Announcements. Learn about SAIC's own Graduating Fellowship Program from Romi Crawford, Director, Visiting Artists Program and Denenge Akpem, Fellowship & VAP Coordinator Thursday, February 12 at 12:15 in 112 S. Michigan Building, Room 816. Class website. - PowerPoint PPT Presentation
Announcements Announcements ● Learn about SAIC's own Graduating Learn about SAIC's own Graduating Fellowship Program from Romi Fellowship Program from Romi Crawford, Director, Visiting Crawford, Director, Visiting Artists Program and Denenge Artists Program and Denenge Akpem, Fellowship & VAP Akpem, Fellowship & VAP Coordinator Thursday, February 12 Coordinator Thursday, February 12 at 12:15 in 112 S. Michigan at 12:15 in 112 S. Michigan Building, Room 816 Building, Room 816
Transcript
AnnouncementsAnnouncements
● Learn about SAIC's own Graduating Fellowship Learn about SAIC's own Graduating Fellowship Program from Romi Crawford, Director, Visiting Program from Romi Crawford, Director, Visiting Artists Program and Denenge Akpem, Artists Program and Denenge Akpem, Fellowship & VAP Coordinator Thursday, Fellowship & VAP Coordinator Thursday, February 12 at 12:15 in 112 S. Michigan February 12 at 12:15 in 112 S. Michigan Building, Room 816Building, Room 816
Class websiteClass website
● If there are any questions about the course If there are any questions about the course material or assignment, check the web page:material or assignment, check the web page:
● Answers are on the web pageAnswers are on the web page
Comments on assignment, quiz, Comments on assignment, quiz, marking:marking:
● Quiz: Lots of problems with the lifetime question Quiz: Lots of problems with the lifetime question (how does lifetime of civilization play a role in (how does lifetime of civilization play a role in the Drake equation)the Drake equation)
● If civilizations are very short-lived on average, If civilizations are very short-lived on average, there will be fewer that are still alive today, so N there will be fewer that are still alive today, so N drops.drops.
● If civilizations are very long-lived on average, If civilizations are very long-lived on average, most will still be alive, so N is longer.most will still be alive, so N is longer.
● L / LL / LMWMW is the fraction of civilizations born that is the fraction of civilizations born that
are still alive if civilizations are born throughout are still alive if civilizations are born throughout the lifetime of the Milky Waythe lifetime of the Milky Way
Comments on assignment, quiz, Comments on assignment, quiz, marking:marking:
● Assignment: Lots of problems with `scientific-Assignment: Lots of problems with `scientific-sounding claim problem'.sounding claim problem'.
● Need to provide either tests of a theory, or Need to provide either tests of a theory, or theories to explain an observation.theories to explain an observation.
Summary of Last Class: Distance LadderSummary of Last Class: Distance Ladder
● Four `realms', with their own scales and methods Four `realms', with their own scales and methods of measurementof measurement
● When saying something is far away, important to When saying something is far away, important to ask – compared to ask – compared to whatwhat
Summary of Last Class: Drake EquationSummary of Last Class: Drake Equation
● Can use to estimate number of civilizations in Can use to estimate number of civilizations in galaxy todaygalaxy today
● Process of going through the estimate shows Process of going through the estimate shows what we know fairly well (number of stars) and what we know fairly well (number of stars) and what needs more work (everything else, but gets what needs more work (everything else, but gets worse as you go on)worse as you go on)
● Will use to structure rest of the course.Will use to structure rest of the course.
Feedback:Feedback:● Most unclear item from last week's readings?Most unclear item from last week's readings?
Feedback:Feedback:● How much time was spent on readings?How much time was spent on readings?● How much time was spent on homework?How much time was spent on homework?
