ASTR100 (Spring 2008) Introduction to Astronomy Properties of Light and MatterSummary So Far…Three Types of SpectraContinuous SpectrumEmission Line SpectrumAborption Line SpectrumHow does light tell us what things are made of?Chemical FingerprintsSlide 9Emission LinesSlide 11Absorption LinesSlide 13PowerPoint PresentationSlide 15Example: Solar SpectrumParts of a SpectrumSlide 18Slide 19Slide 20Slide 21Slide 22How does light tell us the temperatures of planets and stars?Thermal RadiationProperties of Thermal RadiationWhich is hotter?Slide 27Why don’t we glow in the dark?Slide 29How does light tell us the speed of a distant object?Doppler Effect for SoundSlide 32Doppler Effect for LightMeasuring the ShiftThe amount of blue or red shift tells us an object’s speed toward or away from us:Doppler shift tells us ONLY about the part of an object’s motion toward or away from us:Thought QuestionSlide 38ASTR100 (Spring 2008) Introduction to Astronomy Collecting Light with TelescopesHow do telescopes help us learn about the universe?Bigger is betterSlide 42Angular ResolutionBasic Telescope DesignSlide 45Keck I and Keck II Mauna Kea, HISlide 47Slide 48Different designs for different wavelengths of lightX-ray telescope: “grazing incidence” opticsWant to buy your own telescope?ASTR100 (Spring 2008) Introduction to AstronomyProperties of Light and MatterProf. D.C. RichardsonSections 0101-0106Summary So Far…Light is an electromagnetic wave.Visible light is tiny part of EM spectrum.Matter is made of atoms, which are made of protons, electrons, & neutrons.Matter and light interact: emission, absorption, transmission, reflection.Three Types of SpectraContinuous SpectrumThe spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption.Emission Line SpectrumLow-density gas clouds emit light only at specific wavelengths that depend on their composition and temperature, producing bright emission line spectra.Aborption Line SpectrumA gas cloud 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Each type of atom has a unique set of energy levels.Each transition corresponds to a unique photon energy, frequency, and wavelength.Energy levels of hydrogenChemical FingerprintsDownward transitions produce a unique pattern of emission lines.Emission LinesChemical FingerprintsAtoms can also absorb photons with these same energies to produce a pattern of absorption lines.Absorption LinesChemical FingerprintsEach type of atom has a unique spectral fingerprint.Chemical FingerprintsObserving the fingerprints in a spectrum tells us which kinds of atoms are present.Example: Solar SpectrumParts of a SpectrumWhich letter(s) label absorption lines?A B C D EParts of a SpectrumWhich letter(s) label absorption lines? … C & DA B C D EParts of a SpectrumWhich letter(s) label the peak in infrared?A B C D EParts of a SpectrumWhich letter(s) label the peak in infrared? … EA B C D EParts of a SpectrumWhich letter(s) label emission lines?A B C D EParts of a SpectrumWhich letter(s) label emission lines? … AA B C D EHow does light tell us the temperatures of planets and stars?Thermal RadiationNearly all large or dense objects emit thermal radiation, including stars, planets, people…An object’s thermal radiation spectrum depends on only one property: its temperature.Properties of Thermal Radiation1. Hotter objects emit more light at all frequencies per unit area.2. Hotter objects emit photons with a higher average energy.Which is hotter?a) A blue star.b) A red star.c) A planet that emits only infrared light.Which is hotter?a) A blue star.b) A red star.c) A planet that emits only infrared light.Why don’t we glow in the dark?a) People do not emit any kind of light.b) People only emit light that is invisible to our eyes.c) People are too small to emit enough light for us to see. d) People do not contain enough radioactive material.Why don’t we glow in the dark?a) People do not emit any kind of light.b) People only emit light that is invisible to our eyes.c) People are too small to emit enough light for us to see. d) People do not contain enough radioactive material.How does light tell us the speed of a distant object?The Doppler Effect!Recall for sound waves:Approaching sound emitter has higher pitch (shorter wavelength).Receding sound emitter has lower pitch (longer wavelength).Doppler Effect for SoundHow does light tell us the speed of a distant object?The Doppler Effect!Recall for sound waves:Approaching sound emitter has higher pitch (shorter wavelength).Receding sound emitter has lower pitch (longer wavelength).Same thing for light!Approaching light emitter looks bluer (shorter wavelength).Receding light emitter looks redder (longer wavelength).Doppler Effect for LightMeasuring the ShiftWe generally measure the Doppler effect from shifts in the wavelengths of spectral lines.StationaryMoving awayAway fasterMoving towardToward fasterThe amount of blue or red shift tells us an object’s speed toward or away from us:Doppler shift tells us ONLY about the part of an object’s motion toward or away from us:Moving along line of sight  full speed measuredMoving across line of sight  no Doppler effectIntermediate caseEarthThought QuestionI measure a line in the lab at 500.7 nm.The same line in a star has wavelength 502.8 nm. What can I say about this star?A. It is moving away from me.B. It is moving towards me.C. It has unusually long spectral lines.Thought QuestionI measure a line in the lab at 500.7 nm.The same line in a star has wavelength 502.8 nm. What can I say about this star?A. It is moving away from me.B. It is moving towards me.C. It has unusually long spectral lines.ASTR100 (Spring 2008) Introduction to AstronomyCollecting Light with TelescopesProf. D.C. RichardsonSections 0101-0106How do telescopes help us learn about the universe?Telescopes collect more light than our eyes  light-collecting area.Telescopes can see more detail than our eyes  angular resolution.Telescopes/instruments can detect light that is invisible to our eyes (e.g., infrared, UV).Bigger is better1. Larger light-collecting area.2. Better angular resolution.Bigger is betterAngular Resolution•The minimum angular separation that the telescope can