Intro To Stars And Galaxies
Intro To Stars And Galaxies PHYS 1060
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Date Created: 09/30/15
Chapter 14 Our Galaxy 141 What does our galaxy look like The Milky Way galaxy appears in our sky as a faint band of light Dusty gas clouds obscure our view because they absorb visible light This is the interstellar medium that makes new star systems We see our galaxy edge on Primary features disk bulge halo globular clusters If we could view the Milky Way from above the disk we would see its spiral arms How do stars orbit in our galaxy Stars in the disk all orbit in the same direction with a little up and down motion Orbits of stars in the bulge and halo have random orientations Thought Question Why do orbits of bulge stars bob up and down They re stuck to the interstellar medium The gravity of disk stars pulls them toward the disk Halo stars knock them back into the disk Sun s orbital motion radius and velocity tells us mass within Sun s orbit 10 x 1011Msun Orbital Velocity Law The orbital speed v and radius r of an object on a circular orbit around the galaxy tell us the mass M within that orbit 142 How is gas recycled in our galaxy Star gas star cycle Recycles gas from old stars into new star systems High mass stars have strong stellar winds that blow bubbles of hot gas Lower mass stars return gas to interstellar space through stellar winds and planetary nebulae X rays from hot gas in supernova remnants reveal newly made heavy elements A supernova remnant cools and begins to emit visible light as it expands New elements made by supernova mix into interstellar medium Multiple supernovae create huge hot bubbles that can blow out of disk Gas clouds cooling in the halo can rain back down on disk Atomic hydrogen H2 gas forms as hot gas cools allowing electrons to join with protons Molecular clouds form next after gas cools enough to allow atoms to combine into molecules Molecular clouds in Orion Composition Mostly H2 About 28 He About 1 CO Many other molecules Gravity forms stars out of the gas in molecular clouds completing the star gas star cycle Radiation from newly formed stars is eroding these star forming clouds Summary of Galactic Recycling Stars make new elements by fusion Dying stars expel gas and new elements producing hot bubbles 106 K Hot gas cools allowing atomic hydrogen clouds to form 100 10000 K Further cooling permits molecules to form making molecular clouds 30 K Gravity forms new stars and planets in molecular clouds Thought Question Where will the gas be in 1 trillion years Blown out of galaxy Still recyclingjust like now Locked into white dwarfs and lowmass stars We observe the star gas star cycle operating in Milky Way s disk using many different wavelengths of light Infrared light reveals stars whose visible light is blocked by gas clouds X rays are observed from hot gas above and below the Milky Way s disk 21 cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk Radio waves from carbon monoxide CO show locations of molecular clouds Long wavelength infrared emission shows where young stars are heating dust grains Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds Where do stars tend to form in our galaxy Ionization nebulae are found around short lived high mass stars signifying active star formation Reflection nebulae scatter the light from stars Why do reflection nebulae look bluer than the nearby stars For the same reason that our sky is blue Halo No ionization nebulae no blue stars 3 no star formation Disk Ionization nebulae blue stars 3 star formation TABLE 141 Typical States of Gas in the Interstellar Medium State of Gas Atomic Hot Hydrogen Molecular Bubbles Clouds Clouds Primary Ionized Atomic Molecular Constituent hydrogen hydrogen hydrogen Approximate 1000000K 100 10000K 30K Temperature Approximate 001 1 100 300 Density atoms per cm3 Description Pockets of The most Regions gas heated common of star by stellar form of lormation winds or gas lling supernovae much of the galactic disk Much of star formation in disk happens in spiral arms Spiral arms are waves of star formation 1 Gas clouds get squeezed as they move into spiral arms 2 The squeezing of clouds triggers star formation 3 Young stars flow out ofspiral arms 143 What clues to our galaxy s history do halo stars hold Halo Stars 002 02 heavy elements 0 Fe only old stars Disk Stars 2 heavy elements stars of all ages Halo stars formed first then stopped Disk stars formed later and kept forming How did our galaxy form Our galaxy probably formed from a giant gas cloud Halo stars formed first as gravity caused the cloud to contract The remaining gas settled into a spinning disk Stars continuously form in the disk as the galaxy grows older Detailed studies Halo stars formed in clumps that later merged 