AST 115 1st Edition Lecture 32 Outline of Last Lecture I. The Distribution of Galaxies in the Universe continuedII. Interacting GalaxiesIII. Dark Matter in Other GalaxiesOutline of Current Lecture I. Some methods of determining distancesII. Redshifts in the Spectra of GalaxiesIII. The Hubble LawIV. The “Look – Back” PrincipleV. QuasarsCurrent Lecture Some methods of determining distances:1. Surveying or triangulation ( d=1p )2. Variable stars (RR Lyr; Cepheid)3. Novas, Supernovas(other methods also exist)o Methods 2 and 3 make use of the “standard light bulb” technique. Measuring the apparent magnitude of an object, then measuring/estimating the absolute magnitude of that same objects. ( m - M ) Knowing the m and M leads to m – M (distance modulus) and thus the distance. Redshifts in the Spectra of GalaxiesThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o Slipher discovered that 38 of 41 bright “spiral nebulas” (galaxies) had redshifts in their spectra; they are moving away from us.Redshift=z =[WL(observed)−WL(rest)]WL(rest ) WL = wavelength z values can range from zero to maximum The dimmer and presumably more distant galaxies showed greater redshifts (The “Law of Redshifts”). The Hubble Lawo Hubble and Humason also measured the distances (using Cepheids) to the galaxies they got redshifts for.o They converted z (redshift) to V (velocity) and graphed it versus the distances they had measured. They found a linear relationship. The slope of the line is called the H (for Hubble and Humason).H=change∈velocitychange∈distance → The Hubble “ratio”, where units are km/sec per Mpc (or km/sec per MLY). Thus V =H × d=Hd → “The Hubble Law”; or…- “The farther a galaxy is, the greater its recessional velocity is.”o Assume H = 75 km/sec per Mpc, and solve for d (distance).d=VH=czH=30,000 z75c = speed of lightd=4,000 z(¿ Mpc)for z<0.1 Example: - Find d if a galaxy has z = 0.05d=4,000 ×0.05=200 Mpc~ 650 MLYo The use of Hubble’s Law to measure distances is the biggest “meter stick”.o We can use this when no objects of any kind can be resolved in a galaxy, when only the spectrum can be obtained.o The discovery of the Hubble Law also means that the universe is expanding. The “Look – Back” Principleo “The farther you look into space, the earlier back in time you see.”o We see the Sun not as it is now, but as it was about 8 minutes ago.o We see M31 not as it is now, but as it was about 2.5 million years ago.o We see a galaxy with a redshift of 0.05 not as it is now, but as it was about 650 million years ago. Quasarso Quasars = new kind of object based on radio emission (1960s).o Optical photos showed “star-like” dotso 3C 48 = one of the first to be discoveredo Stars are not strong radio sources, they are not quasars.o QUAsi – StellAR Radio Sources “3C 48” was the first Quasar discovered.o They had emission lines with large redshifts (M. Schmidt 1963).o They contain hot gas, are moving away from us rapidly, and are very far away. They must have great luminosity to be seen at distances of billions of LY.o Some quasars vary in times as short as a few days, implying that they region responsible for the great luminosity must be about a few light days across: about 10 times the size of the solar
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