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by: Carlotta Dare DVM


Carlotta Dare DVM
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This 98 page Class Notes was uploaded by Carlotta Dare DVM on Saturday September 12, 2015. The Class Notes belongs to Physics 20 at University of California - Irvine taught by Staff in Fall. Since its upload, it has received 44 views. For similar materials see /class/201921/physics-20-university-of-california-irvine in Physics 2 at University of California - Irvine.




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Date Created: 09/12/15
The Milky Way Galaxy Chapter 15 Topics to be covered 1 Contents of our Galaxy Interstellar MediumISM and Stars Nebulae 2 Distribution of galactic clusters and center of our Galaxy 3 Structure of our Galaxy Central bulge Disk and Spiral arms 4 Methods of observations through dust Ir Radio Xrays gammarays 5 General description of Galactic Center Spiral arms birthplace of stars 6 The HI and H11 regions of hot and cold hydrogen 7 The 21cm radio spectral line and mapping the galactic Hydrogen 8 Cradle of new stars Giant Molecular Clouds GMC Our galaxy contains different objects which show their presence by different observational techniques I note some important points 1 Type of radiation emitted depends on the temperature of the emitter What is qualitatively the relation between T and peak wavelength Higher the temp smaller the peak wavelength Hot stars violet UV Xrays as temp increases Cooler stars visible IR microwaves Still cooler environs Radio waves 21 cm line for neutral cold H 2 Transit from source to earth modifies the signal obscuration reddening Visible light obscured by dust particles IR radio and Xrays generally unobscured by dust Satellite observatories WMAP microwave IRAS IR COBE DIRBE uvir Avoid atmospheric absorption Ground based observatories Adaptive optics corrects for atmospheric uctuations large multimirror Telescopes Rapidly Orienting telescopes Interferometry and VLA and VLBA radio telescopes These have made us learn a great deal about our milkyway galaxy The galaxy contains stars gas dust magnetic fields black holes and Dark Matter We Will survey this knowledge Our galaxy in 23 Giga Hz microwaves as seen by WMAP satellite emu YhomsanEmole cm Galactic disk is clearly seen Map is in coordinate system Whose X axis is longitude away from the galactic center from 180 to 180 deg and y axis is the latitude away from the plane of the galactic disk Microwave radiation from the galactic disk is in red Composite of galaxy in Visible light Obscuration by Dust is clearly seen 2004 ThomsonBrookscola A View of a region of the galaxy not Visible to the eye is shown below Perseus spiral arm of the galaxy ln radio and IR light false image mam womanEmle Col Drawing of Milky W Galaxy seen in line of sight of 200000 ly Glnbular clusters i Bulge Nucleus y inL t39minutgs V 5quotquot Ea r i Mercury Venus 39 EuzooI lmmsanEmls Cola Notice the distribution of globul clusters 11 erical region around the disk and distribution of gr 39 ting matter extending in surrounding Halo a at View The Whir1p001 galaxy Where would the sun be Open Clusters meIYMnswdEmoVs Ooh A11 39 8 drawing of our Milky Plan View Spiral arms sho W11 Position of sun indicated 0 9quot st PersausArm in nun iy gt lt It was Harlow Shapley who first discovered that distribution of globular clusters of stars was not centered on the solar system but around a point some 28000 light years distant from the sun surrounding what we now know as the central bulge of the galaxy There are regions observed in the galaxy which are called nebula They are large diffuse regions They are of three types Bright nebulae Gas heated by radiation from hot young stars and the gas in turn emits radiation which we observe Dark nebulae Regions populated by dust which absorbs visible light and which remits in the Infra RedIR wavelength region Re ection nebulae Region which contain material which re ects the light from nearby stars Bright and Dark Nebulae Shows the dark absorbing region of horsehead shape Lot of glowing gas Provide evidence for presence of ga and dust in the galaxy reflection dark bright mom fhonsunEmols col A collection of young OB stars are seen in this image Pleiadis Open cluster Star forrning region x Visible a i 39 hotyoung stars his a xray image 1 o i Viewing the galaxy in different wavelength bands is one of the ways in which we explore the structure of the galaxy This is shown in the next figure u mam Ymnsmt moks we Reveals distribution of stars gas dust and high energy sources ALF Iccfl Pan s Inm39feJMk 11539 a m HI Nauru 21 Lam HI Iom39zoJ Hot stars shock V is and dust grains HALO spherical and large GALALTIC wryD Hot re ions HII g 392 I39ll5H nahave T h If 39K BI3uamp Vgt70Z Mauvex MO NODuS T 30 kpc 4 H1 gLouDs I LvlIDOu 