Step 4: Apply The Gaussian Blur Filter To The Noise

Now that we've filled our Stars layer with noise, we need to blur it out just a little bit. Go back up to the Filter menu and this time, choose Blur, then choose Gaussian Blur:

This opens the Gaussian Blur dialog box. We need only a very subtle amount of blurring, so increase the Radius value to 0.3 pixels:

Click OK to close out of the dialog box. The noise in the document window will look a little bit softer.

Step 5: Hide The Stars Layer Temporarily

We've done everything we need to do on the Stars layer for the time being. At this point, we need to see our original photo again. Problem is, our original photo is being blocked from view by the Stars layer above it, so we need a way to temporarily hide the Stars layer. We can do that by clicking on its layer visibility icon, which looks like a little eyeball on the left side of the layer in the Layers panel. Once you've clicked on it, the eyeball will disappear, leaving only an empty square icon, indicating that the layer is now turned off. The original photo will once again be visible in the document:

Step 6: Select The Sky Area In The Photo

With the noise temporarily hidden, use the selection tool of your choice to draw a selection around the sky so that only the sky (the area where we want the stars to appear) is selected, while everything below the sky (the area where we don't want the stars to appear) is not selected. Depending on your image, you may be able to get away with simply clicking inside the sky area with the Magic Wand, or you may need to use a different selection tool, like Photoshop's Lasso Tool, Quick Selection Tool or the Pen Tool. In my case, since everything below the sky is a building or tower made up mostly of straight lines, I'll use the Polygonal Lasso Tool. Here's my image with the sky selected:

Step 7: Turn The Stars Layer Back On

With the sky now selected, click once again on the layer visibility icon for the Stars layer in the Layers panel to turn the noise back on in the document:

Step 8: Add A Layer Mask

Click on the Layer Mask icon at the bottom of the Layers panel:

This adds a layer mask to the Stars layer, and a layer mask thumbnail appears in the Layers panel. Photoshop fills the selected sky area with white on the layer mask, while the unselected area below the sky is filled with black:

With layer masks, white-filled areas of the layer remain visible in the document while black areas are hidden from view, and we can see with my image that the noise now appears only in the sky:

And we're done! Doesn't that look great?!

No? Hmm... You're right, we're not quite done yet. Let's turn all that noise into some stars using a Levels adjustment layer, which we'll do next!