M101- A Different View

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All Images Copyright Steve Cannistra

Please click here for the original image of M101 by Rob Gendler.

M101 is a well-known spiral galaxy in Ursa Major.  From Earth, we see this galaxy face-on, but through the magic of 3D processing in Photoshop CS4 Extended, I have portrayed M101 as it might look if viewed at an angle.  The conventional, face-on image of M101 was graciously provided to me for this project by Rob Gendler, and the background starfield was obtained from my own image of M103.  The processing technique that I used is fairly straightforward and is nicely explained in this video tutorial by Russell Brown.  Although this is not a completely realistic portrayal of the galaxy's rotation (because the outer arms would not rotate in synch with the inner arms), it provides an interesting perspective of this familiar object.  The above image is accelerated for visual effect- the period of revolution for a typical galaxy like the Milky Way is approximately 225 million years!

Vera Rubin was the first astronomer to note that the rotation speed of the outer arms of galaxies was faster than predicted by Newtonian dynamics.  In particular, Newtonian dynamics predicts that orbital velocity should be inversely proportional to the square root of the distance from the center of gravity, which is how planets in our solar system behave.  And yet, when Rubin determined the speed of rotation in galaxies by studying Doppler shifts in the radio emission line of hydrogen, she found that the outer portions of galaxies were spinning more rapidly than expected, suggesting the presence of additional, unseen matter ("dark matter").  It is now generally accepted that the visible portion of galaxies like M101 and others comprises only a small fraction of the total galactic mass, with the remainder of the mass consisting of dark matter, which forms a dark matter "halo" that extends well beyond the conventional visible light boundary of the outer galactic arms.

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