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E:\2012-2013\SSU\PHY 207 spring 2012\Stellar Evolution\Document1 1 of 42 pages PHS 207 Astronomy for r Monday, March 4 examining and expanding on Moche s ch 3.18 -21 & ch. 5. By the end of class, you should be able to determine a star s location on the Hertzsprung-Russell Diagram when you know the star s radius and are given its luminosity or temperature explain a star s birth to death cycle by using specific terms such as interstellar medium (ISM)and dwarf starting with protostar, and know the different types of binary stars. E:\2012-2013\SSU\PHY 207 spring 2012\Stellar Evolution\Document1 2 of 42 pages Sample Stellar Evolution TEST QUESTIONS Multiple Choice Questions 1. Protostars are difficult to observe because a. the protostar stage is very short. b. they are surrounded by cocoons of gas and dust. c. they radiate mainly in the infrared. d. all of the above e. they are all so far away that the light hasn't reached us yet. 2. The nuclear reactions in a star's core remain under control so long as a. b. c. d. luminosity depends on mass. pressure depends on temperature. density depends on mass. weight depends on temperature. e. temperature depends on mass. 3. Interstellar gas clouds may collapse to form stars if they a. have very high temperatures. b. encounter a shock wave. c. rotate rapidly. d. are located near main sequence spectral type K and M stars. e. all of the above. 4. The region of the sun just below the photosphere a. is undergoing thermonuclear fusion using the proton-proton chain. b. is undergoing thermonuclear fusion using the CNO cycle. c. is transporting energy to the photosphere by convection. d. is not in hydrostatic equilibrium. e. a and c above E:\2012-2013\SSU\PHY 207 spring 2012\Stellar Evolution\Document1 3 of 42 pages 5. ______________ is the thermonuclear fusion of hydrogen to form helium operating in the cores of massive stars on the main sequence. a. The CNO cycle 100,000 b. c. d. e. The proton-proton chain Hydrostatic equilibrium The neutrino process none of the above 6.. The Great Nebula in Orion a. is a Herbig-Haro object. b. is a reflection nebula. c. is an emission nebula. d. contains only young low mass stars. e. is believed to be about 5 billion years old. 7. The diagram to the right is an HR diagram. The line indicates the location of the main sequence. Which of the five labeled locations on the HR diagram indicates a luminosity and temperature similar to that of a T Tauri star. a. b. c. d. e. 1 2 3 4 5 5. 10,000 1000 100 40 10 Luminosity 1 0.1 0.01 10 20,000 ' 1 10,000 5000 Temperature 2500 Document1 4 of 42 pages 8. What causes the outward pressure that balances the inward pull of gravity in a star? a. The outward flow of energy. b. c. d. e. The opacity of the gas. The temperature of the gas The density of the gas c and d 9. Convection is important in stars because it a. b. c. d. e. increases the temperature of the star. mixes the gases of the star. transports energy outward in the star. carries the neutrinos to the surface of the star where they can escape. b and c 10. The main sequence has a limit at the lower end because a. low mass stars form from the interstellar medium very rarely. b. low mass objects are composed primarily of solids, not gases. c. pressure does not depend on temperature in degenerate matter. d. the lower limit represents when the radius of the star would be zero. e. there is a minimum temperature for hydrogen fusion. 11.- The lowest mass object that can initiate thermonuclear fusion of hydrogen has a mass of about a. b. c. d. 1 M0. 60 Mo. 0.5 Me. 0.08 M o . e. 0.001 M o . 12. What is the lifetime of a 10 Me star on the main sequence? a. 3.2 x 107 years b. 320 years c. 3.2 x 10 12 years d. 1 x 109 years e. 1 x 1011 years Document1 5 of 42 pages 13. The extinction of starlight due to the interstellar medium I. II. III. IV. is the greatest in the ultraviolet. is the greatest in the infrared. is caused by ionized hydrogen. is caused by dust particles. a. I & II b. II & III c. I& IV d. II & IV e. only IV . 14. Stars are born in a. reflection nebulae. b. dense molecular clouds. c. MI regions. d. e. the intercloud medium. the local bubble. Fill in the Blank Questions 1. The condition of _______________ means that the force due to gravity pushing down on a layer is exactly equal to the pressure pushing outward on that layer. * 2. Stars with masses greater than 1.1 solar masses use the ___________ to convert hydrogen into helium and produce energy 3. Energy transport by is important when photons cannot readily travel through a Document1 6 of 42 pages gas. True-False Questions 1. Ninety percent of all stars fuse helium to form carbon and lie on the main sequence. 2. Nuclear fusion in stars is controlled by the dependence of density on mass. 2. The sun has a core in which energy travels outward primarily by radiation. 