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Astronomy week 5

by: Lauren Price

Astronomy week 5 Astronomy 154

Lauren Price
GPA 4.0

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Lessons 11-13
Stars/Galax/Cosmology Lecture
Sean Lindsay
Class Notes
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This 11 page Class Notes was uploaded by Lauren Price on Monday September 19, 2016. The Class Notes belongs to Astronomy 154 at University of Tennessee - Knoxville taught by Sean Lindsay in Fall 2016. Since its upload, it has received 4 views. For similar materials see Stars/Galax/Cosmology Lecture in Physics and Astronomy at University of Tennessee - Knoxville.


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Date Created: 09/19/16
Astronomy 9/14/16 The Electromagnetic Spectrum ­Visible portions of the Em Spectrum are very small ­Scale is logarithmic, i.e., each labeled tick is marked 10 times the previous, not added  10 ­Stretched out to a linear scale, this graphic would stretch many light­years in size ­You need to know the names of the pieces of the EM Spectrum in the correct order Atmospheric Opacity ­The spectral windows are the wavelengths of the EM Spectrum that reach the ground ­Determines what light of the universe we can see from Earth’s surface ­The universe looks very different depending on what wavelength “color” of light you are observing it in Thermal Radiation ­Temperature is a measure of the average microscopic motion ­All thermal motions theoretically cease at absolute zero ­Defined to be 0 Kelvin, not degrees Kelvin, just 0 Kelvin ­Kelvin = degrees celsius + 273 ­Blackbody: An idealized body that absorbs and reradiates all wavelengths of light  perfectly ­Y­axis is Intensity, or simply how much light, the brightness. Larger values,  means more of that color ­X­axis is either frequency or wavelength ­So together, a blackbody spectrum tells us how much of each wavelength of  light a body with temperature, T, is giving off, emitting ­Characterizes the electromagnetic radiation light, emitted by sufficiently dense  objects ­Reflects the internal thermal motions (moving particles with charge) ­Sufficiently dense covers dense gases, liquids, and solids ­As a function of temperature, every temperature has a unique blackbody  spectrum (curve) Radiation Laws: Wein’s Laws ­Peak of emission (in wavelength) is inversely proportional to temperature  Will be on quiz  The Colors of Stars  Explained ­The colors of the stars  reflect their surface  temperature  The Radiation Laws ­Stefan­Boltzmann Law: The total energy per unit area per second, flux, emitted is  proportional to the fourth power of temperature  ­Power/Area ­The constant is equal to 5.67 x 10^­8 W/m2 K4. SI units for Flux: watts/meter2 = joules/ (sm2) and joules are the Si unit for energy; watts are the Si unit for power ­Power = energy per second, for emitting objects this is called luminosity  Astronomy  9/16 Doppler Effect ­How the frequency (wavelength) of a wave changes with respect to relative motion ­If one is moving toward an object the wavelengths seem shorter (higher frequency)  “bluer” ­If one of moving away from an object the wavelengths seem longer (lower frequency)  “redder” ­The Doppler Effect depends on relative motion ­Motion needs to have radial (toward/away) component Chapter 4 Spectroscopy  ­Spectroscopy is the study of interaction between light (electromagnetic waves) and  matter (atoms, molecules, etc.) ­The interaction depends strongly on the type of matter (specific atom or molecule) and  the wavelength (or frequency) of light ­So to study it we take part of the electromagnetic spectrum, i.e., visible light, and break it into it’s component wavelengths (color) ­The tool we use to measure the Intensity/Brightness of each wavelength (color) is  called a spectrometer ­Brightness as a function of wavelength; we saw this with the Blackbody Curve Types of Spectra 1. Continuous spectra ­Created by a blackbody: a  sufficiently dense object ­e.g., star, planet, light bulb  filament, hunk of iron, very dense  gas 2. Emission spectra ­Distinct lines of “color” in emission 3. Absorption spectra  ­Distinct lines of “color” removed  (absorbed) Absorption Spectra ­Every element has unique set of lines ­Look like a continuous spectra with specific wavelengths removed Emission and Absorption Spectra ­They occur at precisely the same wavelengths ­Every element