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High School Teacher Program 2008 Are we really made of stars? Are we really made of stars? Recommended for 12-18 yrs olds Summary Are we really made of stars? This lesson starts from studying light emissions from exploding stars, called supernovas. Those observations and analyses can tell us the origin of the elements found on Earth and throughout the universe. This explains how the elements of the periodic system, we find on Earth, are formed in stars. Concepts Hydrogen, Helium, periodic system of the elements, fusion, pressure, supernova, spectroscopy Short introduction Quicktime movie about the periodic system of the elements. Link to the curriculum This lesson can cover the following topics: • The periodic system of the elements • Matter • The Earth in the universe Hints for the teacher First discuss with your students what they think about this question. Let the students work in small groups and let them present their ideas. Then watch the little movie. After the movie you can ask feed-back about how the students feel about this topic. Show them the table of the elements, explain the different properties of atoms, how they are build in stars, … Give the students time to discuss in their groups what they are taught and let them formulate some questions. Let the students summary what they learned in this lesson. Are we really made of stars? Periodic system All liquids, gases, and solids found on our planet are made from one or more of 92 naturally occurring elements. From what they have observed, scientists have determined that these same 92 elements are found throughout our universe. This suggests that a common process leads to their creation. But how are these elements created? How did they get so widely disseminated? ©2008 HST-2008 Page 1 High School Teacher Program 2008 Are we really made of stars? Currently, the most popular theory states that the nuclei of hydrogen and helium, the lightest and most abundant elements in the visible universe, were created in the moments following the Big Bang, 13.7 billion years ago. All other naturally occurring elements were — and continue to be — generated in the high temperature and pressure conditions present in stars. Elements are composed of tiny particles called atoms that are indivisible under normal conditions. However, when exposed to high heat and pressure, atoms can either break apart or fuse together. Under these conditions, the nucleus of one element can fuse with the nucleus of a different element, creating the nucleus of a heavier element. When elements lighter than iron form, the mass of the new nucleus is less than the combined mass of the two original nuclei. The difference in mass between the two is released as energy. In stars, this kind of reaction is referred to as stellar nucleosynthesis, but it is more commonly known as nuclear fusion. Nuclear fusion is used today on Earth in the nuclear explosives called hydrogen bombs. Many people hope that one day nuclear fusion will be used for peaceful energy production. Stars are fuelled by nuclear fusion reactions, which take place in their deep interiors, or cores. Hydrogen nuclei fuse, forming helium nuclei. The energy produced by these fusion reactions prevents the star from collapsing under its own gravity. Mature stars contain enough hydrogen nuclei to last billions of years. When a star's hydrogen fuel supply is spent, however, its core begins to contract. The contraction is so intense that it creates conditions under which helium nuclei fuse. In this way, helium becomes the star's next fuel source. The fusion of helium nuclei produces carbon and oxygen nuclei, and in the process sufficient energy is released to temporarily sustain the star. Once helium runs out, the nuclei of carbon, oxygen, and other elements begin to fuse. These new fuel sources are depleted at faster and faster rates. Since the heaviest element created in a star by nuclear fusion reactions is iron, a large iron core eventually forms at the center of everything. At this point, gravity becomes overwhelming, the core collapses, and an explosion occurs, during which outer layers of gas and heavy elements are ejected to space. Such explosions, called supernovas, occur about once a century in our galaxy. The energy created by supernovas produces nuclei heavier than iron: silver, gold, platinum, and all the other heavy elements all the way up through uranium. This process is known as supernova nucleosynthesis. Exploding, the star seeds these elements into space. Some of the elements are picked up in second-generation stars. These will live for a shorter time since they start by bearing heavy elements within them. Some of the elements the star spews out will collect into planets, which is where the Earth eventually comes in. Life in Universe Observations of many stars and galaxies have shown similar chemical abundances: 98% of the mass is hydrogen and helium, and all other elements compose the remaining 2%. That 2% may not seem like much, but it is enough to create all living things on Earth. One of the most Page 2 ©2008 HST-2008 High School Teacher Program 2008 Are we really made of stars? common of the remaining elements is carbon. Organic molecules have even been observed in interstellar clouds and found in comets and meteorites. While it is still not clear how life on Earth originated from basic organic molecules, the fact is that life exists. If basic organic molecules were able to create life on Earth, and they are available elsewhere in the universe, it is not unreasonable to wonder if life has also developed elsewhere. If many of the atoms within us are 13.7 billion years old, and as "we" are our bodies, not just our minds, then, in a way, we also are 13.7 billion years old. How do we know… How do we know H en He were present in the Universe from the really beginning? Scientists study the universe by studying the light they catch in their telescopes. By