AST—Jan 24 Lecture + Chapter Notes

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Astronomy & Astrophysics
Michael Reid

ASTSpectroscopy Redshift and the Doppler Effect Tuesday January 24 201254Learning From Light y Matter leaves its fingerprints whenever it interacts with light y Redshift absorbs all visible photons except those in red part of the spectrum it must contain a special dye with these absorbing characteristics y Spectroscopyprocess of obtaining a spectrum and reading the information it containsy Spectra show the amount of radiation or intensity at different wavelengths o Intensity at a given wavelength is proportional to the number of photons observed at that wavelength times the energy of those photonsWhat are the tree basic types of spectra y A continuous spectrum looks like a rainbow of light y An absorption line spectrum specifies which colors are missing from the rainbow y An emission line spectrumwe see light only with specific colors against a black background 1 Continuous Spectrum y A spectrum of light that spans broad range of wavelengths without interruption by emission or absorption lines 2 Emission Line Spectrum y A spectrum that contains emission linesemission is the process by which matter emits energy in the form of light o Emission lines look like spikes on a background with little or no intensity o A warm gas cloud produces an emission line spectrum 3 Absorption Line Spectrumy A spectrum that contains absorption linesabsorption is the process by which matter absorbs radiative energyo Absorption lines appear as dips on a background of relatively high intensity lightHow does light tell us what things are made of y Emission or absorption lines occur only at specific wavelengths that correspond to particular energy level transitions in atoms or molecules Every kind of atom ion and molecule produces a unique set of spectral lines so we can determine composition by identifying these linesy Emission Line Spectrao Atoms in any cloud of gas are always colliding with one another exchanging energy in each collision o Most collisions send the atoms off in new directionsA few collisions transfer the right amount of energy to bump an electron from a low energy level to a higher energy level o Electrons cant stay at higher energy levels for longalways fall back down to the ground stateThe energy the electron loses when it falls to a lower energy level must go somewheregoes to emitting a photon of light
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