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nucleons thesis kd-lesson2-stellarevolution.ppt
- 1. A n in t r o d u c t i o n t o t h e b e g i n n i n g o f t h e u n i v e r s e a n d t h e f o r m a t i o n o f s t a r s . Stellar Evolution
- 2. The Big Bang TheOrigin of the universe is thought to be from the Big Bang, which occurred ~13.9 billion years ago. Since then, the universe has been continually expanding. The first stars began to form around 400 million years ago.
- 3. Hertzsprung Russell Diagram TheH-R diagram shows the relation between temperature and luminosity in stars. Main sequence stars are found along the main branch that extends from the top left to the bottom right of the diagram. The main sequence stars exhibit the typical evolution of a star.
- 4. Birth of Stars Nebulaare clouds of dust and gas. Stellar nebula are specifically the beginnings of stars. As they being to accrete, the dust and gas will coalesce and eventually form a star. The Eagle Nebula (taken via the Hubble Telescope, NASA)
- 5. Birth of Stars Starsform through the collapse of nebula, after which the gas and dust begin to accrete and coalesce. The accretion begins to form a protostar. Protostars eventually become full-sized stars. Formation of a protostar from a nebula
- 6. Life Cycle ofStars All stars have a life cycle that begins with a stellar nebula, but can lead to different fates. Sun-sized stars evolve along the main sequence, and then become red giants. More massive stars become red supergiants and eventually end in supernova explosions.
- 7. The Big Bangoccurred ~13.9 billion years ago and the universe has been expanding ever since. The Hertzsprung Russell diagram describes the relation between temperature and luminosity in stars. Stellar nebula are clouds of gas and dust that eventually form stars. Stars form through the collapse of stellar nebula and subsequent accretion. The Life Cycle of Stars describes the evolutionary paths for stars. Low-mass stars eventually become white dwarf stars. High-mass stars result in Type II supernova. Summary
Editor's Notes
- #2 AUTHORS: Jasmeet K Dhaliwal, Scripps Institution of Oceanography, UCSD Jason Moore, San Diego High School: School of the Arts SUMMARY: This lesson concentrates on presenting information about the origin of the universe to the students. It is more of a teacher-led discussion, with ample opportunities for students to take notes and ask questions. CONTEXT FOR USE: This is an introduction to the chemistry unit, which draws on stellar nucleosynthesis as a context for explaining how atoms are built. In a Chemistry class, this may have to be shortened in the interests of time. MISCONCEPTIONS: Preconceived notions about the origins of the universe, what stars are (including difference from shooting stars), and how elements are formed. Common misconceptions are: The stars are smaller than the Sun The Sun is not a star Stars can fall Elements are formed under high pressure in the earth
- #3 MAIN POINT: The Big Bang and the origin of the universe TEACHING NOTES: This diagram shows the expansion of the universe shortly after the big bang. The time increases from left to right, with important events identified on the image. The events are represented by stacked graphs on a time-continuum. Two of the most important events with respect to this lesson are the first stars (~ 400 Million years ago) and the subsequent development of galaxies and planets. Under the current cosmological model for the beginning of the Universe, the “Big Bang” occurred ~13.8 billion years ago. Under this model, the Universe was extremely hot and dense and an “explosion” caused it to begin expanding rapidly. After the initial expansion, it then began to cool allowing the energy and matter to condense to form subatomic particles, such as proton, neutrons and electrons. A few thousand years later, the first atoms (with stable atomic nuclei) formed. These “primordial elements” consisted of hydrogen and helium, with some lithium. These elements later condensed under the force of gravity to form stars, which then formed heavier elements, either through fusion or during supernovae (details in nucleosynthesis lesson); the first stars began forming about 400 million years after the Big Bang. The occurrence of the Big Bang has been inferred from telescopic observations of remote objects such as galaxies. These observations revealed a “redshift,” which is an electromagnetic phenomenon in which, as an object moves away, its wavelength increases. When its wavelength increases, it becomes shifted towards the red end of the electromagnetic spectrum. Therefore objects with the greatest amount of redshift are considered to be the farthest away from our point-of-view (the Earth). This is evidence for an expanding universe. The redshift in our telescopic observations of the universe suggests that there is continued expansion of the universe. REFERENCES: Big Bang: http://en.wikipedia.org/wiki/Big_Bang Red Shift: http://en.wikipedia.org/wiki/Redshift PICTURE/GRAPHICS CREDITS: http://upload.wikimedia.org/wikipedia/commons/6/6f/CMB_Timeline300_no_WMAP.jpg
