Spiral Structure 101: (APOD 2009 April 14)

[bazillion]

Another non-astronomer here ... Is it possible to take a photograph like this one and filter out the images of stars known to be in the foreground? Is the parallax between summer and winter, say, enough to give a clean view of what's in the background?

[Chris Peterson]

Another non-astronomer here ... Is it possible to take a photograph like this one and filter out the images of stars known to be in the foreground? Is the parallax between summer and winter, say, enough to give a clean view of what's in the background?

All the stars in the image are in the foreground. Many of the objects that appear as stars are actually active knots in M101, globular clusters around M101, or background galaxies. For most or all of the foreground stars, annual parallax is insignificant. However, many could be identified by looking closely at their profiles (although this relatively low resolution image might present some problems), or their intensity through different filters. Once identified, they could be removed aesthetically by a combination of manual and semi-automated image processing. I've seen this done with several nebula images (perhaps even on APOD). The area behind the stars, of course, would simply be faked in by cloning.

Scientifically, there's no reason to do this, but for aesthetic purposes it might be interesting.