What we're going to cover todayWhat we're going to cover today● Observing the Universe: The Electromagnetic Observing the Universe: The Electromagnetic
Spectrum (Ch 2)Spectrum (Ch 2)
– The EM spectrumThe EM spectrum– Inverse Square LawInverse Square Law– Processes by which light is emitted/absorbedProcesses by which light is emitted/absorbed– SpectraSpectra
● Galaxies and The Expanding Universe (Ch 6)Galaxies and The Expanding Universe (Ch 6)
– Types of GalaxiesTypes of Galaxies– Expanding UniverseExpanding Universe– Dark MatterDark Matter
Observing the Universe: Observing the Universe: Electromagnetic wavesElectromagnetic waves
● At end of this lecture/reading, should be able to:At end of this lecture/reading, should be able to:
– Explain the inverse square law and use it to Explain the inverse square law and use it to solve (non-algebraic) problemssolve (non-algebraic) problems
– Define the electromagnetic spectrumDefine the electromagnetic spectrum– Explain what's meant by blackbody Explain what's meant by blackbody
radiation and line spectrumradiation and line spectrum– Explain the wave nature of lightExplain the wave nature of light– Describe how spectra are used to determine Describe how spectra are used to determine
the composition (and speed) of astrophysical the composition (and speed) of astrophysical objectobject
● This is how we must observe the distant UniverseThis is how we must observe the distant Universe● Only now beginning to be able to observe Only now beginning to be able to observe
gravitational wavesgravitational waves● Most of our observations come from Most of our observations come from
Inverse Square LawInverse Square Law● Electromagnetic (and most other Electromagnetic (and most other
kinds) of radiation obey the kinds) of radiation obey the Inverse-Inverse-Square LawSquare Law
● Intensity of radiation (brightness) Intensity of radiation (brightness) falls off with the square of the falls off with the square of the distancedistance
– Doubling the distance to Doubling the distance to something makes it appear something makes it appear four times as dim (¼ as bright)four times as dim (¼ as bright)
– Tripling the distance makes it Tripling the distance makes it appear nine times as dim (1/9 appear nine times as dim (1/9 as bright)as bright)
● Electromagnetic radiation Electromagnetic radiation from a source is in the form from a source is in the form of wavesof waves
● Both Electric and Magnetic Both Electric and Magnetic componentscomponents
● Wave travels at speed of Wave travels at speed of lightlight
WavesWaves
Speed
wavelength
frequency
WavesWaves
● (speed) = (wavelength) x (frequency)(speed) = (wavelength) x (frequency)● Higher frequency – more energyHigher frequency – more energy● Higher amplitude – more energyHigher amplitude – more energy● Have to look on scales ~ wavelength to see Have to look on scales ~ wavelength to see
the wavethe wave
Electromagnetic WavesElectromagnetic Waves
● (speed) = (wavelength) x (frequency)(speed) = (wavelength) x (frequency)
– But speed is fixed (all EM waves travel But speed is fixed (all EM waves travel at the speed of light)at the speed of light)
– So given frequency, you can know the So given frequency, you can know the wavelength and vice versa.wavelength and vice versa.
● Higher frequency – more energyHigher frequency – more energy● Higher amplitude – more energyHigher amplitude – more energy
– But EM waves come in bundles But EM waves come in bundles (`photons') with fixed amplitude(`photons') with fixed amplitude
● Wave nature usually only noticeable on scales Wave nature usually only noticeable on scales ~ wavelength~ wavelength
Electromagnetic WavesElectromagnetic Waves
● Wave nature of light usually not noticeableWave nature of light usually not noticeable● Wavelength of light ~ 1/40000Wavelength of light ~ 1/40000thth of an inch of an inch● In common experience light behaves as if it In common experience light behaves as if it
were made up of particles or rays which were made up of particles or rays which emanate from sourceemanate from source
– Newton's `corpuscular' (particle) theory Newton's `corpuscular' (particle) theory of lightof light
Electromagnetic WavesElectromagnetic Waves
● Light is one facet of the entire electromagnetic spectrumLight is one facet of the entire electromagnetic spectrum● Our eyes have dedicated cells which are sensitive to Our eyes have dedicated cells which are sensitive to
electromagnetic radiation in this rangeelectromagnetic radiation in this range● Eyes most sensitive to yellow light – this is where the sun Eyes most sensitive to yellow light – this is where the sun
emits the peak amount of energyemits the peak amount of energy
CB Radio AntennaCB Radio Antenna ~27 MHz; wavelength~ 36 ft~27 MHz; wavelength~ 36 ft
~9'
Satellite TV dishSatellite TV dish ~12 GHz; wavelength ~9”~12 GHz; wavelength ~9”
~4.5”
What Generates Electromagnetic Waves?What Generates Electromagnetic Waves?