144 What lies in the center of our galaxy Infrared light from center Radio emission from center Swirling gas near center Orbiting stars near center Stars appear to be orbiting something massive but invisible a black hole Orbits of stars indicate a mass of about 4 million Mm X ray flares from galactic center suggest that tidal forces of suspected black hole occasionally tear apart chunks of matter about to fall in Chapter 15 Galaxies and the Foundation of Modern Cosmology Hubble Deep Field Our deepest images ofthe universe show a great variety of galaxies some of them billions of light years away Galaxies and Cosmology A galaxy s age its distance and the age of the universe are all closely related The study of galaxies is thus intimately connected with cosmology the study of the structure and evolution ofthe universe What are the three major types of galaxies Spiral Galaxy Disk Component stars of all ages many gas clouds Spheroidal Component bulge and halo old stars few gas clouds Barred Spiral Galaxy Has a bar of stars across the bulge Lenticular Galaxy Has a disk like a spiral galaxy but much less dusty gas intermediate between spiral and elliptical Elliptical Galaxy All spheroidal component virtually no disk component Red yellow color indicates older star population Irregular Galaxy Neither spiral nor elliptical Thought Question Why does ongoing star formation lead to a blue white appearance A There aren t any red or yellow stars B Shortlived blue stars outshine others C Gas in the disk scatters blue light How are galaxies grouped together Spiral galaxies are often found in groups of galaxies up to a few dozen galaxies per group Elliptical galaxies are much more common in huge clusters of galaxies hundreds to thousands of galaxies 152 How do we measure the distances to galaxies Brightness alone does not provide enough information to measure distance Step 1 Determine size of solar system using radar Step 2 Determine distances of stars out to a few hundred light years using parallax Luminosity passing through each sphere is the same Area of sphere 4n radius2 Divide luminosity by area to get brightness The relationship between apparent brightness and luminosity depends on distance Luminosity Brightness 4n distance2 We can determine a star s distance if we know its luminosity and can measure its apparent brightness Luminosity Distance 4n gtlt Brightness A standard candle is an object whose luminosity we can determine without measuring its distance Step 3 Apparent brightness of star cluster s main sequence tells us its distance Knowing a star cluster s distance we can determine the luminosity of each type of star within it Thought Question Which kind of stars are best for measuring large distances A Highluminosity stars B Low luminosity stars Cepheid variable stars are very luminous Cepheid Variable Stars The light curve of this Cepheid variable star shows that its brightness alternately rises and falls over a 50 day period Cepheid variable stars with longer periods have greater luminosities Step 4 Because the period ofa Cepheid variable star tells us its luminosity we can use these stars as standard candles Step 5 Apparent brightness of a white dwarf supernova tells us the distance to its galaxy up to 10 billion light years What is Hubble s law The Puzzle of quotSpiral Nebulae Before Hubble some scientists argued that quotspiral nebulae were entire galaxies like our Milky Way while others maintained they were smaller collections of stars within the Milky Way The debate remained unsettled until someone finally measured their distances Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles Hubble also knew that the spectral features of virtually all galaxies are redshifted 3 they re all moving away from us By measuring distances to galaxies Hubble found that redshift and distance are related in a special way Hubble s law velocity H0 gtlt distance Distances ofthe farthest galaxies are measured from redshifts How do distance measurements tell us the age of the universe Thought Question Your friend leaves your house She later calls you on her cell phone saying that she s been driving at 60 miles an hour directly away from you the whole time and is now 60 miles away How long has she been gone A 1 minute B 30 minutes C 60 minutes D 120 minutes Thought Question You observe a galaxy moving away from you at 01light years per year and it is now 14 billion light years away from you How long has it taken to get there A 1 million years B 14 million years C 10 billion years D 14 billion years Hubble s constant tells us the age of the universe because it relates velocities and distances of all galaxies Age N 1Ho The expansion rate appears to be the same everywhere in space The universe has no center and no edge as far as we can tell One