“With an eye made quiet by the power of harmony, and the deep power of joy, we see into the life of things.” -William Wordsworth
You've seen the amazing photos of spiral galaxies, with the vast, sweeping arms, the massive, central bulges and the great underdense regions in between the arms. As you move farther and farther towards the outskirts, the stellar density drops away to practically nothing.
Image credit: Vicent Peris (OAUV), José Luis Lamadrid (CEFCA), Jack Harvey (SSRO), Steve Mazlin (SSRO), Ivette Rodríguez (PTeam), Oriol Lehmkuhl (PTeam), Juan Conejero (PixInsight), via http://pixinsight.com/gallery/M74-CAHA/.
But if you see these galaxies at an angle, rather than face-on, you're likely to notice that there's not only the presence of dust, but that one "side" of the galaxy almost always appears "dustier" than the other!
Image credit: Bill Snyder (at Sierra Remote Observatories), via http://apod.nasa.gov/apod/ap140313.html.
Why is that? Thanks to the first 3D dust map, now we know!
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Superbly clear explanation.. Thanks!
Excellent presentation, Ethan. I would ask if either 'side' of a galaxy viewed more edge-on would show a shift in red as the approaching, & receding sides differ in velocity with respect to the observer?