ASTR 101 Lecture 8 Outline of Last Lecture I Gravitational Potential Energy II Conservation of Energy III The Force of Gravity Outline of Current Lecture I Light The Cosmic Messenger II Learning from Light a Chemical Fingerprints b Thermal Radiation Current Lecture Chapter 5 Light The Cosmic Messenger Wavelength and Frequency Wavelength x Frequency speed of light constant lxf c l wavelength f frequency c 3 00 x 108m s speed of light 5 2 Learning from Light What are the three basic types of spectra Emission line spectrum Continuous spectrum Absorption line spectrum spectra of astrophysical objects are usually combinations of these three basic types How does light tell us what things are made of Chemical Fingerprints Each type of atom has a unique set of energy levels Each transition corresponds to a unique photon energy frequency and wavelength Because those atoms can absorb photons with those same energies upward transitions produce a pattern of absorption lines at the same wavelengths Each type of atom has a unique spectral fingerprint Observing the fingerprints in a spectrum tells us which kinds of atoms are present Thermal Radiation Nearly all large or dense objects emit thermal radiation including stars planets and you An object s thermal radiation spectrum depends on only one property its temperature Properties of Thermal Radiation 1 Hotter objects emit more light at all frequencies per unit area 2 Hotter objects emit photons with a higher average energy 17814524077 Interpreting an actual spectrum By carefully studying the features in a spectrum we can learn a great deal about the object that created it Between blue and green on the spectrum reflected sunlight Continuous spectrum of visible light is like the Sun s except that some of the blue light has been absorbed the object must look red Thermal radiation infrared spectrum peaks a wavelength corresponding to a temperature of 225K Carbon dioxide absorption lines are the fingerprints of CO 2 in the atmosphere Measuring the Doppler Effect shif Doppler shif tells us ONLY about the part of an object s motion toward or away from us
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