Visible Spectrum

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Starlight and Atoms

Visible Spectrum• Prism splits Visible light into Visible Spectrum• Newton discovered that white light is made upof many colors in 1666• Newton thought light was corpuscles/particles• Young and others showed light was a wave (inthe ether?)


Electric Field• Electrons & Protons are thenegative & positively chargedelementary particles• They are surrounded by anelectric field• Which changes as they move


Electromagnetism• Changing electric fields (currents) make magnetic fields• Changing magnetic fields make electric currents• Changes in one induce changes in the other Maxwell 1860• All electromagnetic disturbances travel at the Speed of Light


Waves Have a Frequency f, Period P,Wavelength λ (lambda) & Amplitude• Frequency countswaves• Frequency = 3 cycles/sec or 3 Hertz• Has 1/3 sec. period• Period of 1 second• Frequency of 1 cycle/sec= 1 Hertz• Has three timeswavelength λ• Both have same heightor Amplitude


All Electromagnetic WavesMove at the Speed of Light=c• C= λ ×f Distance between waves times the number of waves• frequency f =ν=nu• If c=300,000km/sec and the ‘The Q’ has frequency of100Megahertz, what is the wavelength λ?


Electromagnetic Spectrum


Radio Waves• Hertz discovered radio waves in1888• Marconi and Tesla made firstradio• First radio telescope built byKarl Jansky 1932


Microwaves• Microwave ovens usewavelengths of 30cm• 15meter JCM Telescope studiesinterstellar dust gas, solar system& distant galaxies• Atacama Large Millimeter Arraymost expensive ground basedtelescope


Infrared Region• Bag is transparent to IR light(=heat) - Glasses are not


X-ray Region• First discovered by Roentgen in 1895• Wife’s hand with wedding ring• Produced by gas at 10 million Kelvin• NuSTAR X-ray telescope launchedJune 2012


Gamma Rays• Produced by very hot gas fallingonto a neutron stars & black holes• Produced by radioactive elements• Brightest gamma-ray burst ever;3.6billion light years away;27Apr13


Cosmic Rays• Very high energy subatomic particles• Raining down on us from all directions –no big worry• Probably produced by supernova remnants• AMS-02 installed on ISS May2010


Atmospheric Opacity• Opacity is the opposite of transparency• It measures how much light /electromagnetic radiation is blocked


Temperature• Temperature is a measure of the average kinetic energyof the atoms in gas, liquid or solid = speed of atoms• Animation is of Helium atoms at 20C & high pressure.• Speed of 1500 km/hr slowed down two trillion fold withsome colored red to make them easy to see


Heat or Thermal Energy• The hotter it is; the faster its atoms move• The amount of heat or thermal energy dependson temperature AND mass/number of atoms• The atoms cease moving at absolute zero =zero Kelvin (record=0.45nK)• Bowtie nebula -272C=1K


Brownian Motion Shows Atoms Exist• Microscopic view of milk fat droplets moving randomly due toimpacts from molecules of water – higher temperature faster motion• Einstein published mathematical explanation in 1905


Blackbody Radiation• A blackbody is opaque = stars, incandescent light bulbs• Radiative properties (spectrum) depend only on temperature• When charged particles are accelerated they emit photons


Blackbody Curve• Because the particles are going different speedsthe photons have a range of energies


Wein’s Law• As the temperature goes up there are more collisions & more violent• So more photons AND more energetic photons• Higher temperature shorter wavelength of peak emission=• Higher temperature bluer color


Stefan-Boltzmann Law• E= σ T 4• The amount of energy radiated is proportional totemperature to the fourth power• Twice the temperature give 2X2X2x2=16 times the energy


Blackbody Curve• After attempts by Wien, Stephan, Boltzmann, Rayleigh …• Max Planck finds equation (1900) which matches - BUTenergy must be quantized => Quantum Mechanics


Stellar Energy Distributions• Star temperatures vary from 100,000K to 1500K• Maximum of the curve tells us star’s temperature• Albireo composed of hot blue star & cool red one


Planet Impact?• Star HD172555 has a very bright disk• Disk has spectral signatures ofvaporized rock, lava, gravel and dust• Observations consistent with planetaryimpact


Stationary Source• Speed of waves equalsthe wavelength λtimes the frequency f• C=λf


Moving Source• Wavelengths in direction ofmotion are compressed• Wavelengths when sourceis moving away fromobserver are stretched• Speed of source determineshow much stretching


Doppler Effect• V r / C = (λ - λ o ) / λ o = Δ λ/λ o• If the source is receding (moving away) then it is a redshift• If the source is approaching then the light is blueshifted


Relative Motion• Stationary observer sees wave with emitted wavelength• Moving observer sees wavelength decreased due to his relativemotion


A-toms = Not Cutable• Democritus 420BC popularized thetheory matter was made of Atoms:• Too small to be seen• Indivisible• Surrounded by a void• Solid• No internal structure


Picturesof Atoms• 48 iron atoms oncopper• Made with ascanning tunnellingmicroscope


Electrons• JJ Thompson discovers the electron (1897)• How does a CRT work?


