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Lecture 29

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PHYS 183
Tracy Webb

PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013 Lecture 29: March 27 , 2013  there are 2 main types of galaxies: spiral galaxies & elliptical galaxies  also have irregular galaxies which don’t fit into the other 2 categories  how are galaxies grouped together? galaxy clusters  galaxies are held together by gravity  galaxy groups contain very low mass galaxies (mostly spiral galaxies)  tiny villages  galaxy clusters contain very high mass galaxies (mostly elliptical galaxies)  large cities  you can’t measure distances galaxies by how far away they look  could just be faint  use a distance ladder/distance scale to determine how far away galaxies are  apparent brightness of an object is b = L / 4πd  this equation is the fundamental equation that the distance ladder is based on  can use radar and stellar parallax to get direct distance measurements up to about 1 light year away  past this distance we use standard candles method  direct measurements are similar to the techniques we use on earth to measure distances  can use radar for our solar system, and parallax to a maximum of 10 light years  standard candles requires some kind of assumption/inference that isn’t from a straight, direct measurement o standard candle: an object whose intrinsic luminosity is known (either measurement or constant) o we know how brightness falls with distance based on brightness = luminosity / 2 4πdistance o example: at home the brightness of a tea candle is dependant upon how far it is from you  you may be able to use it to read if it was beside you, but not if it was across the room o standard
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