3. The sun makes most of its energy by the CNO cycle. 4. Energy flows by radiation or convection inside stars but almost never by conduction. 5. Hydrostatic equilibrium refers to the balance between weight and pressure. 6. The Orion region contains young main sequence stars and an emission nebula,. 7. The thermal motions of the atoms in a gas cloud can make it collapse to form a protostar. 8. The pressure of a gas depends on the temperature and density of the gas. Essay Questions 1. Why do nuclear reactions in a star occur only near its center? 2. Explain what keeps the nuclear reaction in a star under control. 3. Why are interstellar absorption lines so narrow? 4. Why are massive stars more luminous than low mass stars? That is, why is there a mass-luminosity relation? Document1 7 of 42 pages 5. Why is there a lower end to the main sequence? THE END 1. Let s create a table using the NAAP Hertzsprung-Russell Explorer of a star s luminosity versus its temperature for the radius equal to our Sun NAAP H-R Explorer is at http://astro.unl.edu/naap/hr/animations/hr.html Radius = 1 Ro You can best move the red X in the H-R Explorer by dragging it. Note in the table below how the luminosity (in Absolute Magnitude M)and temperature change as you move the red X to the different temperature values indicated in the H-R window in the lower left. Document1 8 of 42 pages At a given temperature determine its Spectra Class by changing x-axis scale in the window below the H-R diagram Luminosity Luminosity Luminosity at 1 Ro at 2 Ro at 10 Ro Temperature K degrees 2300 5000 10000 20000 40000 http://astro.unl.edu/naap/hr/hr_background1.html Spectral Classification of Stars Spectral Class Type for a given temperature Document1 9 of 42 pages Note that these are all different ways of talking about the surface temperature of a star. Types of Spectra Astronomers are very interested in spectra – graphs of intensity versus wavelength for an object. They basically tell you how much light is produced at each color. Spectra are described by Kirchoff s Laws: 1. A hot opaque body, such as a hot, dense gas (or a solid) produces a continuous spectrum – a complete rainbow of colors. 2. A hot, transparent gas produces an emission line spectrum – a series of bright spectral lines against a dark background. 3. A cool, transparent gas in front of a source of a continous spectrum produces an absorption line spectrum - a series of dark spectral lines among the colors of the continuous spectrum. It should be noted that the bright lines in the emission spectrum occur at exactly the same wavelengths as the dark lines in the absorption spectrum. Thus, one can think of the absorption spectrum to the right as the continuous spectrum minus the emission spectrum. Document1 10 of 42 pages Absorption Spectra From Stars The light that moves outward through the sun is what astronomers call a continuous spectrum since the interior regions of the sun have high density. However, when the light reaches the low density region of the solar atmosphere called the chromosphere, some colors of light are absorbed. This occurs because the chromosphere is cool enough for electrons to be bound to nuclei there. Thus, the colors of light whose energy corresponds to the energy difference between permitted electron energy levels are absorbed (and later reemitted in random directions). Thus, when astronomers take spectra of the sun and other stars they see an absorption spectrum due to the absorption of the chromosphere. Spectral Type Document1 11 of 42 pages NOAO/AURA/NSF OBAFGKM Astronomers have devised a classification scheme which describes the absorption lines of a spectrum. They have seven categories (OBAFGKM) each of which is subdivided into 10 subclasses. Thus, the spectral sequence includes B8, B9, A0, A1, etc. A traditional mnemonic for the sequence is Oh, Be, A Fine Girl/Guy, Kiss Me! Although based on the absorption lines, spectral type tells you about the surface temperature of the star. One can see that there are few spectral lines in the early spectral types O and B. This reflects the simplicity of atomic structure associated with high temperature. While the later spectral types K and M have a large number of lines indicating the larger number of atomic structures possible at lower temperatures. Annie Cannon Image Source: Harvard College Observatory Website Annie Cannon Most of the early work on stellar spectra was done early in the 20th century at Harvard University. The principal figure in this story was Annie Jump Cannon. She joined Harvard as an assistant to Observatory Directory Edward C. Pickering in the 1890 s to participate in the classification of spectra. She quickly became very proficient at classification examining several hundred stars per hour. She completed a catalogue of spectral types for hundreds of thousands of stars. Document1 12 of 