750 k Ema Gram 5 1m V4107 MasswwoLa 311577 su my 1 5L0 mm L SN 3am WA 3H5quot 2153 5M Mass Eu wh m 1M0 I Observing the galactic center region Dust makes the galactic center region obscured in visible light It can however be observed in IR X rays and Radio wavelengths It reveals a very active and exciting features The center is at a location close to that of Saggitarius A Maps of the galactic center region by different techniques is shown next Why is the dust transparent to IR waves Short wavelength waves less than or equal to dust grain W size are scattered or absorbed by the dust grains They usually re emit in longer wavelengths Long wavelength waves are unaffected by the presence of dust grains which are much smaller in size than the wavelength and are transmitted through Consider deep sea waves passing by a small boat The boat is lifted and then lowered by crests and troughs respectively but the deep sea waves keep on going Ripples on a pond will get scattered in all directions by the presence of the boat in the pond Sgr D Hll Sgr D sun SNR 0310 1 Sgr 32 39 SgrB1 Am gt Mouse SNR 3590403 2004 Thomsona39Bruoks Cole New SM 0 34110 Threads New feature 7 I The Cane 39 M Background Galaxy gt Threads New thread The Pelican Sgr Coherent structure L u SgrE sun 359 14105 39 39a a 2004 vnommamm Cole Hubble Telescope IR of center Galactic plane Tornado ISNHPI Wide field VLA map at 90 cm wavelength A detailed look at the Arc in the previous figure at right angles to the d k We believe there are high energy electrons in the filaments and they produce X by collisions with a million solar Inass cloud of cold gas Ealactic plane Stars Revolve around the galactic center quot223 19942 19955 2005 ThurmanEmu Dal Other stars orbiting the go are shown in Fig 15 15 b in the text An analysis of these star orbits show that there must be a giant black hole at the location of Sag Aistar with a mass about 4 Million solar masses There is an extended source of X rays as observed by Integral mission at the galactic center See Figure 15 16 in text Spiral Arms As we are located in the galactic plane it is dif cult to trace out the spiral arms By analyzing distances and directions to objects of various types we can infer the spiral structure We examine other spiral galaxies some of which we see in their plan View and nd that young starsO and B type and gas and dust are preferentially distributed in spiral arms which are the cradles of star formation By plotting out directions and distances to open clusters and H11 regions we can trace out bits of three spiral arms We can map HI regions and dust regions by techniques of radio astronomy In particular using the 21 cm line of neutral hydrogen Spiral Arms continued We can determine that our sun revolves around the center of the galaxy with an enormous speed 200 km sec It completes one revolution around the galactic center in 240 million years As the sun is about 46 billion years in age it would have completed some 18 revolutions These spiral arms are caused by spiral density waves A matter compression and rarefaction wave similar to sound wave along the direction of such waves probably originating in or near the galactic center and spiraling out Where matter is compressed conditions are appropriate for star formation and a cluster of stars are formed These open cluster stars tend to migrate So at different epochs one has different stars along the spiral arms Interstellar medium ISM The most predominant element in our galaxy is Hydrogen If Hydrogen is cold it can either eXits in atomic or molecular state Cold hydrogen regions are called H I regions and they have higher density than hot hydrogen regions which are called H 11 H 11 regions are formed by energetic radiation from hot stars which disassociate neutral hydrogen atoms into a P and an e When such a dissociated atoms recombines to form neutral hydrogen again it emits characteristic H emission lines Thus can be identified This is shown schematically in the next slide O I l 39 Q 39 G w 39 I H II JHelium atom Q I 9 D 0 V Hydrogen atom u G G G Q G Interstellar medium ISM H 11 regions are formed by energetic radiation from hot stars which disassociate neutral hydrogen atoms into a P and an e When such a dissociated atoms recombines to form neutral hydrogen again it emits characteristic H emission lines Thus can be identified A special quantum property of neutral hydrogen makes it detectable by radio astronomy This property has to do with intrinsic spin of protons and electrons and leads to the emission of fixed wavelength radio wave of wavelength 21 cm Energy eV Lya1216A Y kpc O 5 10 X kpc 9 am Hammocks Co s The sun is hot fireball of gas We observe its outer surface called the photosphere We determine the temperature of the photosphere by measuring its spectrum The peak intensity of light as a function