example of something that expands but has no center or edge is the surface of a balloon Cosmological Principle The universe looks about the same no matter where you are within it Matter is evenly distributed on very large scales in the universe No center and no edges Not proved but consistent with all observations to date Distances between faraway galaxies change while light travels Astronomers think in terms of Iookback time rather than distance Expansion stretches photon wavelengths causing a cosmological redshift directly related to lookback time 153 How do we observe the life histories of galaxies Deep observations show us very distant galaxies as they were much earlier in time old light from young galaxies How did galaxies form Our best models for galaxy formation assume that Matter originally filled all of space almost uniformly Gravity of denser regions pulled in surrounding matter Denser regions contracted forming protogalactic clouds H and He gases in these clouds formed the first stars Supernova explosions from the first stars kept much of the gas from forming stars Leftover gas settled into a spinning disk Conservation of angular momentum Why do galaxies differ Conditions in Protogalactic Cloud Spin Initial angular momentum of protogalactic cloud could determine the size ofthe resulting disk Density Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk Collisions were much more likely early in time because galaxies were closer together Many of the galaxies we see at great distances and early times indeed look violently disturbed The collisions we observe nearby trigger bursts of star formation Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies Starburst galaxies are forming stars so quickly that they will use up all their gas in less than a billion years The intensity of supernova explosions in starburst galaxies can drive galactic winds 154 What are quasars If the center of a galaxy is unusually bright we call it an active galactic nucleus Quasars are the most luminous examples The highly redshifted spectra of quasars indicate large distances From brightness and distance we find that luminosities of some quasars are gt1012L5un Variability shows that all this energy comes from a region smaller than the solar system Thought Question What can you conclude from the fact that quasars usually have very large redshifts A They are generally very distant B They were more common early in time C Galaxy collisions might turn them on D Nearby galaxies might hold dead quasars Galaxies around quasars sometimes appear disturbed by collisions Quasars powerfully radiate energy over a very wide range of wavelengths indicating that they contain matter with a wide range oftemperatures Radio galaxies contain active nuclei shooting out vast jets of plasma that emit radio waves coming from electrons moving at near light speed The lobes of radio galaxies can extend over hundreds of millions of light years An active galactic nucleus can shoot out blobs of plasma moving at nearly the speed of light The speed of ejection suggests that a black hole is present Characteristics of Active Galaxies Luminosity can be enormous gt1012L5un Luminosity can rapidly vary comes from a space smaller than solar system They emit energy over a wide range of wavelengths contain matter with wide temperature range Some drive jets of plasma at near light speed What is the power source for quasars and other active galactic nuclei The accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of quasars Energy from a Black Hole The gravitational potential energy of matter falling into a black hole turns into kinetic energy Friction in the accretion disk turns kinetic energy into thermal energy heat Heat produces thermal radiation photons This process can convert 10 40 of E me2 into radiation Jets are thought to come from the twisting of a magnetic field in the inner part of the accretion disk Do supermassive black holes really exist Orbits of stars at center of Milky Way stars indicate a black hole with mass of 4million MSW Orbital speed and distance of gas orbiting center of M87 indicate a black hole with mass of 3 billion Mslm Black Holes in Galaxies Many nearby galaxies perhaps all of them have supermassive black holes at their centers These black holes seem to be dormant active galactic nuclei All galaxies may have passed through a quasar like stage earlier in time Galaxies and Black Holes The mass of a galaxy s central black hole is closely related to the mass of its bulge Galaxies and Black Holes The development of a central black hole must somehow be related to galaxy evolution The mass of a galaxy s central black hole is closely related to the mass of its bulge The development of a central black hole must somehow be related to galaxy evolution Chapter 