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Light Astronomy 315 Professor Lee Carkner Lecture 4

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Scale Exercise What is scale for solar system (0.0016 ly)? measure ball diameter = real/model = scale scale = 0.0016/2 = What is the model value for the distance to Sirius (9 ly)? real/scale = model 9 /0.0008= 11250 cm = Distance to other end of Science Building Change scale so that ball equals 9 ly new scale = 9/2 = Find size of galaxy in model (100000ly) 100000/4.5 = 22222 cm = Distance to Old Main

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Disturbing the Universe Can’t visit directly or send probes Would take ~100000 years to get to nearest star Can do some simulations in the lab But how do we know if they are right?

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Light What is light? How do these properties give us information about the object that emitted the light?

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What is Light? EM radiation can be thought of in two different ways: As a stream of photons (particle) Light is both a particle and a wave We use what ever formulation is most useful

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Properties of Light When we examine a light emitting object, what do we want to know? Energy Photon Flux How much total energy is emitted by an object depends on how much energy each photon has and how many of them are emitted

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Wavelength Each photon has a wavelength Energy is inversely related to the wavelength ( ) Long wavelength = Short wavelength = We will often measure wavelength in meters or nanometers (1 billionth of a meter, or 1X10 -9 m)

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Waves

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Speed and Frequency c = 3 X 10 8 m/s = 186,000 miles/s We can use this speed to write the frequency: c = f Frequency is directly related to energy High frequency = high energy Low frequency = low energy

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Color This is called visible light Short wavelength, high energy = blue Long wavelength, low energy = red

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A Spectrum

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Star Colors Stars come in 4 basic colors

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How is Light Produced? Every object in the universe emits blackbody radiation that depends on its temperature Given in degrees Kelvin Room temp = 300 K Higher T means more radiation

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Spectrum The radiation is a continuum of wavelengths called a spectrum We can describe the spectrum as a curve on the intensity versus wavelength diagram

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Peak Wavelength and Temperature A higher temperature produces a spectrum that peaks at shorter wavelengths Wien’s Law: max = 3,000,000/T Where T is in Kelvin and is in nanometers

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Intensity and Temperature A higher temperature means more total energy emitted Stefan-Boltzmann law: P = AT 4 is the Boltzmann constant (5.67 X 10 -8 W/m 2 K 4 ) A is the surface area of the object (in m 2 ) T is the temperature in Kelvin

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Using Radiation Laws Wien’s Law If you can find the peak wavelength you can find the temperature Stefan-Boltzmann law Hot objects emit more energy then cool objects The intrinsic brightness of a star depends on both its temperature and size

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Alberio This is the double star Alberio Two stars orbiting around each other Both are the same distance from Earth Size of star image proportional to brightness What is the relative temperature and size of the stars?

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The Electromagnetic Spectrum Light can have a wide range of wavelengths This corresponds to a wide range in energies Today we call the range of wavelengths the electromagnetic spectrum

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The EM Spectrum

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The EM Spectrum and You You see in visible light, feel infrared as heat and get a sunburn from ultraviolet Microwave and radio have long wavelengths and low energy

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Next Time Read Chapter 5.1-5.8

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