of wavelength of light received at earth Using Wien39s law AWT029cmXK 0 3 2239 gt E 9 E E 7500 K E 5 a ll 5 E 9 as 6000 K 4500 K l l x 5000 A 10000 A 1 0m 2 pm Wavelength mmmmmmmmm out 1A one Angstrom l X 10 86111 Its surface temperature is 5700 deg K Composition of the photosphere This is determined by examining the dark lines in the spectra caused by absorption by different elements and bright lines due to emission by elements First observed by Fraunhofer 197 r r I 39 The Most Common Elements in the Sun s Photosphere Atomic Symbol Number for emch 1000000 mm or hydrng39em here are H 1 98000 atoms of mlium He 2 030 atoms or mygcn o s 400 itnms nl carbnn C 6 120 atoms 01 neon Ne 10 100 Alon ul nitrogui N 7 47 atoms of run Fl 16 Mac mommanets we Analysis of these spectral lines leads used to nd composition Element Helium was rst discovered by this type of observation before we isolated it on earth The percentage of elements heavier than helium is called metallicity of the star and for the sun it is 17 Structure of the sun Parts of solar atmosphere and interior The core is Where sun39s energy Coronal is generated by nuclear fusion streamer the pp chain Its temperature is 15 million degrees K The corona is at high temperature supplied by magnetic energy and contains prominences filaments streamers and coronal holes Sunspots are magnetic with N and south pole strucures emu TmmsmIENDB Colo Amount of energy radiated by the sun is 3 X 1026 Joules sec 01 watts Sunspots and Solar activity First observed with telescope by Galileo in 1610 earth 2004 womansmote om They are magnetic disturbances with cooler gas hence dark They come in pairs north and south magnetic polar Flares coronal mass ejections filaments prominences Flares are tremendous eruptions ejecting energetic particles and radiation Temperaturesreach 5 million degrees Coronal mass ejections CME cause magnetic storms on the earth could wipe out communications in northern latitudes Produce what is called solar wind which travels out of the sun at supersonic speeds and produces an excited region called the Heliosphere which extends up to 100 AU Voyager space craft has finally reached the end of the heliosphere called the heliopause Some examples are Solar Corona during total eclipse Superposed is an image of the sun taken at the same time by SOHO spacecraft an UV image mzoo ho unEmoks Cole COIOna The UV image is used to correlate coronal streamers back to their roots on the Sun s surface Solar prominence and coronal loop sunsp ots cooler regions rw umuw u Coronal 1V During the 1 Solar Activity Cycle The number of sunspots varies with an 1 1 year cycle The Sun like the earth is a magnet with north and south poles and this magnet reverses polarity every 1 1 years So after 22 years the north pole is north pole again When it reverses polarity we have a minimum number of sunspots This is shown in 180 160 120 Annual Mean Sunspot Number mtmmo ooooo O 1750 1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1870 1880 1590 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 Year Year mom womanEms we 2004 Thumsunl rmks Dole In the Maunder minin1um 1645 1715 there were probably no sunspots It was also the time when weather in Europe was extremely cold Sunspots weather connection Annual Mean Sunspnl Number The outer layer of the sun Chromosphere and the Corona can be best observed during solar eclipse when the main photosphere of the sun is covered exactly by the moon Moon and the sun subtend the same angle at the earth and if the moon is in front of the sun and centered then we get full occultation of the sun or total solar eclipse This outer layer can be observed in a range of wavelengths from radio to X rays either from the earth or by satellite instruments The chromosphere seen through hydrogen filter shows a lot of activity its temperature varies from 7000 to 15000 deg K See Figure 10 8 Mass of the Sun Newton39s version of Kepler39s III law can be used to find the mass of the sun This version is given on page 105 of the text Figure it out 52 If P is the period of revolution of the earth around the sun 2 1 year R the radius of the orbit which is 1 AU P and R are related by 2 4 1T2 P Z XR3 CM m sun earth as m is ltlt M we can solve for M earth sun sun neglecting meath and get 2 3 4 IT R Msun X For the earth Plyear3gtlt107sec G P Earth s orbit R1AU15gtlt10 m 30 N 2 puttlng 1n numbers we get M m 2 X 10 kg Newton s constant G66gtlt10 ll m kg Important Numbers for the Sun Solar Radius Rmn696gtlt108meters Solar Mass Msun22gtlt1030kg Surface Temperature Tsurface5700 deg K SOlaf Luminosity LW 2 2 86 X 1026 watts Joules per sec Temperature of Core Tom 15 X 107 deg K Mostly Hydrogen and Helium Average density pm I 1400 kg m3 More about the sun Core is the region Where a large fraction of solar mass resides Central pressure in