1 Astronomy Scientific study of nature outside of Earth the oldest of the sciences we have always looked to the sky what are those points of light what time is it day month season where are we and where are we going what is our place in the universe how did we get here What is our place in the universe Our quotCosmic Address Our place p3 in space E 1 planet of 8 orbiting the Sun Sun an ordinary star one of 100 billion in our Galaxy the quotMilky Way Milky Way typical galaxy one of 10s or 100s of billions of galaxies in observable Universe Universe totality of all space time matter amp energy in time Sun Earth 46 billion years old Milky Way 1113 billion years old the scale of things Solar System all 9 planets lie within about 40 astronomical units AU of the Sun 1 AU m 150 million km 93 million miles distance between stars often measured in light years Iv speed of light 300000 kms 946 trillion kmyr 1 light year 946 trillion km 588 trillion miles our Galaxy is 100000 ly across nearest large galaxy to our own is 25 million ly away How can we know what the universe was like in the past Light travels at a finite speed 300000 kms Thus we see objects as they were in the past The farther away we look in distance the further back we look in time Example This photo fig 13 shows the Andromeda Galaxy as it looked about 2 12 million years ago Question When will we be able to see what it looks like now At great distances we see objects as they were when the universe was much younger Can we see the entire universe fig 14 Thought Question Why can t we see a galaxy 15 billion lightyears away Assume the universe is 14 billion years old Because no galaxies exist at such a great distance Galaxies may exist at that distance but their light would be too faint for our telescopes to see Because looking 15 billion lightyears away means looking to a time before the universe existed How big is the Universe Now let s step through the Universe in powers of 10 video 13 How is Earth moving in our solar system Contrary to our perception we are not quotsitting still We are moving with the Earth in several ways and at surprisingly fast speeds Earth orbits the Sun revolves once every year Our Sun moves randomly relative to the other stars in the local Solar neighborhood More detailed study of the Milky Way s rotation reveals one of the greatest mysteries in astronomy How do galaxies move within the universe Hubble discovered that all galaxies outside our Local Group are moving away from us the more distant the galaxy the faster it is racing away Are we ever sitting still CHAPTER 4 Universal Laws of Motion Objects in Motion speed rate at which an object moves ie the distance traveled per unit time ms mihr velocity an object s speed in a certain direction eg quot10 ms moving east acceleration a change in an object s velocity Le a change in either speed or direction is an acceleration msz The Acceleration of Gravity As objects fall they accelerate The acceleration due to Earth s gravity is 10 ms each second or g 10 msz The higher you drop the ball the greater its velocity will be at impact The Acceleration of Gravity g Galileo demonstrated that g is the same for all objects regardless of their mass This was confirmed by the Apollo astronauts on the Moon where there is no air resistance Forces Forces change the motion of objects momentum the mass x velocity of an object force anything that can cause a change in an object s momentum Units are Newtons N kg x m squot2 As long as the object s mass does not change the force causes a change in velocity or an acceleration s Mass the Same Thing as Weight mass the amount of matter in an object weight a measurement of theforce which acts upon an object Sir Isaac Newton 16421727 Invented the reflecting telescope Philosophiae naturalis principia mathematica Isaac Newton one smart dude Born year of Galileo s death 1642 16651666 bubonic plague swept England and Isaac went home from college and developed science of optics 3 laws of motion form basis of Physics invented Calculus derived the Universal Law of Gravity Newton s Laws of Motion 1 A body at rest or in motion at a constant speed along a straight line remains in that state of rest or motion unless acted upon by an outside force 2 The change in a body s velocity due to an applied force is in the same direction as the force and proportional to it but is inversely proportional to the body s mass F ma 3 For every applied force a force of equal size but opposite direction arises Depending on its initial velocity the cannonball will either fall to Earth continually freefall orbit or escape the force of Earth s gravity angular momentum the momentum involved in spinning circling mass xvelocity x radius torque anything that can cause a change in an object s angular momentum