INTRODUCTION TO ASTRONOMY
INTRODUCTION TO ASTRONOMY AST 301
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This 29 page Class Notes was uploaded by Keegan Goyette on Sunday September 6, 2015. The Class Notes belongs to AST 301 at University of Texas at Austin taught by Staff in Fall. Since its upload, it has received 14 views. For similar materials see /class/181761/ast-301-university-of-texas-at-austin in Astronomy at University of Texas at Austin.
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Date Created: 09/06/15
Wednesday Nov 5 Syllabus class notes and homeworks are at wwwasutexasedu 9 courses 9 AST 301 Lacy Reading for this week chapter 12 The Wednesday help session is in GRG 424 at 500 for the entire semester Topics for this week Describe the Milky Way Galaxy Describe the Standard Candle method of determining distances and how Cepheid variable stars are used as standard candles Describe how astronomers measure the distribution of mass in the Milky Way and what they find Explain why we might expect the spiral arms in the Milky Way to become more tightly wrapped and how density wave theory solves this problem Describe the various types of galaxies both normal and active and how they differ from the Milky Way Our place in the Milky Way Harlow Shapley about 1920 Determined the constant of proportionality between period of variation and luminosity so the luminosity of a Cepheid variable star could be determined from its penod Then he could use Cepheid variables as standard candles He mapped out the distribution of globular clusters He found that they formed a distribution centered some distance from us in the direction of Sagittarius He concluded that we are not at the center of the Milky Way and that the Milky Way is more than 10000 ly across We now know it is more than 100000 ly across g zuuz amok Cole Puhnsmng a mvlslnn cnnumsnn Learning Components of the Milky Way Galaxy disk the stars in the band of light we see a flat arrangement of stars orbiting on nearly circular paths bulge an elliptical distribution of stars near the center halo a large diffuse distribution of stars surrounding and overlapping the disk bar an elongated ridge of stars toward the centers of some galaxies including the Milky Way ring a ring of stars around a bar Distribution of Mass in the Milky Way At least for other galaxies we can measure the distribution of luminous matter by simply taking a picture But is that a fair sample of all of the matter in a galaxy Some stars emit much more light than others We can measure the distribution of mass in a galaxy by observing how fast stars orbit in the galaxy Near the center the stars are orbiting around the black hole Fartherout they orbit around whatever is inside of their orbits black holes stars gas dust We can use Kepler s 3rOI law to determine the mass of whatever is inside of their orbits 300 Keplerian motion i 275 10 g 250 z 8 3 225 5 3 3 o 200 175 150 O a 2mm Snacks cm Publishing 7 a divislun olTnnmson Learning Radius kpc Rota on curve Orbital speeds and mass All stars seem to orbit around the center of the Milky Way with speeds of 200250 kms Kepler s 3rcl law then tells us GMinside0rbit vorbital Forbit Of 2 M vorbitalrorbit inside0rbit G so if the orbital speed is approximately constant M 0C1 inside 0rbit orbit There is mass where we see almost no stars We conclude that Minside ofradius r or r This means that there is as much mass between 11 and 12 kiloparsecs from the center as there is between 1 and 2 kpc This is surprising because there is much more light coming from the stars between 1 and 2 kpc from the center The amount of mass we calculate is also surprising It is several times more than the mass we calculate of the stars we can see Most of the mass in the Milky Way must be dark manna M5 hwhirlvnnl alaxyrnztstav m H 3 r Hap m Nay seen stars here The 1k spots are star vaum15 16511119 wucx C luzw abma auxu 121 vuauly solar systems are born The bright object to the left is another galaxy a small barred spiral captured by M51 EVentually theyfwill merge making a quot single large galaxy probably very distorted 39om its present state Both galaxies are about 35 million lightyears away Spiral arms The brightest stars and most young stars in spiral galaxies are arranged in a spiral pattern in their disks Could the stars in spiral arms be moving along the arms toward the centers of their galaxies Or could stars stay where they are in spiral arms as a spiral galaxy rotates The Doppler shifts of the stars give us information about their motions Doppler shifts show that stars in disks of spiral galaxies follow nearly circular orbits they do not spiral in along the arms Rotation of spiral arms Doppler shifts also show that stars in a spiral galaxy all orbit with about the same speed Would this allow stars to remain in spiral arms and the galaxy to rotate like a pinwheel If a galaxy rotated like a pinwheel how would the speeds of stars as they orbit depend on distance from the center of the galaxy Think it over and discuss the question with your neighbors Monday Oct 20 Syllabus class notes and homeworks are at wwwasutexasedu 9 courses 9 AST 301 Lacy Reading for this week chapter 10 The second exam will be this Friday If you didn t get a review sheet on Friday there are more in the homework return boxes The Wednesday help session is in GRG 424 at 500 for the entire semester Topics for this week Describe how a star or protostar changes