What’s an Atom?-Nucleus & Electron(s)• Rutherford (1911) fired helium nuclei at gold foil and avery few bounced straight back• Nucleus is 100,000 times smaller than the atom• Nucleus of atom is like pinhead in a Stadium


Periodic Table of Elements• Chemical properties determined by number of electrons• Which match number of protons• Increasing by 1 proton/electron for each element


Neutrons & Isotopes


Radioactive Decay of Carbon 14• Isotopes have same chemical properties• Carbon 14 has 6 protons & 8 neutrons• 1 Carbon 14 ( 14 C) in 10 13 carbon atoms• Half the Carbon 14 will decay in 5700 years• Your body is radioactive (thousands ofdisintegrations/second)


Bohr Atom• Tiny positive nucleuscontains most of mass• “Orbited” by negativelycharged electron(s)• Held in orbit by Coulomb=Electromagnetic Force


Quantum Mechanics• Because electron orbits; it accelerates; it should radiate photons• But energy is quantized so only certain orbitals/energies allowed(just like stairs, piano keys, bookshelf)• Electron can be in lowest energy (=ground state) can raised to higherenergy (=excited state)


Photon Emission/Absorption• The electron jumps to ahigher energy level whena photon is absorbed =absorption• Excited state• The electron jumps to alower energy when itemits a photon =emission• Ground state


Photon: Light is a Particle & a Wave• Einstein showed light is quantized (1905).• Each quantum of light is called a photon• Planck`s constant relates the energy of a photon to frequency. E=hf• Photons are like money; Radio photons are pennies, X-rays-dollars


Electron: is a particle & a wave• Because an electron is a wave as well as a particle we can notsay exactly where it is due to its quantum mechanical nature• Heisenberg’s uncertainty principle “you can’t know everythingabout anything”


Hydrogen Emission Lines• Difference in energy level determines photon energy=Wavelength• Visible lines are Balmer series; H-Alpha, H-Beta, H-Gamma• Ultraviolet = Lyman series and Infrared = Paschen series• If electron gains enough energy to leave atom it is Ionized


EmissionMatchesAbsorptionSpectrum


Kirchoff’s Laws• Continuous – solid, liquid or dense gas will radiate at all wavelengths• Emission - a low density gas will emit light at specific wavelengths• Absorption - results from a continuous spectrum passing through alow density gas resulting in dark spectral lines


PhotonAbsorptionandEmission• Electron canusually only stayin excited state fora nanosecond• Electrontransitions cantake place due tocollisions


Electron Shells• More protons require more electrons• More populated energy levels• More possible transitions• More complicated spectra


Spectra ofMolecules• Carbon Monoxide has• A) Electronic transitions• B) Rotational transitions• C) Vibrational transitions• Spectrum of Hydrogen molecule in (a)• Spectrum of Hydrogen atoms in (b)


Solar Spectrum• Fraunhofer discovered lines in solar spectrum 1817• D=Sodium, C&F&h=Hydrogen, H&K=Calcium• Different elements have a different set of lines like a bar code


Annie Jump Cannon• Originated the modern stellarclassification scheme in 1901• Based on strength of H lines• Found to be temperature sequenceClassified 400,000 stars forthe Henry Draper Catalogs


Balmer Thermometer• Balmer lines originate at the n=2 energy level• If it is too cool then all atoms in n=1 level• If it is too hot then all hydrogen atoms are ionized• Use Calcium, Helium and molecules as well


Cecilia Payne-Gaposchkin• Found the abundances of the different elements of the stars• First PhD in astronomy from Harvard/Radcliffe 1925• First woman full professor at Harvard and chair in 1956


Composition of StarsLine strength and line profile depend on abundance of elementAnd temperature (which energy levels are populated)Sun is composed of:Element MassHydrogen 71%Helium 27%All the Rest 2%Most other stars SAME as sun!