twistingforce Conservation of Angular Momentum In the absence ofa net torque the total angular momentum ofa system remains constant A Universe of Matter and Energy quotThe eternal mystery of the world is its comprehensibility Thefact that it is comprehensible is a miracle What are Matter and Energy matter is material such as rocks water air energy is what makes matter move Energy is measured in many different units The metric unit of energy used by scientists is Joule Three Basic Types of Energy kinetic energy of motion potential stored energy radiative energy transported by light Energy can change from one form to another Kinetic Energy Amount of kinetic energy of a moving object mv if mass m is in kg amp velocity v is in ms energy is in joules On the microscopic level the average kinetic energy of the particles within a substance is called the temperature it is dominated by the velocities of the particles Temperature Scales Temperature vs Heat Temperature is the average kinetic energy Heat thermal energy is the total kinetic energy Potential Energy gravitational potential energy is the energy which an object stores due to its ability to fall It depends on the object s mass m the strength of gravity g the distance which it falls d Potential Energy energy is stored in matter itself this massenergy is what would be released if an amount of mass m were converted into energy E mcquot2 Conservation of Energy Energy can be neither created nor destroyed It merely changes it form or is exchanged between objects This principle or law is fundamental to science The total energy content of the Universe was determined in the Big Bang and remains the same today Universal Law of Gravitation Between every two objects there is an attractive force the magnitude of which is directly proportional to the mass of each object and inversely proportional to the square of the distance between the centers of the objects G is the quotgravitational consta nt 667 x 103911 N m2 ng examples of gravity mutual force Like other forces effect of gravity is quotmutualquot The force of gravity exerted by Sun on Earth Fgrav Mew Bream cc Mew X Mmd2 is exactly equal to that of Earth exerted on the Sun Fgrav Msun Dsun 06 me M nhd does Earth or Sun have larger acceleration why Orbital Paths Extending Kepler s Law 1 Newton found that ellipses were not the only orbital paths possible orbital paths ellipse bound parabola unbound hyperbola unbound Newton s Version of Kepler s Third Law Using the calculus Newton was able to derive Kepler s Third Law from his own Law ofGravity In its most general form P2 412 aa G m1 m2 fyou can measure the orbital period of two objects P and the distance between them a then you can calculate the sum of the masses of both objects m1 m2 Changing Orbits orbital energy kinetic energy gravitational potential energy conservation of energy implies orbits can t change spontaneousy An object can t crash into a planet unless its orbit takes it there An orbit can only change if it gainsloses energy from another object such as a gravitational encounter If an object gains enough energy so that its new orbit is unbound we say that it has reached escape velocity Light The Cosmic Messenger power the rate at which energy is usedemitted It is measured in units called watts 1 watt ljoule per second A 100 watt light bulb radiates lOOjoules of energy every second Light info from the universe Distances vast in Universe virtually all information arrives in the form of light Light consists of an electromagnetic wave wave a pattern of oscillation quotup and down motion other examples water waves in pond sound waves electromagnetic two waves electric amp magnetic oscillate together and travel through space Light as a Wave A wave is a pattern which is revealed by its interaction with particles Light behaves as a wave 3 basic properties Of wave are wavelength A frequency f and speed c c 7 El f speed of light c m 3 x 105 kms that s fast that s 3 x 108 ms special wave properties of light light waves can travel through a vacuum the speed of light wave is constant independent of the source or observer speed Wavelength and Frequency Our eyes recognizefor A as color and a particle light is strange behaves as wave or particle depending on how measured particles of light are called photons may be thought of as quotbundles of light wavesquot Light as a Particle The energy carried by each photon depends on its frequency color E hf hcA quothquot is called Planck s Constant Bluer light carries more energy per photon The Electromagnetic Spectrum Most wavelengths of light cannot be seen by the human eye Thought Question The higher the photon energy the longer its wavelength the shorter its wavelength Energy is independent of wavelength What is matter nature of atoms atoms building