How exactly did you do that?
The procedure is based on a mixture of method and improvisation. First I import the primary photo into GIMP, e.g. the Vienna Parliament building. This is the first of two layers. I intensify the contrast a little and sharpen the picture, but also only a little. Then comes the important step of coloring the background black, either when it is even, with the watering can function, or otherwise with a large brush. Afterwards the motive (e.g. the building) is colored by different color filter functions and strengthened again with the contrast function. Then I use the transparency function for this layer and set it to about 60 percent. This makes the image slightly transparent.
Thus you get a picture like the one below.
Now I create a second layer with a background motif, in my style this is mostly a picture from the Hubble telescope photo series. The imported image is sharpened and given a strong contrast. The second layer must be below the first layer with the main motif (e.g. building), so that the second layer with the stellar motif shines through (because the first layer is partly transparent). Afterwards, when both layers merge, the contrast and the brightness of both layers have to be regulated in such a way that the image appears harmonious, i.e. not composed of two layers.
That was the methodical part. Now comes the part with the improvisation. The stellar motif is now processed with certain effects, especially the kaleidoscope effect. This effect is a gigantic toy because it offers virtually millions or billions of possibilities that are available to you in a few seconds when you use the tool just as a pianist plays the piano. You can create either symmetric or asymmetric shapes, with a variety that quickly makes you lose track and you run the risk of not being able to decide because there are so many possibilities. Therefore, experience and an unerring eye are important for a successful variation, i.e. you must make the decision at the right moment not to continue, but to keep and save the variant you have just found.
This can take a minute or ten minutes, but it doesn't matter. In any case, you can save several variations and then decide which to use. Now you have the problem that the second layer shines not only through the black background (as it should be), but also through the main motif (e.g. building). This can be interesting, but it is better to remove the lower layer at the place where the main motif lies on the upper (first) layer. This can be done by using a brush to paint black over these areas on the second layer (in my case the stellar layer) so that they do not shine through. Therefore no stars shine through the parliament building, I simply painted them black over and made them invisible.
That is in principle the procedure.
Here's a similar picture I made last year (my photo). The motive is Nymphenburg Castle in Munich. The difference is that I didn't use a kaleidoscope effect, but only worked on the starry sky with contrast filters.
That's a Neptune figure in the castle park (my photo).
This is the front of a baroque church in downtown Munich (my photo). I just inverted the colors, that gives the mystic effect.
That is, somewhat off topic, a Japanese ...., the photo is edited according to the procedure described above, but I used more than 2 layers and let the background partially shine through the main object.
