Lab 3 - PHSC12000 V1.0 PHSC 12000 Lab 3 Galaxies...

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PHSC12000 V1.0 PHSC 12000 Lab 3 Galaxies: Morphological Types and Spectra 1 Introduction In the 1920s University of Chicago alumnus Edwin Hubble 1 (S.B., 1910; Ph.D., 1917) solved four fundamental questions in astronomy and cosmology 2 : 1922-1926. Hubble proposed a classiFcation system for nebulae, both galactic (which we still call “nebulae”) and extragalactic (which we know call “galaxies”). The classi- Fcation system of galaxies has become known as the Hubble morphological sequence of galaxy types. In 1924 Hubble decisively showed that spiral “nebulae” (or galaxies) are huge “island universes” of stars similar to our own Milky Way, with his discovery of Cepheids in the nearby galaxies NGC 6822, M33 and M31 (The Andromeda galaxy) which allowed him to measure distances to them. In 1929 Hubble with a lot of technical help from his assistant Milton Humason dis- covered a linear relation between redshifts of lines in galaxy spectra and their distance from the Milky Way: the more distant a galaxy, the greater its redshift. This discovery led to the notion of the expanding universe and is the most fundamental discovery and observational fact that we know about our observable Universe. Having established the fact that galaxies are distant stellar systems like the Milky Way, Hubble used their distribution in various areas on the sky to show that the overall distribution of galaxies in space on large scales is quite homogeneous. This was a crucial fact as it justiFed the cosmological principle of homogeneity and isotropy used to obtain dynamical models of expanding universe from Einstein’s equations of general relativity. In a sequence of labs we will redo (in a simpliFed way) Hubble’s analysis that resulted in the Frst, third, and fourth of these fundamental discoveries using modern tools. We will also Fnd out that that the distribution of galaxies is not as uniform when looking at smaller scales, and that galaxies show a tendency to cluster. ±rom the study of galaxy clusters, we 1 You can fnd detailed inFormation about the liFe and work oF Edwin Hubble on this website: http://www.spacetelescope.org/about/history/the man behind the name/ 2 See the essay by Hubble’s student, Alan Sandage at http://antwrp.gsfc.nasa.gov/diamond jubilee/d 1996/sandage hubble.html 1
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PHSC12000 V1.0 2 will fnd evidence For the existence oF dark matter in the universe - one oF the most intriguing mysteries oF modern cosmology.
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Lab 3 - PHSC12000 V1.0 PHSC 12000 Lab 3 Galaxies...

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