if it generates more or less energy by nuclear fusion than it radiates from its surface Describe the changes that occur near the center of a star as it changes from a mainsequence star into a red giant Describe how a red giant becomes a planetary nebula and a white dwarf How does the pressure inside a white dwarf differ from normal gas pressure How does nuclear fusion inside of very massive stars differ from the fusion that will occur inside of the Sun How do the processes that occur inside of massive stars lead to supernova explosions Pump the handle H rSlmclzs 0 mm 5mm 0 Grmnly O 7 Luis Innls s OMInns Atmncell ounmns gtgt R2521 I m n HemSheues r qun Shzcles ppm UM hm Annnrmn Wman Thermal Equilibrium in Stars Protostars are not in thermal equilibrium They lose energy by radiation from their surfaces but they aren t hot enough inside to ignite nuclear fusion to replace the lost energy As a result they contract and heat up Once they are hot enough inside about 107 K fusion can replace the energy they are losing They are then in a stable thermal equilibrium if fusion slowed down they would contract and heat up causing fusion to speed back up until it balances the energy they are losing Because of this stable equilibrium the Sun will hardly change for 1010 years until it uses up all of the hydrogen in its core Quiz If a star or a part of a star loses more energy than it generates it will A contract and heat up B contract and cool off C expand and heat up D expand and cool off The loss of energy will cause a loss of pressure leading to contraction But contraction will convert gravitational energy to heat energy and the star will end up hotter than it was at first Becoming a Red Giant The complete explanation for how a mainsequence star becomes a red giant is complicated and I m not really giving you the whole story But the conclusion is right Don t worry if you don t follow all of the explanation When all of the hydrogen in the core of a mainsequence star is all turned into helium fusion will stop in the core and the core will contract and heat up Fusion will continue in a shell around the helium core and will generate more energy than fusion in the core did The extra energy going out from the coreshell will make the envelope expand and cool off Red Giants When the Sun becomes a red giant its radius will increase to about 12 AU and it will become more than 100 times more luminous than it is now Life will not be pleasant on Earth The core of the Sun will be mostly helium and will continue to contract and heat up When the temperature in the core reaches about 108 K about 1 million years after the Sun leaves the main sequence helium will begin to fuse to make carbon A Red Giant with helium fusion When helium fusion starts generating energy in the core of a red giant the core expands and hydrogen fusion in the shell around the core slows down As a result less total energy is being generated and the envelope contracts and warms up some But pretty soon all of the helium in the core is converted into carbon and fusion stops again in the core Then the core again contracts surrounded by two shells one with He 9 C fusion and one with H 9 He fusion The envelope again expands and cools off ls stellar evolution just a theory Or is it even a scientific theory For a hypothesis to be a scientific theory it must be testable Almost all of stellar evolution occurs so slowly that we cannot watch it happen Our tests of stellar evolution must be indirect The primary test involves observing stars in a star cluster which we have reason to believe all formed at the same time so have the same age Stellar evolution theory predicts the pattern they should form on an HR diagram L L m 104 20000 e zuuz Evaaks cm pumxsn mg a nivisinn mnnmson Learning NGC 2264 10000 5000 3000 Temperature K a 2m Brooks Cole Publismng a uiwslun mThumson Learning M 67 Age 4 x109 yr 10 1 20000 10000 5000 3000 Temperature K Have we proved the theory right The theory of stellar evolution provides a very good explanation for the HR diagrams of star clusters We have tested the theory and it passed the test But for a hypothesis to be a scientific theory it must be testable and a test only makes sense if it is conceivable that the theory will fail In that sense we can never prove a scientific theory right If a hypothesis can be proved right it may be a valid mathematical or logical theorem but it s not a scientific theory We are confident that the theory of stellar evolution is essentially correct but we will keep testing and refining it Friday Oct 10 Syllabus class notes and homeworks are at wwwasutexasedu 9 courses 9 AST 301 Lacy Reading for next week chapter 9 The Wednesday help session is in GRG 424 at 500 for the entire semester Topics for this week How do astronomers use parallax to measure the distances to stars Why does parallax vary inversely with distance Describe and explain the relationship between a star s apparent brightness or flux its absolute brightness or luminosity and its distance from us Describe and explain the relationship between a star s luminosity its radius and its temperature and how this relationship is used to measure radii of stars Sketch an HR diagram showing the location of main sequence stars red giants and white dwarfs Explain how astronomers measure masses of stars Describe how the luminosities of main sequence stars are related to their masses
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