I ran across this story about a debate over the number of spiral arms in the galaxy (2 versus 4):

http://news.sciencemag.org/space/2013/12/scienceshot-milky-ways-missing-arms

The paper is here.

Thing is, I remember seeing charts showing pretty clearly there are more than two. I dug up the refs:

Taylor & Cordes 1993
and Cordes 2004 which you can see on page 6 here.

But I couldn't dig up a ref for the study that found only two arms.

For no particular reason I made a gif of the structures given by the 3 papers above:

It is hard to determine from our point of view what the exact shape of our galaxy is. For a long time it was thought that the Milky Way resembled the Andromeda galaxy but was somewhat smaller.

I think now the commonly held theory suggests the shape is a barred spiral.

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Bob DeWoody

My motto: Never do today what you can put off until tomorrow as it may not be required. This no longer applies in light of current events.

Nature World News says:

"The 12-year-long study of massive stars in the Milky Way conducted by Red MSX Source (RMS) survey led by University of Leeds astronomers scoured the galaxy for massive young stellar objects, which are uncommonly large. These massive stars [live] shorter lives than their lower mass counterparts, which astronomers utilized when conducting their survey. As these massive stars die relatively quickly, they are only found in the arms of the galaxy where they formed; they have no time to move elsewhere."

"Research done in the 1950s concluded that our galaxy has four spiral arms, based on the clouds of gas in which new stars form. But a later analysis by the NASA Spitzer Space Telescope that scoured the Milky Way with its infrared scope revealed about 110 million stars, but only evidence of two spiral arms."

Both surveys were looking for different things though.

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