ASTR 1102 Lecture : Lecture30
Document Summary
Population i: metal rich, typically younger stars (coded blue) Population ii: metal poor, old stars (coded red) We cannot fly up and look down on it. Infrared is better than optical, but still some dust. Radio waves are not affected by dust at all. Radio waves also let us do something very neat: we can measure position, and distance of hydrogen clouds. Optical hydrogen lines require quite hot gas (like emission nebula) Should see 21cm radio waves from interstellar hydrogen. Strong emission from the plane of the galaxy. So far we have only seen two-dimensional map on sky. Very hard to get distance of hydrogen gas directly. If gas is approaching or receding, line is blue or red-shifted (doppler effect) If we understand how galaxy rotates, we can deduce how far gas cloud is. Very untidy - incomplete arms, disjointed arcs . Other spiral galaxies can also be untidy viewed this way.
