Discuss how astronomers know that our galaxy contains spiral arms in its disk

Question

What will be an ideal response?

Answer 1

Radio astronomers use the strong spectral line emission from carbon monoxide (CO) to map the location of giant molecular clouds in the plane of the galaxy. Giant molecular clouds are sites of active star formation. If you point a radio telescope at a section of the Milky Way, you will receive a combination of signals from gas clouds in the direction you are looking that lie at various distances across the galaxy. Signals from those different clouds can be separated by measuring the Doppler shifts of their spectral lines. Astronomers then use a simple model of orbital velocities at different distances from the center of the galaxy in order to untangle the observations and locate the individual clouds. Maps constructed from such observations reveal that the giant molecular clouds, like O and B stars, are located along segments of spiral arms.