42 pages Three blackbody curves at different temperatures. Planck Curves The outward appearance of stars depends more strongly on the underlying continuous spectrum coming from the inner parts of a star than the absorption at its surface. Continuous spectra for stellar interiors at different temperatures are described by Planck Curves shown in the figure to the left. Note that as the temperature increases the total amount of light energy produced (the area under the curve) increases and the peak wavelength (the color at which the most light is produced) moves to smaller more energetic wavelengths. Classification O0 Temperature 40,000 K Max Wavelength 72.5 nm Color Blue Document1 13 of 42 pages B0 20,000 K 145 nm Light Blue A0 10,000 K 290 nm White F0 7,500 K 387 nm Yellow-White G0 5,500 K 527 nm Yellow K0 4,000 K 725 nm Orange M0 3,000 K 966 nm Red This table lists corresponding values of color, spectral type, and peak wavelength. Note that these are all different ways of talking about the surface temperature of a star. Practice Exercise Document1 14 of 42 pages One can experiment with the relationships between spectral type, temperature, and color with the simple animation below. Use the animation to answer the following questions: Drag the slider through different spectral types to see the change in temperature, color, and brightness. What are the surface temperatures and colors of: a hot O2 star? a cool M3 red dwarf? a G2 star like the sun? What is the spectral type of a star with: Document1 15 of 42 pages a surface temperature of 10,000 K ? a surface temperature of 5,000 K ? What is the color of a star with: spectral type A0? surface temperature 4,000 K? A Star s Life Cycle depends almost entirely on its initial mass. From Holland and Williams article that is a little later. Stellar evolution is the necessary fundamental building block and distributive method of most common elements in the universe. Within the interior of stars, fusion creates new elements from the basic elements (H, He). While this process takes billions of years as measured by human standards, the life of a star is minor in comparison to the age of the universe. Much like its life span, the outcome of one star�s life is insignificant. When we add the efforts of one star, however, to the production of the billions of stars that have existed, exist and will exist again they are invaluable to the creation of life. Without the elements produced by stellar evolution (the processes within the interiors of stars), Carbon-based life as we know it would not be possible. The next time you are star gazing, remember, you are a product of the stars. Document1 16 of 42 pages http://www.cosmosportal.org/files/47101_47200/47109/file_47109.jpg Document1 17 of 42 pages http://essayweb.net/astronomy/images/Stellar_Evolution_large.jpg Document1 18 of 42 pages http://intouniverse.weebly.com/uploads/5/5/9/8/5598621/star_evolution.jpg Document1 19 of 42 pages http://media-1.web.britannica.com/eb-media/50/62750-004-223BCC72.jpg Document1 20 of 42 pages Stellar Evolution : The Life and Death of Our Luminous Neighbors by Arthur Holland and Mark Williams Introduction and Why You Should Care Observations of the stars with calculations of stellar models have give astronomers a comprehension of stellar evolution. Stellar evolution is the process in which the forces of pressure (gravity) alter the star. With these forces acting upon stars, their characteristics change dramatically over the period of their existence. Stellar evolution is inevitable as stars deplete their initial fuel sources. The search for new fuel sources affects the properties of stars as they evolve. This evolution is a process that consists of many different stages with fuel consumption as the dominant life cycles of an evolving star. Stellar evolution, in the form of these fuel consumption stages and their finality, is important because it is responsible for the production of most of the elements (all elements after H and He). Moreover, stages in the life cycle of stars are a vital part in the formation of galaxies, new stars and planetary systems. Time scales of Stellar Fuel Consumption The time scales of stellar evolution depend on the mass of the star. The rule governing stellar evolution is the more mass present, the faster the evolution for the star through the fuel consumption stages. Another property directly linked to the mass and evolution of a star is its luminosity. The mass-luminosity relation demonstrates that the main sequence on an H-R diagram (a chart plotting the the luminosities of stars against their surface temperatures) is a progression in mass as well as in luminosity and surface temperature (Kaufmann,1991, pg. 356). The Hertzsprung- Russell diagram is two possible plots of spectral types dependent on absolute magnitude when compared to temperature and luminosity. Document1 