PowerPoint PresentationMost kinds of e-m radiation cannot penetrate the Earth's atmosphereImagingSlide 4Slide 5Slide 6An Object’s SpectrumSpectra of galaxiesThe Four Basic Parameters of StarsHow to measure the surface temperature of a star?Slide 14Spectral Types A classification of the stellar black bodyTemperature, Luminosity, and Size – pulling them all togetherSlide 18The Doppler Effect: other information contained in spectrumIn general …Doppler EffectBut the spectrum tells us about the motion of sourcesSlide 23The Doppler shiftSlide 25Slide 26Intensity(spatial distribution of the light)Spectra(composition of the objectand the object’s velocity)There are three basic aspects ofthe light from an object that we can study from the Earth.Variability(change with time)Light is all we see in AstronomyMost kinds of e-m radiation cannot Most kinds of e-m radiation cannot penetrate the Earth's atmospherepenetrate the Earth's atmosphereImagingImagingImagingImagingAn Object’s SpectrumAn Object’s SpectrumEncoded in an object’s spectrumis information about the emitter/absorber. This is how we learn what the Universeis made of!Spectra of galaxiesSpectra of galaxies27 COSMOS BzK<z>=1.87The Four Basic Parameters of StarsLuminositySizeMassSurface TemperatureHow to measure the surface How to measure the surface temperature of a star?temperature of a star?1.1.Overall spectral shape (the peak of the Overall spectral shape (the peak of the blackbody continuous spectrum)blackbody continuous spectrum)2.2.More accurately, spectroscopicallyMore accurately, spectroscopicallyRemember: the peak of the B-B radiation lets us know the temperature of the emitting body.Spectral TypesSpectral TypesA classification of the stellar black bodyA classification of the stellar black bodyThe sun has a spectral type: G2For historical reasons, astronomers classify the temperatures of stars on a scale defined by spectral types, called O B A F G K M, ranging from the hottest (type O) to the coolest (type M) stars.Temperature, Luminosity, and Temperature, Luminosity, and Size Size –– pulling them all together pulling them all togetherStefan-Boltzmann LawLuminosityStellarradiusSurfacetemperatureL=4πR2 σT4A star’s luminosity, surface temperature, and size are all related by the Stefan-Boltzmann Law:1) 10 times more luminous2) 100 times more luminous3) 1000 times more luminous4) 1/10th as luminous5) 1/100th as luminousTwo stars have the same surface temperature, butthe radius of one is 10 times the radius of the other.The larger star isL=4πR2 σT4The Doppler EffectThe Doppler Effect::other information contained in spectrumother information contained in spectrum A moving light or sound source A moving light or sound source emits a different frequency in the emits a different frequency in the forward direction than in the reverse forward direction than in the reverse direction. direction. Take a look at the police car to see Take a look at the police car to see how this works.how this works.In general …In general … The “native” frequency at which an object is The “native” frequency at which an object is emitting is called the emitting is called the restrest frequency. frequency. You will see/hear frequencies You will see/hear frequencies higherhigher than than the rest frequency from objects moving the rest frequency from objects moving towardstowards you. you. You will see/hear frequencies You will see/hear frequencies lowerlower than the than the rest frequency from objects moving rest frequency from objects moving awayaway from you.from you.Doppler EffectDoppler EffectThe first crest travels out in circle from the original position of the planeAt a later time, a second crest is emitted from the planes new position,but the old crest keeps moving out in a circle from the planes original positionThe same thing happens again at a later timeLonger wavelength (more red)Shorter wavelength (more blue)But the spectrum tells us about But the spectrum tells us about the motion of sourcesthe motion of sourcesTwo identical stars are moving towards the Earth. Two identical stars are moving towards the Earth. Star A’s emission lines are observed to be at Star A’s emission lines are observed to be at visible wavelengths. The same emission lines visible wavelengths. The same emission lines for Star B are observed to be at ultraviolet for Star B are observed to be at ultraviolet wavelengths. From these observations you wavelengths. From these observations you conclude that: conclude that: Both stars are moving away from the EarthBoth stars are moving away from the EarthStar A is moving towards the Earth faster than Star A is moving towards the Earth faster than Star BStar BStar B is moving towards the Earth faster than Star B is moving towards the Earth faster than Star AStar AStar B is moving away from the Earth while Star B is moving away from the Earth while Star A is moving towards the Earth.Star A is moving towards the Earth.The Doppler shiftThe Doppler shiftAn object shining red light with An object shining red light with =656.3 =656.3 nm is moving at V=5,000,000 m/s toward nm is moving at V=5,000,000 m/s toward you. What is the color of the light that you. What is the color of the light that you see?you see?V/c = V/c = oo- - rr//rr5x105x1066/3x10/3x1088 = 1.67x10 = 1.67x10-2-2 = = oo- - rr//rrr = r = oox(1+1.67x10x(1+1.67x10-2-2) = 667.3 nm) = 667.3 nmTwo otherwise identical stars are rotating at Two otherwise identical stars are rotating at different rates. Star A is rotating slower different rates. Star A is rotating slower than Star B. How do Star A’s spectral than Star B. How do Star A’s spectral lines appear with respect to Star B’s lines appear with respect to Star B’s lines? lines? Star A’s lines are narrower than Star B’s Star A’s lines are narrower than Star B’s lines.lines. Star B’s lines are narrower than Star A’s Star B’s lines are narrower than Star A’s lines.lines.There is no difference in the lines of the There is no difference in the lines of the two stars.two stars.Star A’s lines are stronger than Star B’s Star A’s lines are stronger than Star B’s