Another thing, so you resized the original image... how so? I mean, when you resize an image the result will be... well awkward, and the sizes you are talking about seem astronomic.
I guess you had an already big BIG base image to work on right?, (being best to shrink than to enlarge) IIRC the one I used was 6000 x something.
I used the enlarged image of the galactic plane as an alpha mask and colourization source for a greyscale noise layer. That way, the blurring that results from scaling up the original image is neutralized - all the detail comes from the noise layer, and the enlarged image works only to determine the intensity and colour of the pixels on the noise layer.
Here's an example:
Let's say you have an image of a star cluster or elliptical galaxy, which is very low-res and you want to increase the resolution.
It's so low res that no individual stars are shown. However, it gives you the general intensity at different locations, and gradients scale up relatively well, so let's increase the resolution from 64^2 to 512^2:
Now, this is indeed a bit awkward looking. It doesn't look like a star cluster, it looks like a blob.
So I make a noise layer:
...and apply the enlarged blob as an alpha mask:
Then I apply the enlarged blob as a colour layer:
...and finally I applied the enlarged blob as screen layer, and reduced the opacity of the noise layer to 30%, resulting in this:
At this point it's a matter of calibrating the layers as you like, depending on whether it's an elliptic galaxy (more fuzzy) or a closed cluster (more grainy/noisy as you can see individual stars better).
In a nutshell this is what I did to the galactic plane there.