Thermal Broadening• Because the atoms that make up the star’s surfaceare in motion their Doppler shifts broaden theemission or absorption lines


RotationalBroadening


Pressure/Collisional Broadening• Spectra of Vega & Deneb


Turbulent Line Broadening• Doppler shifts of gas moving on surface ofstar cause line broadening


Discovering Extrasolar Planets• Spectra from largest telescopes have enough precision tosee radial velocity variations of a star moving around centerof mass due to a planet• First was 51 Peg in 1995• Big planets, near their star• Called Hot Jupiters• Easiest to find


Artificial Lighting• Tungsten/Incandescentlamps put out most of theirenergy in the infrared -continuous• Mercury/Fluorescent lampshave emission lines• LED lamps cool andcontinuous spectrum invisible


Electromagnetic Spectrum• All electromagnetic radiation travels at the speed of light• Energy of a photon depends on frequency/wavelength• Light has both wave and particle properties• Electrons and protons have both wave & particle properties• Everything has wave properties so: Heisenberg uncertaintyprinciple “You can not know anything exactly”


Car Horn


Energy LevelDiagram of aHydrogen Atom-If an electron gains more than thebinding energy then it willescape from the atom-This process is called ionization- The atom is a positive ion- Denoted by H +- Called a negative ion if it gainsan electron H -


Spectral Classes / Spectral Sequence• Oh Be A Fine Girl/Guy Kiss Me• Spectral Types have subdivisions of 0-9• Pick a star to be representative/standard star


Pressure• Gas is made of tiny atoms & molecules which are in constant motion• The higher the temperature the faster the particles move and• The more often the gas atoms hit the wall the larger the pressure• Bubble nebula – star wind exerts pressure to expand nebulaCoolHot


Transverse/Radial Velocity• We can measure the radial velocity of objects on the otherside of the universe but• We can measure the transverse velocity of only nearby stars• Thus we can only measure the true velocity of the nearby stars


Relative Radii of Atoms• Atomic radii are all about 0.1-0.2 nanometer• Smaller than the wavelength of visible light


Clocks in Rocks• Clocks start at time of solidifying and reset by melting•238U decays to 234 Th → 206 Pb with half-life of 4.5billion years• We have rock samples of the Earth, Moon, Mars, meteorites• All date to a maximum of ~4.6 Billion years• Time of formation of Solar System


Time to Solar System Formation• Meteorites with chondrules (spherical) formed before Earth• White Calcium Aluminum inclusions were formed 4.57 billion years• Short lived radioactive elements indicate that ~million years elapsedfrom supernova/solar system formation and rock crystallization


Atom• Number of Protons determines the Element• Neutrons hold the nucleus together• Number of electrons determined by protons in nucleus• Chemistry determined by electrons


Transitions of Hydrogen• Difference in energylevels determinesphoton energy• Photon energydepends onwavelength equalsphoton’s color• Balmer lines invisible• Lyman lines in UV• Paschen in infrared


Star’s With Dust Disks• Star radiates light like ahot Blackbody• Some starlight warmsthe disk• Disk radiates light(Infrared) like a coolerBlackbody


Spectral Sequence = Temperature Sequence


Spectrum of a Nebulae


Solar Spectrum• Millions of absorption lines of different elements and ions• Width of lines depends on temperature and abundance


Kirchhoff’s Laws=Kinds of Spectra


Types of SpectraWe usually look at graph of Intensityversus Wavelength• No lines = Continuous• Absorption lines = Dark lines• Emission lines = Bright Lines


HydrogenEnergyLevelDiagram• Lyman Series• Balmer Series• Paschen Series• Ion, Ionized,Ionization andBinding energy• Recombination


(B-V) Color Index• Counting the number of photons which pass through a filter andcomparing to another filter will tell us the temperature of a star• A hot blue star has a (B-V) = -0.1 and (B-V)=1.5 for a cool red star


Carbon Atom in Ground State• Six electrons populatingmany energy levels givesmuch more complexspectrum


Wien’s Law• Wavelength of maximum intensity depends on temperature• λ = 3,000,000/Temp in nanometers & degrees Kelvin• λ = 3,000/Temperature in microns & Kelvin


Image/Line Spectra• We rarely look at bands of color• Usually we graph intensity versus wavelength


Loss ofAtmosphere• First atmosphere of Hydrogenand Helium• Is lost to space because thetemperature / velocity is abovethe escape velocity


Waves• Wavelength λ is distance betweencrests• Frequency f is number of wavesper second which pass a point

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