analysing the spectra of different stars, the American astronomer Edwin Hubble saw that galaxies were moving away from each other at a rate constant to the distance between them. He determined that the greater the distance between a galaxy and Earth, the faster that galaxy was moving away from us — a phenomenon now known as Hubble's law. These findings signalled that the universe is expanding and laid the foundations for the Big Bang theory, which states that the universe exploded into existence from a single point or a very small region in time and space and has been expanding ever since. Just after the Big Bang, temperatures were so high that particle pairs could be created purely out of the heat energy present. During its early phases the Universe was “γ radiation dominated”, that is the photons dominated the energy and pressure of the Universe. As the Universe expanded, it cooled, T ∼ 1/R, where R is some measure of the "scale of the Universe". During this cooling process, particles were able to form out of energy: first quarks, then neutrons, protons and electrons, and then nuclei and atoms. After 1 million years the temperature was decreased to 4000 K, the temperature where atoms can survive. The strongest evidence that something like the Big Bang really happened is the “Cosmic Background Radiation” predicted by Cosmologist George Gamov in 1948 and discovered by Arno Penzias & Robert Wilson of Bell Labs in 1965. All those γ-rays described above are part of the thermal radiation present in the early Universe because it was hot. As the Universe expanded and cooled, the radiation field cooled along with it. Gamow predicted that the Universe should be filled with this "relic radiation left over" from the Big Bang. Using the peculiar horn-shaped antenna shown in the picture to the right, Penzias & Wilson made the first glimpses of the Cosmic Background Light quite unexpectedly. Since their discovery the evidence has become stronger and stronger that we are seeing the light from the Big Bang. Penzias & Wilson received the Nobel Prize in Physics in 1978. By analysing this light, scientists have found evidence that from the very beginning of our Universe hydrogen and helium were present, in the comparable rates as we ©2008 HST-2008 Page 3 High School Teacher Program 2008 Are we really made of stars? find them in today’s universe: helium is about 25% by mass and hydrogen about 73% with all other elements constituting less than 2%. How do we know H fuses to He in stars en He fuses to Be…? We can’t “see'' further into a star than its outer layers, its photosphere, so how do we know what goes on inside? The structure of a star (i.e., how the quantities temperature, density, and pressure vary through the star) is determined by setting the force of gravity at each layer equal to the available pressure distributed over the surface area of that layer. The sources of pressure inside normal stars are gas (proportional to density and temperature) and radiation (proportional to T4), though the latter is important only in stars much hotter inside than our Sun. Gravity bears inward, pressure pushes outward, and in this struggle an equilibrium arises, setting up the structure of the star. Well, except for one thing.... The resulting conditions at the very centres of stars are such that the fusion of lighter elements into heavier ones occurs naturally, releasing energy through Einstein's most famous equation E = mc2. This energy must be transported to the surface, either by radiation (photons) or convection (upward bulk motion of buoyant gas), and when the energy produced in the core is exactly balanced by the energy lost at the surface as light, then the star can come into full equilibrium. What is the evidence that this is what's really happening inside a star?'' 1. Until recently, we had only indirect evidence. That is, we could build a computer model of a star (or a whole series of model stars for a star cluster) and compare their outward characteristics (luminosity, surface temperature, radius, spectrum) with what we could derive from measurements of a star of a given mass and composition. We could also let our models progress in time, as their cores consumed one fuel and fused it into another, and in this way we could track the evolution of a star, or a whole cluster of stars, and compare the resulting distributions of the model stars with what is observed. All such comparisons between model and observation have met with spectacular success. 2. The observed abundances of the elements on the periodic table are reproduced to good accuracy by models of galactic evolution, which describe how multiple generations of billions of stars produce and recycle the elements heavier than helium. That is, we know WHY carbon, nitrogen, oxygen and iron (for example) are found in concentrations of 3.5 x 10-4, 9.3 x 10-5, 7.4 x10-4, and 3.2 x 10-5 by number relative to hydrogen in our Sun, in stars with ages similar to our Sun's, and in gas clouds in this part of our Milky Way Galaxy. Recently, improvements in experiment, observation, and technology have allowed us a closer, more direct peek at the inside of at least 1 star - our Sun. In two words: helioseismology and neutrinos. 