Bang. Practice Use this resource to answer the questions that follow. http://science.nationalgeographic.com/science/space/universe/origins-universe-article/ 1. Explain the Big Bang Theory. 2. Who suggested the Big Bang Theory? 3. Who discovered cosmic microwave radiation? What does cosmic microwave radiation indicate? 4. What questions are not answered with the Big Bang Theory? 5 1.1. Formation of the Universe - The Big Bang Theory www.ck12.org Review 1. How is the idea that the universe started in a big bang a logical extension from a fact? 2. What evidence is there that the universe began in a big bang? 3. What happened in the first minutes after the Big Bang? Brainpop Review http://www.brainpop.com/science/space/bigbang/ login: robfrost; password: brainpop 6 www.ck12.org Chapter 1. Week 1 1.2 Star Formation • Describe star formation. Can you see stars being born? On a dark night you can practically watch stars being born. The fuzzy area in the sword in the constellation Orion is the Orion nebula. A close up shows the nebula with around 3,000 stars. New stars are being born there. Star Lives Stars have a life cycle, just like people. Stars are born, grow, change over time, and eventually grow old and die. Most stars change in size, color, and class at least once in their lifetime. What astronomers know about the life cycles of stars comes from data gathered from visual, radio, and X-ray telescopes. Nebulae Stars are born in a cloud of gas and dust called a nebula (Figure 1.4). Our Sun and solar system formed out of a nebula. The Pillars of Creation (Figure 1.5) are within the Eagle Nebula. The pillars are thought to be a large region where stars are forming. The dark areas may be stars that are about to begin. Star Formation For a star to form, gravity pulls gas and dust into the center of the nebula. As the material becomes denser, the pressure and the temperature increase. When the temperature of the center becomes hot enough, nuclear fusion 7 1.2. Star Formation www.ck12.org FIGURE 1.4 Stars are forming in the Rosette Nebula. FIGURE 1.5 In the Pillars of Creation, gas and dust come together as a stellar nursery. begins. The ball of gas has become a star! Our Sun was probably born in a nebula, like Orion in the top image. Summary • Stars form in a nebula. • A nebula is a cloud of dust and gas. • Nebulae can be spotted with the naked eye or simple telescopes. Review 1. What is a nebula? 2. How do stars form? 3. Why do scientists think the Pillars of Creation are where stars are forming? 8 www.ck12.org Chapter 1. Week 1 1.3 Where did all the elements come from? WHERE DID ALL THE ELEMENTS COME FROM? In the very beginning, both space and time were created in the Big Bang . It happened 13.7 billion years ago. Afterwards, the universe was a very hot, expanding soup of fundamental particles. The universe expanded rapidly during inflation and expands at a more or less constant rate now. As it grows, it cools. At first, the universe was dominated by radiation. Soon, quarks combined together to form baryons (protons and neutrons). When the universe was 3 minutes old, it had cooled enough for these protons and neutrons to combine into nuclei. This is known as the time of nucleosynthesis. Hydrogen, helium, lithium, and beryllium were produced. Today, about 90% of the universe is still hydrogen. Remember that only the nuclei of these atoms were created at this time. The universe was still far too hot to allow these nuclei to attract electrons and form atoms. That didn’t happen for another 300,000 years, at the time of recombination. At this time the universe had cooled sufficiently for atoms to exist. Echoes from these first atoms can still be seen in the Cosmic Microwave Background . But this process only created the lightest four elements. How did the other 88 natural elements come about? To understand this, we need to understand fusion. All protons have positive charge and therefore repel one other. How then, can they be packed tightly into a nucleus? What holds them there? The answer is another force, the strong force . When protons collide with enough energy, their electric repulsion can be overwhelmed by this new force and they will be bound together. Similar processes take place between neutrons and protons and between small nuclei. This building up of heavier nuclei is called nuclear fusion. Three important fusion processes are: the proton-proton chain which details how helium is made in our sun, the CNO cycle which explains how hydrogen is fused in hotter stars, and the triple alpha process which accounts for the helium fusing that occurs in mature stars. Sketches of these three processes are shown below. In each case, the sketch is incomplete. These reactions occur in chains with more than one possible path. Only the most probable paths are shown below. Proton-proton chain 9 1.3. Where did all the elements come from? www.ck12.org Triple alpha process Thanks to Michael Richmond for the following image Another important concept is the behavior of an ideal gas. When a gas is compressed, its temperature increases. When a gas expands, its temperature drops. This is why a bike pump warms up when in use. It is also why air rushing out of a balloon feels cool. Now back to our story. The atoms left over by the big bang were gravitationally attracted to one another and condensed into huge clouds. The gravitational pressure on the centers of these clouds heated them to temperatures of millions of degrees. This led to the fusion of hydrogen into helium. Stars were born. As the fuel in their cores is used up, the cores shrink and warm. This causes hydrogen fusing to occur in the outer parts of the star, its shell. This new source of energy causes the star to expand and cool, turning it into a red giant . 