blocks of all matter composed of protons amp neutrons no charge in nucleus surrounded by electron quotcloudquot molecule two or more atoms bound together number of protons in nucleus determine the atom eg hydrogen helium carbon electromagneticforce binds electrons to nucleus atoms and molecules too atom is neutral if Nelectrons Nprotons atom is called ion if Nelectrons 5 Nprotons Four Ways in Which Light can Interact with Matter emission matter releases energy as light absorpti n matter takes energy from light transmission matter allows light to pass through it reflection matter repels light in another direction moving microscopic matter ALL light is emitted by charged particles acceleration of charged particles ions electrons deexcitation of atom or molecule Reflection and Scattering Mirror reflects light in a particular direction Movie screen scatters light in all directions Interactions between light and matter determine the appearance of everything around us Thought Question Why is a rose red The rose absorbs red light The rose transmits red light The rose emits red light The rose reflects red light 52 What are the three basic types of spectra Three Types of Spectra Continuous Spectrum The spectrum of a common incandescent light bulb spans all visible wavelengths without interruption Emission Line Spectrum A thin or lowdensity cloud of gas emits light only at specific wavelengths that depend on its composition and temperature producing a spectrum with bright emission lines Absorption Line Spectrum A cloud of gas between us and a light bulb can absorb light of specific wavelengths leaving dark absorption lines in the spectrum How does light tell us what things are made of Chemical Fingerprints Downward transitions produce a unique pattern of emission lines Because those atoms can absorb photons with those same energies upward transitions produce a pattern of absorption lines at the same wavelengths Each type of atom has a unique set ofenergy levels Each transition corresponds to a unique photon energy frequency and wavelength Each type of atom has a unique spectral fingerprint Observing the fingerprints in a spectrum tells us which kinds of atoms are present How does light tell us the temperatures of planets and stars Thermal Radiation Nearly all large or dense objects emit thermal radiation including stars planets and you An object s thermal radiation spectrum depends on only one property its temperature Properties of Thermal Radiation l Hotter objects emit more light at all frequencies per unit area 2 Hotter objects emit photons with a higher average energy Wien s Law Thought Question Which is hotter A blue star A red star A planet that emits only infrared light Thought Question Why don t we glow in the dark People do not emit any kind of light People only emit light that is invisible to our eyes People are too small to emit enough light for us to see People do not contain enough radioactive material Interpreting an Actual Spectrum By carefully studying the features in a spectrum we can learn a great deal about the object that created it Reflected Sunlight Continuous spectrum ofvisible light is like the Sun s except that some of the blue light has been absorbed object must look red Thermal Radiation Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 K Carbon Dioxide Absorption lines are the fingerprint of C02 in the atmosphere Ultraviolet Emission Lines Indicate a hot upper atmosphere What is this object The Doppler Effect 1 Light emitted from an object moving towards you will have its wavelength shortened 2 Light emitted from an object moving away from you will have its wavelength lengthened 3 Light emitted from an object moving perpendicular to your lineofsight will not change its wavelength Doppler shift tells us ONLY about the part of an object s motion toward or away from us Measuring the Shift We generally measure the Doppler effect from shifts in the wavelengths of spectral lines The amount of blue or red shift tells us an object s speed toward or away from us Measuring Rotational Velocity Thought Question I measure a line in the lab at 5007 nm The same line in a star has wavelength 5028 nm What can I say about this star It is moving away from me It is moving toward me It has unusually long spectral lines 53 telescopes overview photons bring information from cosmos celestial objects appear dim even if intrinsically bright telescopes are quotlight bucketsquot primary purpose is to collect photons and bring to them to focus secondary resolve fine detail structure tertiary some image magnification desirable large telescopes gather more light and produce sharper images than small ones The Bending of Light Focus to bend all light waves coming from the same direction to a single point Light rays which come from