- #4 MAIN POINT: Review the Hertzsprung Russel diagram and the life cycle of stars. TEACHING NOTES: The Hertzsprung Russel diagram shows the relationship between the absolute magnitude (luminosity / brightness) of stars and their temperatures. The brightest stars are toward the top of the diagram while the hottest starts are on the left of the diagram. The main band that stretches across the diagram (bottom right to top left) consist of the Main Sequence Stars. These main sequence stars are in hydrostatic equilibrium, meaning that their inward gravitational pressure is balanced by outward thermal pressure (generated by the fusion within the hot core). The main sequence represents the major hydrogen-burning phase of a star’s lifetime. A general rule is that the larger a star, the shorter its life span along the main sequence branch. Following the hydrogen-burning phase, more massive stars can evolve along the red-giant-branch (RGB) or asymptotic-giant-branch (AGB) stars. These are represented by the branch in the top right. RGB stars continue to fuse hydrogen in their cores while AGB stars begin to burn heavier elements such as carbon and oxygen. While these stars will not be discussed in explicit detail, they are important to the formation of elements through stellar nucleosynthesis. In the lower left is a small arm of white dwarf stars. These may result from main sequence stars, which have lost their outer shells. It is essentially a remnant core of a star that is very dense. The low luminosity results from stored thermal energy, rather than active fusion within the star. Depending on their age, stars differ in size and have different sources of energy for fusion; this will be detailed in the next lesson. REFERENCES: H-R Diagram: http://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram Main Sequence: http://en.wikipedia.org/wiki/Main_sequence Red Giant: http://en.wikipedia.org/wiki/Red_giant White Dwarf: http://en.wikipedia.org/wiki/White_dwarf An alternative H-R diagram showing different stars along their evolutionary tracks can be found here: http://en.wikipedia.org/wiki/File:Hertzsprung-Russel_StarData.png PICTURE/GRAPHICS CREDITS: http://upload.wikimedia.org/wikipedia/commons/6/6b/HRDiagram.png
- #5 MAIN POINT: The concept of stellar nebula and how they are related to the formation of stars TEACHING NOTES: Nebula are clouds of dust and gas, which eventually clump together to form larger masses that eventually form stars. Nebula give rise to stars at their center and the remaining material then forms the planets around it. Some famous named nebulae are the Eagle, Crab, Orion and Heart. This image is of the Eagle nebula and is titled “The Pillars of Creation.” Nebula form in high density regions of diffuse interstellar medium (ISM), which is ~70% hydrogen and most of the remainder being made of helium. Just as in a star, these nebula are under hydrostatic equilibrium, in which the kinetic energy of the gases is balanced by the gravitational force. When the stellar nebula cloud becomes to large, however, it will experience gravitational collapse, which will eventually lead to the formation of a star. REFENCES: Nebula: http://en.wikipedia.org/wiki/Nebula PICTURE/GRAPHICS CREDITS: https://upload.wikimedia.org/wikipedia/commons/b/b2/Eagle_nebula_pillars.jpg
- #6 MAIN POINT: The formation of a star through gravitational attraction and accretion. TEACHING NOTES: The formation of a star occurs under gravitational collapse as particle coalesce and the material condenses under its own weight. This results in a dense rotating sphere of gas, which is called a stellar embryo. As it collapses further, the core temperature increases and there is convection and radiation so that the energy can be lost to space. Eventually the gas becomes hot enough for internal thermal pressure to balance gravitational attraction (hydrostatic equlibrium), resulting in a proto-star, which is surrounded by a circumstellar disk of material. The proto-star is an early phase in a star’s lifetime; for a star the size of our sun, it lasts ~100,000 years. As the cloud contracts and material from the circumstellar disk accretes onto the star, it becomes dense. As the density and pressures cause the core to heat-up, more emission begins to emanate from the proto-star, rather than the surrounding material in the disk. The temperature of the proto-star continues to rise as the disk grows smaller (with material accreting to the center). When the accretion disk has dissipated, the star is fully formed. REFERENCES: Star Formation: http://en.wikipedia.org/wiki/Star_formation Proto-star: http://en.wikipedia.org/wiki/Protostar PICTURE/GRAPHICS CREDITS: http://www.physics.hku.hk/~nature/notes/lectures/images/chap14/protostar.jpg
- #7 MAIN POINT: The Life Cycle of Stars TEACHING NOTES: The diagram above describes the basic life cycle of stars. There are different evolutionary paths for low-mass stars (like the Sun) and high-mass stars, but they both begin with growth along the Main Sequence. As mentioned previously, star-forming (stellar) nebula condense to form proto-stars, which then condense further to form full-fledged stars. At this point, the core reaches 10 million degrees Kelvin, which initiates hydrogen fusion, thereby generating energy for the star (this process is explored in more detail in later lessons). The hydrogen fusion maintains the star through hydrostatic equilibrium (with external thermal pressure counteracting inward gravitational collapse). This star is currently on the main sequence, but after the core uses up its hydrogen supply for fusion, a fate of a star will differ and depends on the size of the star. The low-mass stars are represented by the cycle on the left. The star starts along the Main sequence and then becomes a