The Milky Way at far-ir wavelengths Milky Way morphology: early research >1610: Galileo Galilei discovered the Milky Way to be a vast collection of stars Mid 1700s: Milky Way is a stellar disk in which the solar system is embedded Star counts led William Herschel (1738-1822) to put the Sun near the center of the stellar disk. His assumptions: All stars have same absolute magnitude Number density constant Nothing to obscure them No stars too faint to see Glowing dust, heated by stars 21-9 21-10 21-11 21-12 Milky Way morphology: early research Distribution of globular clusters led Harlow Shapley in 1919 to put the sun at 1/3 of the radius of the Milky Way stellar disk, which is correct. However, the calibration of his distance scale was off by a factor ~2 he overestimated the size of the Milky Way by a factor 2
Solar Orbital Period Around the GC CO molecular gas in the Milky Way GIVEN: RSun ~8 kpc VSun ~220 km/s The orbital period is: P = 2 Rsun / Vsun 2 x 108 yr Mass enclosed within the solar orbit is: G M(Rsun ) / R2sun = V2sun / Rsun (for spherical distribution) M(Rsun ) 1011 Msun (almost true for disk) North America Nebula 21-17 Great Orion nebula Galactic center Coal Sack 21-18 Using the HI 21cm line to study our galaxy Rotation curve of molecular gas in the Milky Way Red shifted (receding) Blue shifted 21-19 21-20