● Thermal radiationThermal radiation: Hot things glow.: Hot things glow.● Heat causes atoms to rattle about in an objectHeat causes atoms to rattle about in an object● Atoms contain charged particles (electrons, protons)Atoms contain charged particles (electrons, protons)● Accelerating charged particles emit electromagnetic Accelerating charged particles emit electromagnetic
radiation.radiation.
Thermal RadiationThermal Radiation● If material is dense enough to be opaque, hot body emits If material is dense enough to be opaque, hot body emits
radiation in a characteristic `blackbody' spectrumradiation in a characteristic `blackbody' spectrum
● Hot objects emit Hot objects emit moremore and at and at shorter wavelengthsshorter wavelengths ((higher frequencies)higher frequencies)
High frequency Low frequencyShort wavelength Long wavelength
Temperature and SpectrumTemperature and Spectrum● Can put temperatures on Can put temperatures on
frequency chartfrequency chart● Our suns temp ~6000 K Our suns temp ~6000 K
(~10,000 F) means that the (~10,000 F) means that the peak of its radiation is in peak of its radiation is in yellow part of visible lightyellow part of visible light
● Room temperature: infraredRoom temperature: infrared● X-rays, gamma-rays: so X-rays, gamma-rays: so
high energy that would high energy that would thermal emission is modest thermal emission is modest up thereup there
Line SpectraLine Spectra● For non-opaque materials, For non-opaque materials,
spectra can look quite spectra can look quite different.different.
● Atoms/molecules can emit Atoms/molecules can emit or absorb photons only of or absorb photons only of particular energies.particular energies.
● If dense enough, these lines If dense enough, these lines get blended out into get blended out into blackbody spectrumblackbody spectrum
● If not (like gas in flame) the If not (like gas in flame) the spectrum is composed of spectrum is composed of lineslines
Line SpectraLine Spectra● If electron is an excited state If electron is an excited state
((e.ge.g., from thermal jostling ., from thermal jostling around) can fall back down around) can fall back down and lose a specific amount of and lose a specific amount of energyenergy
● Corresponds to specific Corresponds to specific frequency/colourfrequency/colour
● All possible combinations of All possible combinations of going down levels makes for going down levels makes for the the emission spectraemission spectra of that of that materialmaterial
Line SpectraLine Spectra● If photon is coming towards If photon is coming towards
electron of right energy, can be electron of right energy, can be absorbed by electronabsorbed by electron
● Electron jumps up a levelElectron jumps up a level● After a while, electron will After a while, electron will
`fall' back down, emitting `fall' back down, emitting photon, but maybe:photon, but maybe:
– in another directionin another direction– in stages, emitting at in stages, emitting at
other frequencies other frequencies ● Net effect: Net effect: absorption spectraabsorption spectra
Atmospheric AbsorptionAtmospheric Absorption
● Our own atmosphere absorbs a lot of radiationOur own atmosphere absorbs a lot of radiation● Absorbs UV (ozone): keeps energetic radiation from Absorbs UV (ozone): keeps energetic radiation from
destroying lifedestroying life● Absorbs infrared (greenhouse gasses): good to keep Absorbs infrared (greenhouse gasses): good to keep
things warm, but too much can cause troubles things warm, but too much can cause troubles
Solar SpectrumSolar Spectrum
hot core
Wispier outer layersWispier outer layers
● Central region of sun fairly Central region of sun fairly dense dense
– Emits as blackbodyEmits as blackbody● Outer layers progressively less Outer layers progressively less
densedense
– Line effects start Line effects start becoming noticeablebecoming noticeable
● We see continuum blackbody We see continuum blackbody spectrum from the inner star spectrum from the inner star with absorption features from with absorption features from the outer layersthe outer layers
Solar SpectrumSolar SpectrumSolar SpectrumSolar Spectrum
CalciumCalcium
Hydrogen Hydrogen
SodiumSodiumOxygen MoleculesOxygen Molecules
The Sun throughout the spectrumThe Sun throughout the spectrum
Other Physical Processes Other Physical Processes
● Other, non-thermal processes can generate Other, non-thermal processes can generate electromagnetic radiation:electromagnetic radiation:
– Hot electrons in magnetic fields can Hot electrons in magnetic fields can generate X-raysgenerate X-rays
– Nuclear decays can produce X-rays Nuclear decays can produce X-rays or gamma raysor gamma rays
● Thus, spectra can tell us composition of an Thus, spectra can tell us composition of an object and what physical processes are object and what physical processes are occurring.occurring.