21 of 42 pages These H-R diagrams allow astronomers to conceptualize visually the mass-luminosity relationship as it pertains to the fuel consumption evolution of stars. The H-R diagram allows astronomers to plot visually the different stages in the evolving life cycle of stars. A H-R diagram (the red line represents the Main Sequence Stars). Document1 22 of 42 pages Glossary of bold face items in the following article absolute zero - Temperature of -273 degrees Celsius where all molecular motion stops; the lowest possible temperature accretion disk - A disk of gas orbiting a star or black hole binary star system - Two stars revolving around each other black hole - An object that is so strong that its escape velocity exceeds the speed of light core helium burning - The thermonuclear fusion of helium at the center of a star degenerate - The phenomenon, due to quantum mechanical effects, whereby the pressure exerted by a gas does not depend on its temperature event horizon - The location around a black hole where the escape velocity equals the speed of light; the surface of a black hole fusion - combination of lower mass nuclides into higher mass nuclides helium flash - The nearly explosive beginning of helium burning in the dense core of a red giant star hydrostatic equilibrium - The balance between the weight of a layer in a star and the pressure that supports it luminosity - The rate at which electromagnetic radiation is emitted from a star or other object main sequence star - A star whose luminosity and surface temperature place it on the main sequence on an H-R diagram; a star that derives its energy from core hydrogen burning mass- luminosity relation - The relationship between the masses and luminosities of main sequence stars neutrino - A subatomic particle with no electric charge and little or no mass, yet one that is important in many nuclear reactions neutron star - A very compact, dense star composed entirely of neutrons Document1 23 of 42 pages planetary nebula - A luminous shell of gas ejected from an old, low- mass star plasma - A hot ionized gas pulsar - A pulsating radio source believed to be associated with a rapidly rotating neutron star red giant - A large, cool star of high luminosity red supergiant - An extremely large, cool star of luminosity class I shell helium burning - The thermonuclear fusion of helium in a shell surrounding a star�s core shell hydrogen burning - The thermonuclear fusion of hydrogen in a shell surrounding a star�s core singularity - A place of infinite space-time curvature; the center of a black hole solar mass - The mass of our sun; reference point for the masses of other stars solar radii - The distance from a star�s core to the surface of its outermost plasma layer spectral type - A classification of stars according to the appearance of their spectra supernova - A stellar outburst during which a star suddenly increases its brightness roughly a millionfold thermal equilibrium - The balance between the input and outflow of heat in a system thermal pulse - Brief bursts of energy output from the helium- burning shell of an aging low-mass star thermal radiation - The radiation naturally emitted by any object that is not at absolute zero thermonuclear reaction - A reaction resulting from the high speed collision of nuclear particles that are moving rapidly because they are at a high temperature Document1 24 of 42 pages white dwarf - A low-mass star that has exhausted all of its thermonuclear fuel and contracted to a size roughly equal to the size of Earth white hole - A black hole from which matter and radiation emerge X-Ray - Electromagnetic radiation whose wavelength is between that of ultraviolet light and gamma rays Star formation Triggered Star Formation Since massive stars burn so fast they will often go supernova before the rest of the cloud has started making stars The shock wave from the supernova blast will crush the cloud triggering a new generation of star formation Document1 25 of 42 pages Stages in the Life of an Evolving Star Welcome to the life of a new born star. Main sequence stars are a range of stars based on size and surface temperature starting from the hot, bright, bluish stars in the upper left corner of a H-R diagram to the cool , dim, reddish stars in teh lower right corner of the diagram. Life for new stars begins in the Main Sequence. These mature stars undergo a remarkable transformation after they consume all the hydrogen in their core. With the hydrogen consumed, stars leave the main sequence and expand to form red giants. With this new stage, the fusion of helium begins to form heavier elements like Oxygen and Carbon. This process of expansion- collapseexpansion of stars forms the light elements present in the universe (up to Fe). Document1 26 of 42 pages Life in the Suburbs : a Main Sequence Star. Main sequence stars are stars who s