the sun is 250 billion times atmospheric pressure Central density or the sun is about 100 times the density of water It is still much less than the density of neutrons and protons inside a nucleus Nuclear density is about 1014 grams per cc Density of water is 1 gram per cc Our Sun and the solar system provides two tests of Einstein39s General Theory of Relativity GTR 1 Advance of the perihelion of planet Mecury39s orbit around the sun The distance of closest approach of Mercury39s orbit to the sun varies with time This is with respect to a fixed coordinate system of stars It is called the advance of of the perihelion It is a small angular shift 4311 045 seconds of arc per century Newtonian mechanics at space and absolute time predicts 532 arc sec while General Relativity of Einstein predicts 4303 arc sec 2 De ection of light due to curvature of space time near the sun39s surface In GTR freely moving objects in gravitational fields produced by objects like Sun are described by motion in a curved space making it look like they are being pulled by gravity Time is also warped and the world is a curved space time manifold Imagine a rubber membrane representing space then the presence of a large mass like the sun at some point curves the space as shown in the figure Orbit of Mercury pcfs iin o a Star apparent position Perihelion of Mercury Eanh cammnmvamks cm A stable orbit of Mercury is shown in red Path of light ray from a distant star which goes close to the sun on its way to the earth is shown being de ected by the curvature of space near the sun Basic Concepts of Temperature pressure density brightness gravity hydrostatic equilibrium Matter Matter comes in different states Gas liquid solid Matter is made up of atoms and molecules Molecules are made up of atoms Atoms are made up of a tiny nucleus and a cloud of electrons Nucleus is made up of protonselectrically charged and neutronsneutral Most of the mass in ordinary matter comes from the mass of the nucleus Random motion of molecules red and a that of alarge Dust grain 1 930 r r 105cm I am the Very Model of a lodem Ma or Atom Neutron Proton l I 103912cm Atomic nucleus Vibrations of the atoms in a large molecule 1 Different states of ordinary matter A gas for instance air can be compressed Has no well defined size A liquid or a solid is very hard to compress Liquid ows while solids have a definite form Density represents how densely packed are the constituents Solids have a higher density than liquids which have a higher density than a gas at normal temperatures Measured in kg per cubic meter Water density is 1000 kgcubic meter 2Temperature Temperature is a measure of random motion of constituents In a gas this is rectilinear motion of constituents which only interact when they collide amongst themselves or with walls of their container Fast moving molecules or atoms have a higher temperature than slower ones Measured in degrees Kelvin We consider a simple example to illustrate how a gas behaves A Gas lled into a rubber balloon Fixed quantity mass of gas inside the balloon Quantities connected Pressure Temperature and Volume of gas in the balloon Relation is called Equation of State of an ideal gas PVRT Where R is a constant called the Gas constant Also called the ideal gas laW T State of the gas described by T1 VlP1 and Vloc 1 1 Defined by Temp and Pressure gas in a balloon Pressure of gas pushes outwards on the balloon and elastic forces in the balloon plus pressure of the atmosphere balance this pressure so balloon has a volume and a density Lowering the temperature taking away heat causes the balloon to shrink Q density of the gas is mass of the gasvolume of gas New State of the gas is described by T 2 P2 and V2 and P2ltP1 because balloon is not stretched as much and V2 shrinks as Temperature has dropped I If heat is added to raise its temperature the balloon can be made to expand again to original size Mass of gas is unchanged Why Relevance to a star like our sun Sun is a ball of hot gas With mass M and radius R What keeps the ball of gas together The pull of gravity Which tries to crush the sun to a point The sun does not get crushed because pull of gravity is balanced by gas pressure The pressure has to be very high to balance gravity of such a massive object In the fixed volume of the sun39s ball of gas this means that the Temperature must be very high That is Why the sun is hot Pressure at center 250 billion atmpospheres At its center the sun39s temperature is about 15 million degrees K For a fixed radius a star39s central temperature should depend on its Mass M Which provides the crunch Larger the mass higher the temperature Sun39s core and the rest of the reball gt Tc 15x107 K Pc 34Xloualmospheres 8 Average Temp 10 million degK Dc my lgmm hm L Average density 1400 kg per cubic meter 15 E In a E A At