The Galaxy throughout the spectrumThe Galaxy throughout the spectrum
Break!Break!
Doppler Shift in LightDoppler Shift in Light
Doppler Shift in LightDoppler Shift in Light
Doppler Shift in LightDoppler Shift in Light
Doppler Shift in LightDoppler Shift in Light
Doppler Shift in LightDoppler Shift in Light
But what if it moves?But what if it moves?
Doppler Shift in LightDoppler Shift in Light
Doppler Shift in LightDoppler Shift in Light
Doppler Shift in LightDoppler Shift in Light
Doppler Shift in LightDoppler Shift in Light
Doppler Shift in LightDoppler Shift in Light
● Sound or light from a source moving towards Sound or light from a source moving towards you is shifted to higher frequencies (light is you is shifted to higher frequencies (light is bluer)bluer)
● From a source moving away from you, shifted to From a source moving away from you, shifted to lower frequencies (redder)lower frequencies (redder)
Doppler Shift in LightDoppler Shift in Light
● Effect is fairly modest, but spectra can be Effect is fairly modest, but spectra can be measured measured veryvery accurately accurately
● Astronomers can measure velocities Astronomers can measure velocities towards/away very preciselytowards/away very precisely
GalaxiesGalaxies
● At end of this lecture/reading, should be able to:At end of this lecture/reading, should be able to:
– Describe the differences between spiral, Describe the differences between spiral, elliptical and irregular galaxieselliptical and irregular galaxies
– Sketch the structure of a spiral galaxySketch the structure of a spiral galaxy– Explain what evidence there is for galaxies Explain what evidence there is for galaxies
moving apart (expansion of the universe)moving apart (expansion of the universe)– Describe the evidence for galactic dark Describe the evidence for galactic dark
mattermatter
GalaxiesGalaxies
● Amongst the first things born Amongst the first things born in Universein Universe
● Hubble Deep Field, looks back Hubble Deep Field, looks back up to 12 billion years in past up to 12 billion years in past (Universe ~ 15 billion years)(Universe ~ 15 billion years)
● Already galaxies like todays Already galaxies like todays existed, although not as much existed, although not as much structurestructure
GalaxiesGalaxies
● Island universesIsland universes● Building blocks of the universeBuilding blocks of the universe● Contain millions, billions, or trillions of starsContain millions, billions, or trillions of stars● Also contain gas clouds (from which new stars Also contain gas clouds (from which new stars
can be born), dust, star clusters,…can be born), dust, star clusters,…● Typically several tens of thousands of parsecs Typically several tens of thousands of parsecs
acrossacross● Vary in type and structureVary in type and structure
Spiral GalaxiesSpiral Galaxies
● Flat, disk-shaped Flat, disk-shaped galaxies with spiral galaxies with spiral armsarms
● Rotate (our part of our Rotate (our part of our galaxy rotates around galaxy rotates around the center every ~200 the center every ~200 million years)million years)
● Gas clouds, dust, starsGas clouds, dust, stars
Spiral GalaxiesSpiral Galaxies
Forms very early in Forms very early in universe from huge universe from huge cloud of gascloud of gas
● Some gentle initial Some gentle initial rotation around rotation around centercenter
● As collapses, As collapses, rotation increases, rotation increases, flattens diskflattens disk
● Star clusters form Star clusters form before collapse is before collapse is complete, orbit the complete, orbit the newly-forming disknewly-forming disk
Elliptical GalaxiesElliptical Galaxies
