High proper motion stars mess with Google Sky

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I was alerted to this by a post on the Bad Science forum. There one of the posters had used the position of Barnard’s Star to estimate the date a Google Sky image had been taken. Barnard’s Star has the highest proper motion of any star known. This means it moves across the sky compared to background stars. Looking at the image I wondered why Barnard’s star appeared only once as a single, very blue image. These colour images are made by combining images of the sky taken in different colour filters. These images are often taken years apart so for an object such as Barnard’s star which moves very fast across the sky the positions in each filter will be different. Hence you would expect to see one blue, one green and one red image in three different positions, this doesn’t appear to happen.

I decided to check out a few other high proper motion stars to see what they looked like. Proxima Cent is a bit weird, I can’t seem to identify it on the image, perhaps it is the blue thing on top of a background star, there is certainly a bit of noise where the UKST I plate (Google Sky uses a combination of data from the UKST, POSS telescope and the Sloan Digital Sky Survey plus a few other sources of more detailed images) position from the 1970s is. A better example is Kapteyn’s Star is a better example. Notice the bright very blue object in the upper right, that is the blue image from 1975, while the noisy thing in the middle is around the position of the red image from 1998. You can see a better subtraction for Luyten’s star.

Frankly I’m not sure about the finer points of how Google Sky make their colour images. This is clearly an artifact of the way in which they combine the images. Anybody able to use this to work out why this is happening?