luminosity and surface temperature place it in the main sequence of a Hertzsprung- Russell diagram. A fundamental property of all main sequence stars is thermal equilibrium. Thermal equilibrium is the liberation of energy from the interior of the star balanced by the energy released at the surface of the star. The energy released by a main sequence star is produced by hydrogen burning in its core (the fusion of 4H into 4He). Another fundamental property of a main sequence star evolution is hydrostatic equilibrium. Hydrostatic equilibrium reflects the required pressure in the core of a star to support the weight of the outer plasma layers. The heat produced from hydrogen in the core burning supports this outward pressure upon the outer plasma layers. As a main sequence star depletes the supply of hydrogen in the core, thermal equilibrium unbalances and the pressure in the star�s core lessens. Thermal equilibrium unbalances because the fusion of four hydrogen atoms into one helium atom decreases the number of particles present in the star�s core. The star beings to collapse inward because the fewer particles cannot maintain the pressure needed to support the star�s outer layers. Without the necessary pressure, the star�s core contracts slightly under the weight of its outer layers. This collapse increases the pressure and temperature of the core causing the luminosity of the star s surface to increase. This increase in pressure on the layers just outside the core raises their temperature to the necessary point in which the outer layers of hydrogen begin fusion. This occurrence of hydrogen fusion in the outer layers of a collapsing star is called shell hydrogen burning. Shell hydrogen burning allows the star to remain in the main sequence for another few million years. Despite this struggle to remain on the main sequence, the supply of hydrogen for fusion into helium in a star�s inner most layers depletes, which takes the star from the main sequence and to the next step of solar fuel consumption. The next stage of solar fuel consumption starts when hydrogen burning in the core ceases and ignites hydrogen burning in the star s outer layers. When hydrogen burning ceases in the star�s core, it begins to collapse again. At this point, the star converts gravitational energy into thermal energy because it must maintain thermal equilibrium. Stars sustain thermal equilibrium within their interiors through the ignition of helium burning. The collapse of the outer hydrogen burning shell upon the core raises the temperature and pressure in the core and begins helium burning. Because the temperature and energy needed to ignite helium fusion is greater than that of hydrogen, the energy released by helium fusion in the star�s core is greater than needed to support the weight of the outer layer. Document1 27 of 42 pages This excess energy expands the star�s outer layers beyond its previous radius and star�s volume increases. A star going through this stage of fuel consumption (collapse and expansion) is a Red Giant. The following diagram shows the dramatic expansion of a main sequence star as it begins helium fusion. Our Sun today compared to its future Red Giant self 1 AU is approximately 150 million kilometers or the distance from the sun to the Earth s orbit The Life of a Red Giant : A Star in Old Age. In the final states of hydrogen fusion the hydrogen burning shell adds to the mass of the star�s helium core. This added mass and pressure increases the star�s temperature. Temperature within the core of stars greater than three solar masses soon exceeds the 100 million Kelvin degrees barrier, at which it reaches the temperature needed for the onset of helium fusion. Core helium burning reestablishes the thermal equilibrium needed to support the star�s outer layers preventing further gravitational contraction. With gravitational contraction halted, the heat and energy from the core helium burning again begins the expansion of the star s outer layers. The method by which a star begins core helium burning depends upon the mass of the star. Document1 28 of 42 pages In stars below three solar masses (low-mass stars), helium fusion begins in a more spectacular manner. Helium burning begins explosively and abruptly. This process of quick ignition of helium fusion is a helium flash. The universe does not limit helium flash to low-mass stars; Helium is present in red giants and red supergiants as well. Red supergiants and red giants share the same physical characteristics and properties, with size and luminosity the only major difference between them. To understand this size difference we use our sun as a reference point. Red giants have solar radii up to 10 to 100 times larger than our sun, while supergiants boast a solar radii 1000 times larger than that of our sun. A Low-Mass Stars�last gasp. < 4 Mo Stellar evolution can end in