3. Helioseismology - Due to the convective motions of gases in the outer 30% of the Sun's radius, observed as bubbling granulation cells in the Sun's photosphere our Sun rings like a bell - though very gently. Acoustic (sound) waves of 10 million modes of oscillation propagate through our Sun, with periods of typically several minutes. Detailed analyses of the measurements of these ultra-low amplitude oscillations (with surface velocities of less than 10 cm/s) allow astronomers to determine how temperature, density, and composition vary through the Sun's interior. These vibrations are detected as tiny Doppler shifts in the light emanating at the photosphere. This is Page 4 ©2008 HST-2008 High School Teacher Program 2008 Are we really made of stars? similar to how geologists determine the Earth's interior structure and state through the study of earthquake waves, or loosely analogous to an ultrasound sonogram revealing the interior structure of a human. In this picture showing a model representing the observed oscillations in our Sun, the blue areas are approaching, while red areas are receding. The density, temperature, and even changes in composition may all be measured through an analysis of the millions of modes of oscillation. The latest results show that the best theoretical models predict a structure of our Sun (pressure, temperature, density, relative fractions of hydrogen, helium, heavy elements as functions of distance from the Sun's centre) that differs by 0.1%-0.3% from that determined from helioseismology observations. 4. Solar Neutrinos - Many of the thermonuclear fusion reactions predicted to occur in the core of our Sun produce a sub-atomic particle, known as a neutrino. As its name implies, it has no charge, and its main property is that it does not interact with other matter very much (doing so through the weak nuclear force). Until very recently, we did not know if it had any mass (without mass, like a photon, it would travel at the speed of light). Because it interacts very little with other matter, it was known that most neutrinos should "fly" out of the Sun, and that with special detectors we might be able to detect a fraction of them here on Earth. The total number of neutrinos, recently observed, agrees with the number calculated using the standard computer model of the Sun. Yhis shows that scientists now understand how the Sun shines, the original question that initiated the field of solar neutrino research. Why the fusions stop at Fe? During most of their lives, stars fuse hydrogen into helium in their cores, but the fusion process rarely stops at this point. Massive stars become much hotter internally than stars like the Sun, and additional reactions occur after all the hydrogen in the core has been converted to helium. At this point, massive stars begin a series of nuclear burning, or reaction, stages: carbon burning, neon burning, oxygen burning, and silicon burning. In the carbon burning stage, carbon undergoes fusion reactions to produce oxygen, neon, sodium, and magnesium. During the neon burning stage, neon fuses into oxygen and magnesium. During the oxygen burning stage, oxygen forms silicon and other elements that lie between magnesium and sulphur in the periodic table. These elements, during the silicon burning stage, then produce elements near iron on the periodic table. Massive stars produce iron and the lighter elements by the fusion reactions described above, as well as by the subsequent radioactive decay of unstable isotopes. Elements heavier than iron are more difficult to make, however. Unlike nuclear fusion of elements lighter than iron, in which energy is released, nuclear fusion of elements heavier than iron requires energy. Because there is no system that can deliver this energy, the reactions in a star's core stop once the process reaches the formation of iron. ©2008 HST-2008 Page 5 High School Teacher Program 2008 Are we really made of stars? How do we know supernovae create elements heavier than Fe? A supernova is a stellar explosion. They are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun could emit over its life span. The explosion expels much or all of a star's material at a velocity of up to a tenth the speed of light, driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant. Long ago, scientists learned that they could heat atoms of an element to a glow and direct the light through a prism to produce a set of coloured lines. These spectral lines are unique for each element: No two elements produce the same colours and line positions along a spectrum. By using instrumentation that reads light signatures from far away — a technique known as spectroscopy — scientists today know with great certainty which elements a planet or a star, or even a star's dispersed remnants, contains. How do we know we are 13.7 billion years old? The universe is expanding with the velocity v = Hubble suggested a linear relationship: v = H d ⇒H = v [km / s ] . d [MegaPar sec] d . t H…Hubble Constant, d…distance of an object You can get the velocity because of the redshift (Doppler effect), but it was very difficult to measure the distance. When 2 stars have the same luminosity and are at the same distance they will appear like 2 equal lanterns. When one is farer away that one will seem darker with Luminosity L ∝ 1 . d2 Finally they found kind of a “standard candle”: a binary system of 2 stars: when a white dwarf and a red giant are orbiting around their common centre of mass then it may occur that material from the companion red giant is attracted by the white dwarf until the white dwarf can no longer support its own weight and burns its nuclear fuel so suddenly that it explodes. These explosions always release about the same amount of energy and have almost the same peak of brightness. These explosions emit x-rays through the bremsstrahlung process and are called “Type Ia Supernovae”. They are these standard candles which allow to measure the true distance. Page 6 ©2008 HST-2008 High School Teacher Program 2008 Are we really made of stars? So they could measure the Hubble Constant with H = 70 As v = d = H ⋅d t [km / s ] [MegaPar sec] . 1 t d….distance ⇒ H = …….time that has passed ⇒ age of the universe Bibliography • • • • • • • • http://www.teachersdomain.org/resources/phy03/sci/phys/matter/origin/index.html http://www.amateurspectroscopy.com/Spectroscope.htm http://www.db.dk/bh/lifeboat_ko/SPECIFIC%20DOMAINS/Periodic_system.htm http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/hydhel.html http://cass.ucsd.edu/public/tutorial/BB.html http://homepages.wmich.edu/~korista/starstruct.html http://en.wikipedia.org/wiki/Binary_star http://universe-review.ca/R02-07-candle.htm ©2008 HST-2008 Page 7