these high temperatures the atoms are 700000km o 700000km DISTANCE FROM CENTER all ionized and constitute a state of matter calle plasma 7 positively charged nuclei and negatively charged electrons Heat to maintain such high temperatures is provided by nuclear fusion in the core of the sun Combining 4 H into 1 He nucleus Stars Which are stable and shining because of fusion of H in their center form a special category of stars called the Main Sequence The time taken to burn up 10 percent of H in the core or these stars determines the age of the star It depends on their mass and their radius Which determines the central temperature of the stellar furnace 5 oronal streamer Core Radiative zone mmnmmamm om Temperature at Genter of Earth 5000 Canter of A 39 Jupiter m 5M0 10 M0 15 6 Mm 24Urm atmic mmmr mg 39 11 g 3 L I I be eW mw39 1 33000 miunon I For stars on the main sequence Central temp of a star is related to that of the sun39s central temp by star Msmr Rsun M sun star Surface temperatures of the stars are much cooler as stars radiate away their energy at the surface Low mass large radius stars are coolest High masssmall radius stars are the hottest Hot objects radiate energy Common examples 1 Light emitted by an incandescent bulb 2 Heat produced by the bulb or a ame 3 Streak of lightening Air molecules heated to high temperatures What is the nature of this radiated energy It is electrical energy in the form of electromagnetic waves EM waves How is the color wavelength or frequency related to the temperature of the radiating body Higher the temperature shorter the wavelength Blue stars hotter than red stars The range of Electromagnetic Waves 7250K 4100K Ann nm no In waveiengm menus m4 1039 r gammavays l x rays uluaviolex mmva main 5 1d very hot 10 K 30K very 00 El negumcymem o 1019 7 7 101 ms A mergyelerman volls w 1042 cnmmommmvwmw Waves defined by wavelength frequency and speed speed wavelength x freqeucny Measure of energy radiated A 100 Watt bulb consumes 100 Joules per second of electrical energy Some of it goes into heating the filament and the rest appears as light or EM energy Sun emits a total energy of Luminosity 4 X 1026 watts or Joules per second Using Einstein39s equivalence of mass and energy E Mc2 this corresponds to a mass of 685 million tons of matter Mass of the sun in tons is 2X 1027 tons The fractional loss of mass of the sun per second is M35 gtlt10719 2 X 1027 tons It would take 1011 years to completely use up sun39s mass Star radiates in all directions from its surface Only a tiny amount reaches the earth Observed brightness of a star depends on how much it radiates how far it is and whether some light is absorbed or scattered on its way If L is how much energy it radiates per second or watts The amount of energy received per square meter at a distance R L 13 B at if no light is scattered or absorbed on its way to earth So relating B to L requires knowledge of R and absorption or scattering Stars and their properties Chapters 11 and 12 To classify stars we determine the following properties for stars 1 Distance Needed to determine how much energy stars produce and radiate away by using the inverse square law and determining reddening due to dust 2 Luminosity L This is the amount of energy generated in the star and released as EM radiation 3 Temperature T Found by using colors to determine the surface temperature 4 Radius By using the relation 5 Chemical composition L surface area A 4n R2gtltT4 Determined from absorption line spectra The Milky Way Galaxy has 1011stars and its disc has a diameter of 105 Ly The solar neighbourhood is about 100 Ly in diameter and contains about 1000 stars See closer look ll5 How we measure basic stellar parameters Note that apparent brightness is determined by star39s distance and obscuration by intervening dust The relationship between Luminosity HR HertzsprungRussell diagram 5 RedGmnts Cent quot1191quotan BAmx but large mm L quotGiantquot Luminosity L on log scale Sun 1 Hut quotWmquot ngh Bares but low mm L quotdwarf Blue line is the Main Sequence on this plot Compare with Fig 11 13 in text Stellar Masses Determined by analysis of motion of binary star systems Eclipsing binaries optical binaries or spectroscopic binaries From such measurements we can obtain relation between stellar masses and their luminosities shown below mainsequence stars looooiO 100iO LUMINOSlTY my up i u HUS See Figure it Out 116 for Luminosity Mass relation The one shown here is LocM3 For each position on HR diagram main sequence there is a corresponding stellar mass 5 10 L 40 SnlarMasses e Dentmlrempemmre needs m be HIGHER m suppsrr all the extra weight HigherT means Higher Luminnsiry gt 39E 3 1100 SslarMass srars ster eenrr T 11000 m suppsrr lnwerweighr AsT gses dawn ss dses L 40 000k 300 39 Tempemmre Stars are shine and energy is provided