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Lecture 30

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PHYS 183
Tracy Webb

rd Lecture 30: April 3 , 2013  announcements o observing project due date is the day after the final exam, this gives you 1 extra week to complete it o the final is cumulative; it is the same format as the midterms but longer o there will be one more short assignment, posted tomorrow o still waiting on grades from second midterm  we use distances of galaxies to measure Hubble’s Law  Hubble’s Law dates back to 1920  this is when astronomers were still look through telescopes, and Edwin Hubble did much of his work during this time  1 parsec is 3 light years  all of the measurements in Hubble’s Law are made with Cepheid variable stars but calibrated  a particular galaxy originally emits a certain spectrum of light, but the spectrum we observe is shifted by 5% to longer wavelengths, indicating that the galaxy is moving away from us at 5% of the speed of light  Hubble measured the distances from galaxies to Cepheid variables as well as its velocity relative to us  he saw that the velocity that a galaxy was moving is dependent on the distance it was to us  there is large variability, but a general pattern  Hubble’s law is the line that is drawn through the points on the plot (the general pattern)  Hubble’s law is represented by v = Hox d where v is velocity,oH is Hubble Constant (ranges 20/km/s/million light years to 24 km/s/million light years) and d is distance  we have extended the lines and created a modern Hubble diagram to include thousands more galaxies  every million light years that you go, you gain ~22 km/s of velocity  we can now use redshift as an estimate of distance where distance = velocity/Hubble’s constant  almost all of the distant galaxies are measured from their redshift and H
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