ASTR 1120General Astronomy:Stars & Galaxies!ATH REVIEW:Tonight, 5-6pm, in RAMY N1B23-Due THU, Sept. 10, by 5pm, on Mastering Astronomy "OMEWORK #1CLASS RECORDED STARTED - INFO WILL BE POSTED on CULEARNASTR 1120General Astronomy:Stars & Galaxies#AST CLASS:- Light: general properties- Matter: general properties- Matter: general properties (cont.)- Interaction between light and matter $ODAY:Light and Atoms• Light interacts with atoms in specificways• Allows us to measure properties of thegas such as composition & temperature• The key: the spectrum of an object(intensity as a function of wavelength)• How can we use spectral lines to determinethe composition of a distant object?• How can we determine the temperature ofdistant objects?• Can we use spectra to tell us how fastsomething is moving?Our goals for learning:Energy Levels in AtomsElectrons in atoms do NOT “orbit” around the nucleuslike little planets - their position better described byprobability wavesHowever, they do move in different “energy states” –some electrons in a given atom have more energythan othersThese energy states are “quantized”– there are onlycertain energies that the electrons are allowed tohave. This is quantum physics.Example of electron energystates in a hydrogen atom• Lower level is lowerenergy.• Units: 1 electron-volt (eV) =• 1.6 x 10-19 Joules = TINYEach electron in eachelement has itsown particularpattern of energylevels: elementalfingerprints!Electrons can movebetween levels if theyare given or give out theexact amount of energycorresponding to thedifference in the energylevels.For hydrogen, if anelectron at level 1(Ground state) is givenmore than 13.6 eV ofenergy, the electron willfly free (ionize)Example: Energy jumps A, B and Callowed; D is not possible for thisatom. E ionizes the atom with anenergy gain of >3.4 eVHow do electrons move between levels?Where does that energy come from (energyincrease) or go to (energy decrease)??• The energy changebetween levels isequal to the energyof the photon.• Larger energy jumpswill be SHORTERwavelength photons!PHOTONS!Emission Spectra• Emission for thin, hot gas where electrons are “excited” (in highenergy states). Gas glows in specific colors.– This is our FINGERPRINT of the elements in the gas!• Will eventually lose thermal energy through emitting photons,and cool!• Each atom has a different set of energy levels !different spectrum• spectra using a diffraction gratingSpectrum shows brightemission lines from variouselementsThe Crab nebula:remains of an explodedstar (supernova)Most common visible lightemission line:• Hydrogen Alpha (H!) ^• N=3 to n=2 energy jump at 656.3 nm• The universe is mostly red!!Continuous Spectrum• Hot solids/liquids/dense gases emit acontinuous rainbow of light– Blackbody RadiationAbsorption Spectrum• If light with a continuous spectrum shines through a cloud ofCOOL gas with electrons in low-energy states, the gas canabsorb photons OF THE RIGHT ENERGIES to moveelectrons to excited states• Resulting spectrum shows DARK LINES ofabsorption.– Corresponds to wavelengths where the atom hasabsorbed a photon and excited an electron to a higherenergy state• Why don’t we see those atoms re-emit the samephoton when they de-excite?– Atoms WILL emit these photons again and electronsfall back to ground state, BUT photons will bescattered in all directions and so most will be lost fromour sightWhat causes spectral lines?A. Black body radiation.B. Electron energy transitions in the atom.C. The Doppler shift of moving objects.D. High frequency electromagnetic waves.E. Protons and neutrons spinning in anatom.Clicker QuestionWhat causes spectral lines?A. Black body radiation.B. Electron energy transitions in the atom.C. The Doppler shift of moving objects.D. High frequency electromagnetic waves.E. Protons and neutrons spinning in anatom.Clicker QuestionWhat do we see at position 1?Clicker QuestionA. AbsorptionLineSpectrumB. ContinuousSpectrumC. EmissionLineSpectrum123What do we see at position 2?Clicker QuestionA. AbsorptionLineSpectrumB. ContinuousSpectrumC. EmissionLineSpectrum123What do we see at position 3?Clicker QuestionA. AbsorptionLineSpectrumB. ContinuousSpectrumC. EmissionLineSpectrum123Kirchoff’s Laws1) Hot solid, liquid,or dense gas(continuumspectrum)2) Continuousspectrum viewedthrough a coolergas(absorption linespectrum)3) Thin, hot gas(emission linespectrum)123Solar Spectrum (as seen fromEarth)Where could the dark lines inthe Solar spectrum be comingfrom?A. Absorption in the Sun’s atmosphereB. Emission from the Sun’s atmosphereC. Absorption in the interior of the SunD. Emission from the interior of the SunE. Absorption by the glass mirrors in thetelescope used to collect the lightClicker QuestionWhere could the dark lines inthe Solar spectrum be comingfrom?A. Absorption in the Sun’s atmosphereB. Emission from the Sun’s atmosphereC. Absorption in the interior of the SunD. Emission from the interior of the SunE. Absorption by the glass mirrors in thetelescope used to collect the lightClicker Question"164 millionYear-oldCluster+Ionized Nebula+Survivingcloud 1. Hotter objects emit more total radiation perunit surface area." Stephan-Boltzmann Law" E is proportional to T42. Hotter objects emit bluer photons (with ahigher average energy.)" Wien Law" #max = 2.9 x 106 / T(Kelvin) [nm]Rules for Emission by BlackbodyObjects"uman seen wi% an in&a-red camer'Which is the hottest star?One that appears:A. RedB. YellowC. BlueD. WhiteE. They are all the same temperature.They just look different colorsClicker QuestionWhich is the hottest star?One that appears:A. RedB. YellowC. BlueD. WhiteE. They are all the same temperature.They just look different colorsClicker QuestionQuick guide to thermal spectra(be familiar with these)• 3 K (coldest natural things): #max = 1mm = Microwaves• 300 K (people, planets, warm dust): #max = 10-5 m= 10,000 nm, IR• 3000-30,000K (stars): #max = 10-6 m to 10-7 m = 1000 to 100 nm, IR – visible – UV• 300,000- 30,000,000K: weird and intense places (UV through X-rays/gamma rays)What is this object?• Let’s use its spectral information to determinewhat it is.What is this object?Reflected Sunlight:Continuous spectrum ofvisible light is like theSun’s except that some ofthe blue light has beenabsorbed - object mustlook redWhat is this object?Thermal