several ways. After a long life, an aging star completes its red supergiant stage of evolution and shell helium burning begins. Since this shell of helium burning is thin, the star becomes unstable. This instability in the aging star increases the temperature and energy within the star, which thickens the shell of burning helium surrounding the helium-depleted core. The shell of burning helium thickens until it can support the pressures of the star�s outer layers. As the shell helium burning increases the temperature and energy of the surrounding outer layers of the star, it ignites the outer hydrogen shell into fusion through thermonuclear reactions. This process of outer hydrogen shell fusion is a thermal pulse. During thermal pulses, the star�s luminosity increases by a factor of ten. This final expansion and ignition of outer layers is the star�s final moments before death overcomes the star. In its final moments, a star ejects its outer layers emitting ultraviolet radiation. This emission of ultraviolet radiation ionizes the ejected gases, giving them a glow known as a planetary nebula. Document1 29 of 42 pages A planetary nebula, the death of a low mass star The ejection of stellar remnants is the low-mass star�s supernova. On Earth, we measure the effects of this supernova in the increased luminosity. After a low-mass star�s death (supernova), it often leaves behind material that forms new stellar bodies. This is the end of stars with low masses (less than four solar masses) because they cannot reach the appropriate t temperature levels to begin the fusion of carbon (C) into oxygen (O). These stars burn off or eject their remaining outer layers leaving only the stellar core behind. Stellar remnants, a star�s afterlife: white dwarfs, neutron stars and pulsars. Due to the enormous mass of this remaining stellar core, the core begins to collapse and fuse. As the stellar core collapses, it electrons become degenerate (a phenomenon, due to the quantum mechanical effects, whereby the pressure exerted by a gas does not depend on its temperature) and stop the gravitation collapse. This contracted stellar core is a white dwarf. White dwarf stars are approximately the size of our planet, but their mass and density is much greater. White dwarfs are so dense, when compared to the Earth, that a teaspoon of matter from a white dwarf would weigh 5.5 metric tons on the Earth�s surface! The universe places limits on the life of a white dwarf, familiar to the main sequence star from which it originates. The white dwarf glows for billions of year from the energy released from cooling thermal radiation. Eventually all the radioactive matter of a white dwarf cools until it reaches the temperature of surrounding space which is a few degrees above absolute zero. Document1 30 of 42 pages A Neutron Star Neutron Stars and Pulsars : Sometimes the core remnant of a low-mass star is too dense to form a white dwarf. In these cases, the core�s stellar collapse forms a neutron star. Neutron stars are incredibly dense spheres of degenerate neutrons (a gas in which all the allowed states for particles, electrons or neutrons, have been filled, thereby causing the gas to behave differently from ordinary gases). Some neutron stars have massive Document1 31 of 42 pages magnetic fields that sweep out beams of radiation into space, much like light beams from a lighthouse. Such �pulsating�neutron stars are called pulsars. This measurable �pulse�of radiation is where this type of neutron star derives its name. Astronomical models suggest that the properties of superconductivity and superfluity dominate the cores of neutrons stars. These models also suggest there is an upper l limit in the mass of neutron stars. These upper limits are in the range of the mass of the largest possible white dwarfs. The Little Bang : The death of high-mass stars. This death of a low mass star into a white dwarf contrast with the final stages of highmass, main sequence stars (i.e., greater than four solar masses). Unlike low mass stars, high mass stars can extend their lives through the fusion of elements heavier than carbon. After the fusion of helium ends, high mass stars begin burning carbon as their next fuel consumption stage. Carbon burning begins when the star s core reaches temperatures greater than 600 million degrees Kelvin. The greater the mass of the original main sequence star the longer the star continues to burn heavier elements. Incredibly massive stars continue fusion from helium until they create iron (Fe) in their core. In this fusion process, these massive stars create neon (Ne), magnesium (Mg), oxygen (O), sulfur (S), silicon (Si) and finally iron (Fe). Each stage of fuel consumption parallels a raise in the star�s core temperature. Neon fusion begins when the star�s core temperature reaches one billion degrees Kelvin followed by oxygen at 1.5 