by nuclear fusion see section 122 to 124 Stars have masses greater than 0085 Msun For Brown dwarfs see126 Jupiter which is a planet has a mass Which is 0001 Msun Which is 100 times less than the mass of the lightest star Lifetime of a star depends on its mass 1010 ears TM A 2 M374 Large mass stars live a short life For M 10 Msun Lifetime is only 100 million years A star With 02 Msun Will live for 250 billion years section ll8b in text Usually nuclear fusion takes place towards the center of the star where temperatures are high enough for fusion to occur Energy created is transported to the outside of the star by two main mechanisms Convective and Radiative transfer Convective transfer thoroughly mixes the nuclei which are fusing throughout the convective region while radiative transfer limits the fusion region towards the central core Evolution of stars and stellar deaths Chapter 13 l Smallest stars M lt 03 M M have convection They operate on the pp fusion cycle in their cores until 10 of H is used up 2 Intermediate stars 03 M M lt M lt 15 M M have radiative core and convective envelope They also use the pp fusion cycle until 10 of hydrogen is used up After that they become red giants and move up the RGBRed Giant Branch 3 Stars with mass Mgt 15 M sun have a radiative envelope but a convective core They fuse hydrogen to helium but by the CNO cycle They have higher temperatures in the core They also fuse H to He until 10 of H is used up In the next two figures I show the evolution of different mass stars on the HR diagram 1 Those leading the formation of planetary nebula and white dwarfs See figure 138 in the text and section 131 d and 2 Those leading to the big bang of a Supernova See section 132 in text 0 MS star with lt 15 Misun fusion pp chain Exhaustion of core hydrogen Rapid contraction envelop expansion Shell H burning 7 Red Giant Core helium ignition 7 helium ash for lt 15 sun 7 very fast HRB Core He exhaustion Ef EE H Shell helium burning Super wind removes H 39 M lt 8 sun planetary nebula phase with a nucleus ofC star 5 98quot 9 White dwarf remains Structure of star from 5 to 7 is shown in the next slide Asymptotic Red Giant Branch Structure of Planetary nebula stage in region 8 is shown slide after that Asymptotic Red Giant branch Later the red giant becomes a Horizontal branch star when temperature is high enough and helium fusion can take place in the core with hydrogen burning in the outer shell The star moves horizontally to the luminosity being approximately constant but temperature increasing 7 on the way to becoming a white dwarf l He fusion in core H fusion in shell 1nert H envelope Horlzonnal Branch Star Structure This hot central core and a shell of hot gas which appears like a ring is the planetary nebula stage 2 E K F Hot AGE star cure CarbunfoxygeaneIH dcgnerare 6 support it against gravity Expanding Shah of FHa CID d Halpha photons A Evolution of a massive star Mass gt 8 solar masses SIN6n 10 For stars with Mass gt 8 solar masses Here sequential fusion of C Ne O Si Short time scale Lot of neutrino emission 10 Core collapse after Fe is formed in seconds Supernova Type II What remains is a neutron star and a supernova remnant Sometimes for even more massive stars a black hole is produced instead of a neutron star Evolution of a binary with a compact object and a companion star Supernova Type I In Binary stars the two stars affect each other39s evolution and exchange mass This can lead to instability of the massive star white dwarf leading to the formation of Supernova Type I Main Sequence Red Giant Branch Horizontal branch and White dwarf trajectory 100 000 10 000 1000 J N 5 100 5 5 q Horizontal 9 U E branch 2 10 39a 8 3 White dwarfs 01 001 I l I I I I 30 000 20 000 10 000 6000 4000 3000 Surface temperature K Figure 1314 A schematic Hertzsprung Russell diagram for stars The diagram is also known as a colour magnitude diagram or a luminosity temperature diagram Most stars lie in the regions of the diagram as indicated The names of the various sequences are shown The masses and luminosities of stars are quoted in terms of the mass and luminosity of the Sun which are written Mo and Lo respectively Again a recapitulation of star formation and stellar Evolution Star Formation Trajectory on HR diagram sequence Initially the contracting gas cloud has a very large area and is cool so it starts from top right hand corner of the HR diagram As it contracts its area decreases and so does its luminosity and it gets hotter so the track moves down and left on the HR diagram 49 RsoLar premem sequence rmck leoLar Lumi 05in When the temperature at the center of the star reaches sufficiently high nuclear fusion can start converting hydrogen into helium and the star comes to a halt on the main sequence Main sequence Surface Temperature Sun39s path After it