● SimplerSimpler● No initial rotation on collapseNo initial rotation on collapse● Forms a spherical or elliptical sea Forms a spherical or elliptical sea
of starsof stars● Uses up most of gas in star Uses up most of gas in star
formation at original collapse --- formation at original collapse --- mostly old starsmostly old stars
● Also contains globular clustersAlso contains globular clusters
Irregular GalaxiesIrregular Galaxies
● Neither spiral or ellipticalNeither spiral or elliptical● Shapes varyShapes vary● Probably disrupted in the process Probably disrupted in the process
of forming, or afterwards by of forming, or afterwards by neighboring galaxiesneighboring galaxies
Clusters and superclustersClusters and superclusters
● Under gravitational attraction, Under gravitational attraction, galaxies begin clustering galaxies begin clustering together:together:
– First in clusters (like our First in clusters (like our `local group’`local group’
– These then cluster into These then cluster into superclusters (like our superclusters (like our `Virgo supercluster’)`Virgo supercluster’)
Galaxies moving away from us!Galaxies moving away from us!
● Once `spiral nebulae’ Once `spiral nebulae’ were established as were established as galaxies, Hubble galaxies, Hubble examined their redshifts, examined their redshifts, and distancesand distances
● Found that galaxies were Found that galaxies were all moving away from us; all moving away from us; faster faster
Expanding UniverseExpanding Universe
● Either we are very special Either we are very special and everything is moving and everything is moving away from us, or Universe away from us, or Universe as a whole is expandingas a whole is expanding
● But if universe is steadily But if universe is steadily increasing in size, implies increasing in size, implies that at some time in the that at some time in the past, Universe was a past, Universe was a single point.single point.
● `Start of the Universe’`Start of the Universe’
– Big BangBig Bang
Evidence for the Big Bang: Evidence for the Big Bang: Microwave BackgroundMicrowave Background
At very beginning, Universe would have been very At very beginning, Universe would have been very hot hot
● Now, looking back, it is greatly redshifted Now, looking back, it is greatly redshifted (cooler)(cooler)
● Can calculate what temperature it would be Can calculate what temperature it would be now: ~ 3 degrees Kelvinnow: ~ 3 degrees Kelvin
● Microwave temperaturesMicrowave temperatures
The Microwave backgroundThe Microwave background
● Accidentally discovered Accidentally discovered by radio astronomers by radio astronomers (thought it was noise)(thought it was noise)
● 1980s, COBE satellite 1980s, COBE satellite went up to take careful went up to take careful measurementsmeasurements
● Blackbody temperature Blackbody temperature agrees with predictionsagrees with predictions
● Slight fluctuations; hot Slight fluctuations; hot spots which eventually spots which eventually gave rise to galaxies!gave rise to galaxies!
● Can also predict what nuclei are formed at such temperaturesCan also predict what nuclei are formed at such temperatures● Too cold: can’t form nucleiToo cold: can’t form nuclei● Too hot: large nuclei are torn apartToo hot: large nuclei are torn apart● Prediction: Universe should be mostly Hydrogen, Helium, some Prediction: Universe should be mostly Hydrogen, Helium, some
Lithium: Prediction agrees with observationLithium: Prediction agrees with observation
Reading for next classReading for next class
● Birth, life, and death of starsBirth, life, and death of stars● Chapters 3, 4, 5 (~61 pages)Chapters 3, 4, 5 (~61 pages)