billion degrees Kelvin. Silicon fusion does not begin until the star�s core temperature reaches an amazing 3 billion degrees Kelvin. As the star begins each new fuel consumption stage, its lifetime in each stage becomes shorter. The following table illustrates the amount of time an aging star spends in each fuel consumption stage. Document1 32 of 42 pages A H-R diagram (The red line represents the Main Sequence stars and their evolution toward the top right hand corner of the diagram). These massive stars continue to exist by burning the heavier nuclides (magnesium, silicon, phosphorous et al) until they reach the heaviest nuclide that fusion can produce, iron. The iron rich cores of these massive stars can reach sizes equivalent to the Earth. The size of the surrounding shells burning the various lighter elements (H, He, C, O) dwarf this iron core, as they reach sizes that would extend to the orbit of Jupiter. When the core of a high-mass star consists completely of iron, fusion can no longer take place. At this point, the mass of the daughter (one iron nuclide) is heavier than the total mass of the parent nuclides used to produce iron. With this difference in mass, the process begins to absorb energy instead of releasing it back into the star, whichunbalances thethermal equilibrium of the star. With fusion ceased and the thermal equilibrium unbalanced, a star�s only source of energy is from the contraction of the star�s outer layers. This contraction raises the star�s core temperature to 5 billion Kelvin and increases the pressure to the point that the iron core collapses upon itself. The incredible pressures that the collapse of the star�s iron core force many of the existing (but not all) protons and electrons to combine into neutrons. For a moment the star releases this abundance of neutrons as neutrinos. In this brief escape, many neutrinos Document1 33 of 42 pages bombard the iron core and combine with iron nuclides to form the elements heavier than iron. These escaping neutrinos and the electromagnetic forces that repel protons and neutrons force the star into a final expansion. A Star�s Eulogy, a Supernova. This final expansion sends a shock wave through the star�s outer layers until it reaches the star�s surface creating a supernova. This shock wave ejects all the material of the star, including the core into the cosmos leaving behind only a nebula of cooling gasses. Document1 34 of 42 pages A section of space before the supernova Document1 35 of 42 pages The same section of space after the supernova Creation of Black Holes Stars too big for their britches : Black Holes. Sometimes high-mass stars are too massive to become white dwarfs or neutron stars. A high-mass star this massive also has the gravitational forces to prevent the escape of stellar matter through a supernova. Stars with this great of mass become black holes at the end of their stellar evolution. Document1 36 of 42 pages Black holes are the result of the overpowering weight of the star�s massive outer layers pressing inward from all sides on the core causing the rapid contraction of the dying star. With this great mass pressing inward, the star�s core can no longer support the star�s outer layer structure. The matter within a high-mass star in this process is so condensed that the gravity produced is strong enough to prevent the escape of light energy from the cooling radioactive material. Light cannot escape because the escape velocity at the star�s surface is greater than that of the speed of light. Gravity of this magnitude has profound effects upon the star�s shape and on the structure of the surrounding space. With the star collapsing in upon itself, the star changes from a spherical shape to a broader plane in space. The gravity from the center of this plane curves its surrounding space to create a whirlpool-like drain that absorbs the star itself and all surrounding matter. An Artist rendition of a black hole event horizon (notice the whirlpool like shape that leads to the blackhole s singularity). This plane at the mouth of the whirlpool-like drain is called an event horizon. The immense gravitational forces of the black hole compacts all the matter within the star after it moves past the event horizon of the newly formed black hole. The gravity of the black hole compresses the star�s matter, as well as any other matter captured by the black hole�s gravitational forces through its existence, to infinite density. This infinitely dense matter within the black hole is called a singularity. The gravitational pull of a singularity is so great that it pulls matter from all surrounding areas into the event horizon of the black hole. If light cannot escape black holes, how do we find them? Until recently black holes were only theoretical speculations, but with today�s technology we are now finding black holes by measuring x-ray emissions