arrives on the Main Sequence it continues to remain in hydrostatic equilibrium for 10 billion years until it uses up enough hydrogen so as not to be able sustain hydrogen fusion in the core Then its core contracts gets hotter and its outer shell continues to fuse hydrogen This causes the outer shells to expand and it becomes a red giant moving upwards and to the right on the HR diagram Red Giant Branch RGB High Re 1 Giant B ranc 391 fig Ene Core contraction quot xxquot K rElias shell H fusien 3 I c I J quotA Equi1Therrrlalpre55ure i g isbarely winning I Jr 5 x Prerriain If sequence 39 LLirrlinrmsrtjlI 3quot f2quot contraction 2 quot V 39i 39 g i Energy GPE M31 Equcn39x f ECU 111 Grav j is winning Energy Core H fusion Equil Graii39itglI v 5 thermal pressure Lew Hot Cool Temperatu re Key Concepts 1 Ultimate fate of a star depends on its initial mass 2 Massive stars and accreting white dwarfs or neutron stars end their lives with violent explosions Supernova Two types 11 and I respectively 3 Supernovae explosions distribute heavier elements into galaxies Ashes from which we developed Stars are formed from gas and dust clouds Mass of proto stars depend on density temperature and nucleation in the gas and dust clouds When a large numbers of stars are produced from the same initial gas or dust clouds we get star clusters open clusters in the disk and globular clusters distributed more or less uniformly around the galactic center Elemental composition in order of abundance HHeCOFe Low Z elements Deuterium Lithium Beryllium and Boron Traces of elements With atomic number Zgt26 cooked in SN explosions Light Elements H and He produced in the big bang Stars are consumers of hydrogen and cannot produce enough helium Some questions 1 What supplies energy to the stars so that sun like stars shine for 10 billion years Answer Nuclear Fusion 2Why does nuclear fusion require high temperatures Answer One must overcome the electrical repulsion between nuclei of elements which undergo fusion One needs more than 10 million degrees temperature for fusion to proceed 3 How is this high temperature created Initially when stars form this energy comes from Gravity causing high pressure as matter contracts and hence high temperature 5 What keeps stars like the sun stable Answer Hydrostatic balance between gravitational pull and the push due to gas pressure 6 What keeps stars with very high density stable Answer Quantum pressure of anti social fundamental particles like electrons white dwarfs or neutrons neutron stars 7 What happens when graVity is greater than quantum pressure Answer Matter collapses into a singularity called a Black Hole Synthesis of Elements from H and He occur in stars The process is nuclear fusion Heavier element fusion needs higher temperatures Some fusion processes Step 1 Temperature range 107degrees Kelvin Process Conversion of 4 protons into 1 helium via the p p cycler Releases mass energy 07 mass of a proton Step 2 Temperature range 24 to 36 gtlt107 degrees Kelvin Process Conversion of 4 protons in to 1 Helium via the CNO cycle Step 3 Temperature lOSdegrees Kelvin For some stars there occurs a Helium Flash Process fusing of 3 Helium nuclei into one Carbon nucleus I Step 4 Some stars Which are more massive continue on Without helium ash to make carbon and heavier and heavier until Iron Fe is reached This last process is a runaway process which leads to formation of elements beyond iron through neutron and photon reactions So much energy is released and the material falling in from the outer layers hits the Quantum pressure wall of the electrons white dwarfs or neutrons neutron stars in the core and causes the star to explode It explodes releasing energy close to the mass energy of our sun in a few seconds This explosion is the Super nova It can leave behind a neutron star or a black hole Stars whose mass is in the mass range of the sun39s mass do not end up in an explosion because their central temperatures cannot reach high enough to fuse beyond carbon Their deaths leave behind a white dwarf objects with mass of the sun and radius of the earth Next two slides show the HR diagrams for star clusters Stars in a cluster have two properties which are very important 1 They are all at about the same distance so their relative luminosities can be determined well if the distance to the cluster can be estimated 2 Their turnoff point in the HR diagram is a marker for their age This is because the turnoff point occurs when hydrogen fusion ends From the luminosity and the energy production rate from hydrogen fusion we can estimate when hydrogen fuel is depleted In hydrogen fusion some 25 MeV of energy is released When 4 protons combine to form 1 Helium nucleus It continues until 10 of the hydrogen is used up Then there is not enough hydrogen in the core to sustain the p p chain