ASTR 1020 1st Edition Lecture 3Current LectureLight and Spectra Review Part 2- speed of light= c= 3 x 10^8 m/sec- Light is a wave- Discrete particles called photons- High frequency=low energy- FIGURE 5.7o X-rays –very hot gas, exotic stars (blue)o Optical –visible/normal starso Infrared –warm gas, dust (red)Spectrum- Graph of light intensity vs. color/wavelength/frequency/energyElectronic Transitions- Conservation of energy –means that the change in energy has to go somewhere photons- Emission –electron moves to lower energy level, emits photon- Absorption –photon of the right energy hits atom, electron moves up to higher energy levelEmission line spectra- Electrons drop to lower energy level- Photon of the energy difference released- Thin, hot gasses emit an emission line spectrum=only certain wavelengths Absorption line Spectra- Thin gasses in front of a background continuum source.- Atoms absorb photons if they are of exactly the right energy.Doppler Shifts in Spectra- Shifts in astronomy can be large –pink hydrogen can be red shifted to infrared or even radio wavelengths!- We look for familiar atomic patterns at shifted colors/wavelengths and then convert the red/blue shift into a speed of motiono We use the Doppler effect to measure the speed of bright objects in spaceo Black hole in the center of galaxy M84 was detected using Doppler shifts!Continuous Spectrum- Hot solids (or dense liquid) emit light at all wavelengths- Thermal Radiation (or Blackbody Radiation)Thermal (Black body) Spectra- Smooth, continuous spectrum –light at all wavelengths - Emits in specific shape governed by temperatureShape and intensity of thermal spectra are set by the TEMPERATURE of the object.RULES for Thermal emission:1. Hotter =Brighter a. Hotter objects emit more light at all wavelengths per square meter of surface area –Stefan-Boltzmann Law Intensity T^42. Hotter=Bluera. Hotter objects peak at bluer wavelengths (photons with a shorter wavelength, higher frequency, and higher average energy.) –Wien’s LawBrightness of Hot, Solid Objects- Total energy output is also related to SIZE-Intensity / Surface Area- What’s the surface area of a sphere?Scaling relations again!Luminosity R^2 xT^4- Procyon is 2x the diameter of the Sun and 2x the temperature.- How do the luminosities of the 2 stars compare?Ratio (LPro/LSun) = (RPro/RSun)^2 x (TPro/TSun)^4= 2^2 x 2^4 = 2^6Procyon produces 2^6=64x more energy per second than the Sun!“Inverse square law”- Light travels outward in spheres. Total amount of light L stays constant but is spread over larger area- The area of a sphere =4 x pi x R^2The brightness (F) of an object decreases as the universe square (area) of the distance increases:F = L / 4πR^2F=flux or apparent brightness/how the object appears to usL= luminosity or actual total light outputR=
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