Red Giant, where it begins to fuse helium. After helium fusion in the Red Giant stops, the core collapses and the outer layers of the star form a planetary nebula. The remnant core becomes a white dwarf, a star with low luminosity that is very dense. The high-mass stars (>10 solar masses) are shown on the right. They being along the Main Sequence and then become Red Super-giants. The high energy within this star allows for the fusion of heavier elements. When the core consists of iron and the star can no longer generate sufficient energy through nuclear fusion, it results in a Type II supernova explosion (explained later). Following the explosion, smaller remnant cores form into neutron stars, which are dense objects that consist of tightly-packed neutrons. Larger remnant cores become black holes, where the gravitational forces are stronger than the nuclear forces (which prevent the combination of subatomic particles), and not even light can escape. This provides a more detailed explanation of the diagram Left-side of Diagram Low-mass stars, which are less than 0.5 solar mass, continue burning hydrogen, increasing in temperature and luminosity over time; they eventually collapse directly into the white dwarf stage. Mid-sized stars like our Sun will continue along the main sequence for ~10 billion years, and then expand into Red Giants and begin hydrogen fusion within concentric shells (the core is used up and inert at this point). During this period, the star experiences both gravitational compression and thermal expansion, causing the hydrogen in the shells to fuse faster. Eventually, the star reaches the temperature of helium fusion and this process begins within the core of the star; in a Sun-sized star, the increase to sufficient temperatures may take up to one billion years. When all the fuel is used up, the outer layers of the star are stripped away by strong stellar winds. As the star’s atmosphere expands around it, it forms a cloud of material and the core become exposed. Eventually, the core contracts and becomes a white dwarf, while the surrounding cloud dissipates and becomes a planetary nebula. This planetary eventually may form into a stellar (star-forming) nebula and is considered to be one of the important mechanisms for the recycling of elements in the universe. Right Side of Diagram Massive stars (>5 solar masses) evolve away from the main sequence within a few million years and become red supergiants, and immediately have enough energy to begin helium fusion within their cores. They can also burn progressively heavier elements (onion shell model, which is discussed in a Lesson 4), eventually ending in iron cores. After all the fuel is finished, the core contracts inward and as the density increases, an outward shock wave causes the star to expand and explode, resulting in a supernova. More specifically, this is a Type II supernova, which are distinct from Type I supernova based on their emission spectra. Type I supernova result from the explosion of Wolf-Rayet stars, which are both extremely hot and luminous supermassive stars; they lose mass quickly because of a very strong stellar wind. Following the supernova stage, the gravitational energy causes the core to experience sudden collapse, resulting in very dense neutron stars or black holes. In neutron stars, there are no sufficient forces to maintain individual nuclei, resulting in all matter condensing into tightly-packed neutrons; these are very small and extremely dense. Following a supernova, large stars may become black holes, as they collapse even further and have an incredible gravitational attraction out of which not even light can escape. Even more massive stars (>40 solar masses) are thought to burn very quickly and lose their outer layers, thereby becoming blue supergiants or yellow hypergiants. These stars efficiently mix materials from their surfaces to their cores. It is thought that they may explode as supernova, but their exact fate is not well understood. Following a supernova stage or the expansion into a planetary nebula, some stars return to become star-forming nebula (disperse clouds of gas and dust), which may then evolve to form stars later. In this way, the life and death of stars form a cycle, with material from older, extinct, stars being used to form new, younger stars. RELATED ASSIGNMENT: Following the presentation of the material, the students should form small groups (2-4 people) and design their own diagram of the “life cycle of stars.” The should be provided with large-format poster paper, which can then be hung around the classroom for the remainder of the unit. This may be spread over a few days, ~10-15 min / day at the end of class. The objective of this assignment is to allow the students to explore the material and present it in their own way, thereby enhancing comprehension. The key points that they should understand is the circular nature of stellar evolution, and the fact that stars form from gravitational collapse of a cloud of gas and dust. REFERENCES: Stellar Evolution: http://en.wikipedia.org/wiki/Stellar_evolution Supergiant: http://en.wikipedia.org/wiki/Supergiant Supernova: http://en.wikipedia.org/wiki/Supernova Wolf-Rayet Star: http://en.wikipedia.org/wiki/Wolf%E2%80%93Rayet_star Stars (NASA): http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html PICTURE/GRAPHICS CREDITS: http://imagine.gsfc.nasa.gov/docs/teachers/lessons/xray_spectra/background-lifecycles.html