The outer layers are still burning hydrogen and they get hot and expand Beginning of red giant phase In the next figure Which clusters are older and which are younger http observearc nasa govnasaspacestellardeathstellardeath2b html m l Luminosity gt O Absolute visual magnitude N 4 a mu quotammonia ooh A0 F0 GO K0 Spectral type lt Temperature 10000 100 Luminosity Suns Hor39rzomal 15 A 39 branch RR Lyrasi ga39p 39 l c O Luminosny Apparent visual magniuda y F0 GO Spectral type Temperature Balumuml rmbco e Facmr of 100 In b ghmess Next slides show the life histories of 1 Stars which do not end up as supernovae These develop through the planetary nebula phase and become White dwarfs 2 Massive stars Which end up as Supernovae Type II w My new Wm Mquot mencam n Mmum mm V 0 mm 5th a w Mu hydragenrhumm an daume sheur bummg mm WM 5 m nalmm Dummn r nydrogznrhuml hehumrbummg 3m cave men halmm hy mqsnhuml Subglanb39 a g m cm mmm emu u aca unvpemluva WW h qu smwm mwm Mm m r mm umnqmm wqua mam m w mm m w Mu 39 w w mm mm mummmmw WWW WWW mlumnx mkwwwmngvc mama WA Mmmai uumm wu m mm W Lung m n mgcn39mvmn Wm 3 m anew MW WWW mm m quotmm mammwmmw Au WWW mm ayvv39amuawjhuu w Mmen mmmmwwmmma m WWW Jr4 mumMwmmmm nwwwn mmw m maw nrwru m n mm wannun NW Mm w m mum 9 39E a g o a 339 39a o E E 2 10000 6000 surface temperature Kelvin What provides the energy to keep sun shining Let us discuss the sun We know its mass and its luminosity MW2gtlt 1030 kg and LW4 x1026wazzs A If sun was burning coal it would last 6300 years only B As sun shrinks it releases gravitational energy which depends on its mass and radius At the rate it is emitting radiation luminosity it would last only 16 million years if this was the source of energy in the sun much shorter than the age of the earth What is the age of the earth So we need another source of energy Only in the 20th century the answer was established First some words about forces of nature There are four basic forces Gravity Electricity radioactivity and nuclear Chemical burning like the one just discussed is governed by electrical forces and does not work Gravity also does not work We must turn to the other two radioactivity which involves the weak force and nuclear binding which is due to the strong nuclear force which binds the nuclei of atoms of elements Transforming elements one into another was discovered around 1900 With the observation of radioactivity Study of nuclear physics required bombarding elements With energetic subatomic projectiles and observing What happened Reactions Which transform nuclei just like burning coal require energy however they also release energy If energy released is greater than energy needed you get a net generation of energy or heat Understanding how to estimate this energy release needed Einstein39s famous relation connecting mass to energy predicted by his special theory of relativity E mcz r p 108cm Neutron quot Proton U 339quot quot399 Vl39V Model 1139 I I a Modern Mag or Atom 0 Atomic nucleus We are talking about reactions of the nucleus of the atom Consider a schematic nuclear reaction in which Two nuclei collide A and B and produce two new nuclei C and D the reaction is A B gt C D If total mass of CD is smaller than total mass of AB then Some mass is lost and according to A mMCMD MAMB this appears as energy according to E 2A m c2 This is the source of energy which keeps our sun and all stars shining brightly in the night sky Hydrostatic balance in a star is maintained by stable equilibrium between gas pressure and gravity Gas pressure depends on the temperature of the core and the fusion reaction it supports A star39s energy source is nuclear fusion which builds up heavier elements out of the lighter ones Nuclear burning depends on 1 Temperature of the region Where fusion takes place 2 Chemical composition of the star 3 For any fusion chain it depends on the density of the constituents in the core SL111 core very hot 15 million deg K Nuclear fusion fusing 4 H nuclei into 1 He nucleus takes place in the Core Releasing 64x 1015J0ules kg of H burned Pressure of the hot gas in the core supports the inward force of gravity perfectly as long as there is sufficient hydrogen burning It can be shown that when 10 of the hydrogen in the sun is burned up the reaction shown in next slide does not generate enough energy and the core starts to get crushed Nuclear Energy Sources in stars The pp Chain 3 gtlt106 ltTm lt2 gtlt107 deg K Reaction Energy released Time scale MeV pp gtdev 14 133X1010yrs dp gtHe3y 549 5sec He3He3 gtHe4pp 1286 106years The dp reaction must take place twice to get two Helium threes Total energy released per Helium atom formed is 262 MeV or in more conventional units per helium nucleus formed energy supplied by the fUSiOD is 412gtlt1012 Joules Helium


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