different directions converge at different points to form an image Bigger is better Angular Resolution R The ability to separate two objects eg double stars planetary detail The angle between two objects decreases as your distance to them increases The smallest angle at which you can distinguish two objects is your angular resolution diffraction tendency of light to bend around corners or obstacles sets ultimate limit upon R diffraction limited R oc I D R 001 01 10 arc seconds for D 10 1 01 meter telescopes at 7L 4000A Angular resolution smaller is better Telescope Types Refractor focuses light using lenses Reflector focuses light using mirrors used exclusively in professional astronomy today Refractor Yerkes 40inch telescope largest refractor in the world Reflector Gemini 8m Telescope Mauna Kea Hawaii collecting light All modern telescopes use curved mirrors to collect and reflect light to afocus called reflecting telescopes primary mirror determines size of quotlight bucket largest single mirror 84 meters diameter also segmented mirrors 104 meters diam 36 pieces photon collection rate depends on the collecting area ltgt primary mirror area oc DZ D is diameter of primary mirror light gathering power LGP oc D 10 m telescope has 100 times the LGP of 1 m detecting light 1609 c1860 human eye to sketch pad 1860s 1970s photographic film can collectadd up photons over time data is stored on film but only 2 efficient since late 1970s electronic photon detectors highly efficient 80 90 can store amp analyze info within computers since c1985 quotchargecoupled device or CCD several million picture elements quotpixelsquot found in home video amp digital cameras what we do with light with electronic detectors 2 form images to study structure usually measure brightness within several small wavelength ranges filters 2 form a spectrum light consisting of a wide range of wavelengths is spread out so as to study individual ones rainbow is crude spectrum of Sun s light graph brightness vs wavelength to determine temperature density chemical composition motion more Spectroscopy The spectrograph reflects light off a grating a finely ruled smooth surface Light interferes with itself and disperses into colors This spectrum is recorded by a digital detector called a CCD Keck and Keck Mauna Kea Hawaii Radio Telescopes The wavelengths of radio waves are long So the dishes which reflect them must be very large to achieve any reasonable angular resolution Xray Telescopes Different types of photons behave differently Xrays will pass right through a mirror They can only be reflectedfocused at shallow angles like quotskimming stones Why do we put telescopes into space It is NOT because they are closer to the stars Recall our l to10 billion scale Sun size of grapefruit Earth size ofa tip of a ballpoint pen15 m from Sun Nearest stars 4000 km away Hubble orbit microscopically above tip of a ballpointpensize Earth Observing problems due to Earth s atmosphere 1 Light Pollution 2 Turbulence causes twinkling 2 blurs images 3 Atmosphere absorbs most of EM spectrum including all UV and X ray and most infrared Telescopes in space solve all 3 problems Locationtechnology can help overcome light pollution and turbulence Nothing short of going to space can solve the problem ofatmospheric absorption of light CHAPTER 102 How does nuclear fusion occur in the Sun Fission Big nucleus splits into smaller pieces Nuclear power plants Fusion Small nuclei stick together to make a bigger one Sun stars High temperatures enable nuclear fusion to happen in the core The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus Proton proton chain is how hydrogen fuses into helium in the Sun A 4 protons OUT 4He nucleus 2 gamma rays 2 positrons 2 neutrinos Total mass is 0 7 lower Thought Question What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy A The core would expand and heat up slightly B The core would expand and cool C The Sun would blow up like a hydrogen bomb Solar Thermostat Decline in core temperature causes fusion rate to drop so core contracts and heats up Rise in core temperature causes fusion rate to rise so core expands and cools down How does the energy from fusion get out of the Sun Energy gradually leaks out of the radiation zone in the form of randomly bouncing photons Convection rising hot gas takes energy to the surface How do we know what is happening inside the Sun We learn about the inside of the Sun by making mathematical models observing solar vibrations observing solar neutrinos Patterns of vibration on the surface tell us about what the Sun is like inside Data on solar vibrations agree with mathematical models of solar interior Neutrinos created during fusion fly directly through the Sun Observations of these solar neutrinos can tell us what s happening