in space. Document1 37 of 42 pages As the black holes immense gravity captures surrounding matter, this matter accelerates toward the event horizon and singularity and releases high amounts of x-rays. These x-rays leave a trail through space as the black hole absorbs the matter into its singularity. X-ray trails leave �halos�around the black hole called accretion disks. Another method of detecting black holes in space is their effects on binary star systems. When a black hole occurs close to two stars, its gravity impacts their spatial relationships. Each star�s gravity has a calculable effect upon the other�s orbit, however the invisible black hole affects these orbits. An Artist rendition of a blackhole syphoning plasma gas from a nearby star (this acceleration of matter emits x-rays that form a accretion disk and make the blackhole detectable [x-ray emissions represented by the arrows]). To contrast the idea of black holes being places in the universe with infinite lifetimes where no matter (including light) ever escapes, the universe holds a few exceptions. The British physicist Stephen W. Hawking has proven that black holes do in fact have definite lifetimes. As black holes reach the end of their life, they begin to evaporate. In the final stages of this evaporation, the black hole reverses itself and pours matter back out into the universe. When a black hole begins to eject its matter, it is a white hole. With this transformation in the life of a black hole, the universe appears to maintain the fundamental universal energy-matter law with Document1 38 of 42 pages this process. While the black hole, to physics laws, is an unbalancing factor in universal laws, the white hole exists to restore this matter and balance. Reference 1. Universe (fourth edition). Kaufmann, William J. III. W. H. Freeman and Company New York, 1994. 2. http://www.astro.washington.edu/strobel/evolution.notes/evolutio n.notes.html http://spiff.rit.edu/classes/phys230/lectures/star_age/evol_hr.swf http://rainman.astro.illinois.edu/ddr/stellar/pics/hr.jpeg Document1 39 of 42 pages http://archive.ncsa.illinois.edu/Cyberia/NumRel/BlackHoleAnat.html Expo/Science & Industry/Spacetime Wrinkles | Forward | Back | Up | Map | Glossary | Information | Anatomy of a Black Hole By definition a black hole is a region where matter collapses to infinite density, and where, as a result, the curvature of spacetime is extreme. Moreover, the intense gravitational field of the black hole prevents any light or other electromagnetic radiation from escaping. But where lies the "point of no return" at which any matter or energy is doomed to disappear from the visible universe? Document1 40 of 42 pages The Event Horizon Applying the Einstein Field Equations to collapsing stars, German astrophysicist Kurt Schwarzschild deduced the critical radius for a given mass at which matter would collapse into an infinitely dense state known as a singularity. For a black hole whose mass equals 10 suns, this radius is about 30 kilometers or 19 miles, which translates into a critical circumference of 189 kilometers or 118 miles. Schwarzschild Black Hole If you envision the simplest three-dimensional geometry for a black hole, that is a sphere (known as a Schwarzschild black hole), the black hole s surface is known as the event horizon. Behind this horizon, the inward pull of gravity is overwhelming and no information about the black hole s interior can escape to the outer universe. At the singularity, though, the laws of physics, including General Relativity, break down. Enter the strange world of quantum gravity. In this bizzare realm in which space and time are broken apart, cause and effect cannot be unraveled. Even today, there is no satisfactory theory for what happens at and beyond the singularity. Binary eclipsing stars http://astro.unl.edu/naap/ebs/ebs.html Document1 41 of 42 pages http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/041002a.html The Question Document1 42 of 42 pages (Submitted October 02, 2004) Is it at all possible for a companion star to survive a supernova explosion? Do any of the binary systems that we know of, where one of the bodies is a neutron star or black hole, have the original companion star present, especially where the compact object is near enough to draw surface gas off the visible star? Or have all these systems simply come into existence after the explosion and the nebula have dissapeared? The Answer Absolutely. All X-ray binaries have either neutron star or black hole components. By definition these stars must have experiences a supernova stage. Either their companions survived the supernova event or were captured tidally some time later. Certainly these companion stars did not form close to the compact object after the supernova. Energy from the compact source would prevent any cold gas from contracting in the vicinity.