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Lecture 2

Introduction to Astronomy-Lecture 2 Notes!.docx

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PCS 181
Margaret Buckby

Ionation Essay Topics: Virtual Particles, String Theory. *Know the Hydrogen spectrum for Test # 1. The Doppler Effect in Astronomy:  Star = Light Waves… take spectrum.  If the entire pattern of spectral lines is:  Shifted to the blue end of the spectrum (blue – shifted) =  Shorter distance between wave crests =  Object is moving toward you.  If the pattern is:  Shifted to the red end of the spectrum (red – shifted) =  Greater distance between wave crests =  Object is moving away from you. Direction of Motion:  Take the Spectrum of a stationary (= NOT Moving) Source (e.g., hydrogen) in the laboratory.  Take the Spectrum of a moving object (e.g., a Star).  Compare the two spectra’s.  Hydrogen Spectrum in Lab compared to Stellar Spectrum.  Two Blues, One Green, One Red = Know this Laboratory Spectrum.  Purple, Second Blue Line  Two Blues = Hydrogen Delta, Second Blue Line = 434 nm = Hydrogen Gamma, One Green = Hydrogen Beta, One Red = Hydrogen Alpha. *Test Question: Is the star moving toward you or away from you. Doppler Effect to Determine Velocity: 8  Let C = Speed of Light = 3 x 10 m/s.  Triangle Delta = Wave length difference between lab and stellar (STAR) Spectra.  Delta = Wave length on lab Spectrum. lab  V =rRadial Velocity of Star.  Then,  V =rC Triangle Delta Delta lab  Velocity of Star, 8  V =r(3 x 10 m/s) = (434.1 – 434) nm 434nm 4  V = r.91 x 10 m/s = Divide by 1000 to get in 1000’s. To get meters divide by 1000. 1000 m/s  V = r9.1 Km/s Radial Velocity Problem: = *TEST Question for Sure! = Different Numbers!  A Spectral line of hydrogen has a wavelength of 656.2850 nm in the lab. This same line appears at 656.2927 nm in the Spectrum of Regulus. Is Regulus approaching or receding? What is its radial velocity?  The same line has a longer wavelength in the Spectrum of Regulus, so it is red – shifted = Moving away from us.  V = r Triangle Delta = (3 x 10 m/s) (656.2927 – 656.2850) nm = Deltalab 656.2850 nm V r ~ 3500 m/s =3.5 Km/s 1000 m/s *NOTE: Divide m/s by 1000 to get form in km/s.  Using the Doppler Effect discussed here, how can you tell that our sun rotates? Distance Units in Astronomy:  Light - year = ly: Distance that light travels in one year. 15  1 light-year = 9.5 x 10 m  Parsec = pc: 1 pc (units in millions) = 3.26 light-year.  Astronomical Unit = AU = Average distance between the Earth and Sun. *NOTE: Takes 8 mins for sun light to
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