in the core Solar neutrino problem Early searches for solar neutrinos failed to find the predicted number Solar neutrino problem Early searches for solar neutrinos failed to find the predicted number More recent observations find the right number of neutrinos but some have changed form Chapter 11 Surveying the Stars How do we measure stellar luminosities Brightness of a star depends on both distance and luminosity Luminosity Amount of power a star radiates energy per second watts Apparent brightness Amount of starlight that reaches Earth energy per second per square meter Luminosity passing through each sphere is the same Area of sphere 41radius2 Divide luminosity by area to get brightness Thought Question These two stars have about the same luminosity which one appears brighter A Alpha Centauri B The Sun The relationship between apparent brightness and luminosity depends on distance Luminosity Brightness 41distance2 We can determine a star39s luminosity if we can measure its distance and apparent brightness Luminosity 41distance2 gtlt Brightness Thought Question How would the apparent brightness ofAlpha Centauri change if it were three times farther away A Itwould be only 13 as bright B It would be only 16 as bright C It would be only 19 as bright D It would be three times as bright So how far away are these stars Parallax is the apparent shift in position of a nearby object against a background of more distant objects Apparent positions ofthe nearest stars shift by about an arcsecond as Earth orbits the Sun The parallax angle depends on distance Parallax is measured by comparing snapshots taken at different times and measuringthe shift in angle to Parallax and Distance ppamlnx angle 1 1 m PMS p In msmmds d in ightycars 326 x p in manuals Most luminous stars 105L5m Least luminous stars 10 4L5W LSHn is luminosity ofthe Sun How do we measure stellartemperatures Every object emits thermal radiation with a spectrum that depends on its temperature An object of fixed size grows more luminous as its temperature rises Properties of Thermal Radiation Hotter objects emit more light per unit area at all frequencies Hotter objects emit photons with a higher average energy Hottest stars 50000 K Coolest stars 3000 K Sun s surface is 5800 K Level of ionization also reveals a star s temperature Absorption lines in a star s spectrum tell us its ionization level Lines in a star s spectrum correspond to a spectral type that reveals its temperature Hottest O B A F G K M Coolest Pioneers of Stellar Classification Annie Jump Cannon and the quotcalculatorsquot at Harvard laid the foundation of modern stellar classification Thought Question Which of the stars below is hottest A M star B Fstar C Astar D Kstar How do we measure stellar masses Orbit of a binary star system depends on the strength of gravity Types of Binary Star Systems Visual binary We can directly observe the orbital motions of these stars Eclipsing binary We can measure periodic eclipses Spectroscopic binary We determine the orbit by measuring Doppler shifts About halfofal stars are in binary systems We measure mass using gravity Direct mass measurements are possible only for stars in binary star systems Most massive stars 100M5un Least massive stars 008M5un MSun is the mass of the Sun 112 What is a Hertzsprung Russell diagram An HR diagram plots the luminosities and temperatures of stars Most stars fall somewhere on the main sequence of the HR diagram Stars with lower Tand higher L than mainsequence stars must have larger radii giants and supergiants Stars with higher Tand lower L than mainsequence stars must have smaller radii white dwarfs A star s full classification includes spectral type line identities and luminosity class line shapes related to the size of the star I supergiant bright giant giant lV subgiant V main sequence Examples Sun 32V Sirius A1 V Proxima Centauri M55 V Betelgeuse M2 HR diagram depicts Temperature Color Spectral type Luminosity Radius What is the significance of the main sequence Mainsequence stars are fusing hydrogen into helium in their cores like the Sun Luminous mainsequence stars are hot blue Less luminous ones are cooler yellow or red Mass measurements of mainsequence stars show that the hot blue stars are much more massive than the cool red ones The mass of a normal hydrogenburning star determines its luminosity and spectral type The core temperature of a highermass star needs to be higher in order to balance gravity A higher core temperature boosts the fusion rate leading to greater luminosity Luminosity from brightness and distance 10quot L5un 105LSun Temperature from color and spectral type 3000 K 50000 K Mass from period p and average separation a of binarystar orbit 008M5un 100M5un
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