has a unique set of spectral “fingerprints”  Spectra as Compositional Detectors ­The Fraunhofer lines in the Solar Spectrum, all the missing lines ­The absorption lines correspond to elements found in the Sun’s atmosphere, gives us  the composition of the Sun Kirchhoff’s Laws: Spectrum of Source Depends on How You Look at it ­Absorption spectrum: spectrum taken along “line of sight” of continuous source through a cool gas ­Emission spectrum: spectrum of a hot, diffuse gas on its own ­Continuous spectrum: spectrum of an object close to a blackbody Kirchhoff’s Laws 1. A solid, liquid, or sufficiently dense gas emits light of all wavelengths (blackbody  radiation) and produces a continuous spectrum 2. A low­density, hot gas emits an emission spectrum with emission lines that are  characteristic of the chemical composition of the gas 3. A cool, low­density gas absorbs certain wavelengths from a continuous spectrum,  such that the absorption lines appear at the same wavelengths as the emission lines of the same gas at a higher temperature  Atoms and Radiation ­In the spectrum of a gas, the wavelengths (frequencies) and energies are not  continuously spread out ­Spectral lines appear at very precise wavelengths ­We can understand why lines are restricted to specific wavelength quantities using the  particle nature of light and by understanding atomic structure  Atomic Structure ­The Bohr Model ­The discovery of quantized spectral lines requires a new model for the atom, enter  Neils Bohr (1912) ­Quantized means restricted number of states or values (quantities) ­An atom is constructed of a nucleus at the center orbiting electrons ­The nucleus contains protons and neutrons ­Protons have a positive charge, electrons have a negative charge, and neutrons have  no charge (neutral) Light as a Particle  ­Regardless of light intensity/brightness  ­IR and Red Light give no electric reading  ­Blue light gives weak reading ­UV gives strong reading ­Threshold is based on wavelength of light, not intensity of light , minimum energy to  knock electrons free of surface. If a wave, increasing intensity should get a response  from any wavelength. If a particle, each particle of light (photon) above the threshold  can kick out an electron  The Energy of Photon ­As a particle, a photon has an energy that is directly proportional to frequency  (inversely proportional to wavelength)  ­Photon energy usually measured in electron volts (eV), which is the energy gained by  an electron accelerated through an electric potential of one volt Astronomy Lecture 13 9/19 The Energy of Photon ­As a particle, a photon has an energy that is directly proportional to frequency  (inversely proportional to wavelength  ­Bluer has a shorter wavelength and higher energy ­Redder has a longer wavelength and lower energy  Atomic Structure  ­Electrons are at distinct energy levels called orbitals ­The lowest energy level, n = 1, is called ground state  ­Electrons prefer to be in the ground state, which is the lowest energy state ­Electrons with higher energy move to a more energetic state (n > 1) called an excited  state ­Label excited states as first excited state (n = 2), second excited state (n = 3), etc. ­There is a maximum energy that an electron can have and still be part of the atom ­If an electron achieves or exceeds that maximum, it is no longer bound to the nucleus,  and the atom is said to be ionized ­The atom missing one or more of its electrons is called an ion ­Electrons are not on orbit like the planets, but rather, occupy specific volumes of space  around the nucleus, called orbitals, but have a probabilistic nature  Photons and Atoms ­Each electron orbital has an associated specific, quantized, energy E ­No ‘in­between’ levels ­Absorption and Emission spectra of hydrogen, and other atoms, demonstrated taht  atoms can only absorb or emit specific energies ­The amount of energy E absorbed or emitted corresponds perfectly to the energy  difference between two electron orbitals  ­These packets of quantized energy are particles called photons, with energy E =  constant x frequency of light  Formation of Spectral Lines ­Multiple paths of excitation and de­excitation ­Direct excitation and de­excitation back to ground state  The Hydrogen Atom (1 proton, 1 electron) ­Energy levels of the hydrogen atom, showing two series of emission lnes


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