Lecture 11-Chapter 19-Our Galaxy

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University of Toronto St. George
Astronomy & Astrophysics
Stefan Mochnacki

Chapter 19-Our Galaxy Our Galaxy N Our Milky Way Galaxy holds over 100 billion stars N Is among tens of billions of galaxies in the observable universe. N Is a vast spiral galaxy, with spiral arms. N The galactic is about 100,000 light-years in diameter, 1,000 light-years thick. N Sun is located at about 28,000 light-years from the galactic center. DIAGRAM 19.1 $9,78 7-L9L3 :7*,O,[ N Stars in the disk orbit in roughly circular paths that all go in the same direction in nearly the same plane. N Stars in the bulge and halo soar high above and below the disk on randomly oriented orbits. N Like horses on the merry-go-round, individual stars bob up and down through the disk as they orbit. N The orbit of a star around the galaxy arises from its gravitational attraction toward the galactic center N The bobbing arises from the localized pull of gravity within the disk itself. N A star that is too far above the disk is pulled back into the disk by gravity, but because the density of interstellar gas is too low to slow the star, it will fly too far below the disk on the other side N The ongoing process produces the bobbing of stars. N Up-and-down motions of the disk give the disk its thickness of about 1,000 light- years. N ,.K89,7847-L99,N0824709K,3 2LOOL430,78 N Each up-and-down bob takes a few tens of millions of years. N The orbital velocities of stars near the edge and those near the center are about the same. N Stars closer to the center therefore complete each orbit in less time than stars farther out. DIAGRAM 19.2 Orbits of Halo and Bulge Stars N Orbits of stars in the halo and bulge are much less organized than the orbits of stars in the disk. N These stars travel around the galactic center on more or less elliptical paths, but in random orientation of paths. N Neighbouring halo stars can circle the galactic center in opposite directions. N Halo and bulge stars soar to heights above the disk far greater than the up-and-down bobbing of the disk stars. Stellar Orbits and the Mass of the Galaxy N The sun and its neighbours orbit the center of the Milky Way at a speed of about 220 kmsec. www.notesolution.com
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