10 www.ck12.org Chapter 1. Week 1 Once all the hydrogen in the core has been used up, helium is fused into carbon, nitrogen and oxygen. This causes the core to expand and cool once again. The shell, which is still fusing hydrogen, also cools. When all the helium has been used, the core contracts and warms. This heats up the shell which now starts fusing helium. The star enters a second red giant phase. During these cycles, convection within the star increases. After hydrogen fusing is stopped, the first dredge-up carries oxygen, nitrogen, and carbon from the CNO process to the surface of the star. After helium fusing stops, a second dredge-up carries more of these elements from the triple alpha reactions to the surface. If the star has a mass greater than 2 solar masses, a third dredge-up can occur. This time mostly carbon and carbon molecules are brought to the surface. These stars are called AGB or carbon stars. They have very active solar winds and are typically surrounded by a sooty planetary nebula. Thanks to Michael Richmond for the following image At this point, low-mass stars (below 8 solar masses) eject their outer layers and evolve into white dwarfs . White dwarfs are carbon-oxygen-rich and made of super-dense matter. Most of their hydrogen and helium are lost to the stellar wind. These stars are so dense that they form a new type of “degenerate” or nuclear matter. 11 1.3. Where did all the elements come from? www.ck12.org High-mass stars do not evolve into white-dwarves. The gravitational pressure on these stars does not allow their cores to expand and cool. Their gravity is so great that not even the degenerate electron pressure in the core can cool it. After helium fusing ends in these stars, carbon fusing begins creating oxygen, neon, sodium, and magnesium. If the star has a mass of more than 8 solar masses (called a supergiant) its gravity is strong enough to fuse neon after it has finished fusing carbon. This forms more oxygen and magnesium. Afterwards, oxygen will be fused into sulfur, silicon, phosphorus, and magnesium. Then silicon will be fused to new elements as heavy as iron. Each stage burns more quickly, at a higher temperature, and at a greater density. As each new stage begins, the earlier stages continue their reactions in onion-like shells about the core. The star heats up and expands as each new phase begins. It constantly loses mass to its stellar wind. Some of the reactions which lead to the creation of these new elements also produce neutrons. These neutrons are captured by some of the new atoms to form different isotopes. This process cannot create a nucleus with more than 12 www.ck12.org Chapter 1. Week 1 26 protons (iron) because at that point the energy it takes to overcome the electric repulsion is greater than the energy released by the strong force. Creating elements heavier than iron requires energy, it doesn’t release energy. The heaviest elements are created in supernovae, the fantastic death of supergiant stars. As the core of the supergiant becomes saturated with iron, its pressure and temperature increase. Eventually, the blackbody radiation from the core produces gamma rayspowerful enough to break apart the iron atoms in the core. This further increases the pressure to a point where electrons and protons are fused into neutrons. This releases lots of energy in the form of neutrinos. The core cools and contracts; the inner shells rush to fill the void. As the core reaches nuclear density it become rigid and even bounces back a little. When the onrushing material feels this bounce, it creates a wave. As the wave spreads to outer, less-dense regions, it speeds up. Soon it is a shock wave and combines with the wave of neutrinos. The star is doomed. This process blows the star apart releasing 1046 joules of energy. This shock wave is the only place hot and dense enough to fuse elements heavier than iron, elements up to and including uranium. The supernova just described is termed a type II supernovabecause it contains the emission lines of metals. Type I supernovaehave no such lines. They are caused by a binary star system with a red giant and a white dwarf (remember that 70% of stars are binary). Matter that flows off the red giant collects on the white dwarf and increases the pressure on its core. Eventually carbon fusing begins in the white dwarf, but this time the star is made of “degenerate” matter and cannot expand to cool off. The result is a runaway reaction which ends in a supernova. 13 1.3. Where did all the elements come from? www.ck12.org SO THAT’S WHERE THE ELEMENTS COME FROM!! The first four elements were present after the big bang. Elements up through magnesium were created in red giants. Elements up through iron were created in supergiants and the elements from iron to uranium were created in supernovae. These atoms can meet one another in nebulae, on dust particles, and even in planets. When they do, chemical reactions take place. These reactions eventually led to life and then . . . us. 14 www.ck12.org Chapter 1. Week 1 1.4 References 1. Courtesy of NASA/WMAP Science Team. Artistic rendition of the Big Bang. Public Domain 2. Courtesy of NASA/ESA/S. Beckwith(STScI) and the HUDF Team. Far away objects show us what the Universe was like not long after the Big Bang. Public Domain 3. Courtesy of NASA/WMAP Science Team. One piece of evidence that supports the Big Bang theory is background radiation. Public Domain 4. John Lanoue. Stars form inside the Rosette Nebula . Public Domain 5. Courtesy of NASA, Jeff Hester, and Paul Scowen (Arizona State University). The Pillars of Creation is an ac tive region of the Eagle Nebula where new stars are being formed . Public Domain 15
