Unsharp Mask

I personally recommend limiting the effects of sharpening on images. Sharpening is one of the quickest ways to rob the natural beauty of an object. However I think that if one is careful, sharpening can genuinely enhance the details in an image without turning the image into something that looks like a cartoon. I recommend trying to get as much total acquisition time as possible. The more total acquisition time, the  more sharpening and other processing an image will take before losing too much of it's beauty.

Before image:

The first thing we want to do before diving into the unsharp mask is to deselect the stars from the image. Stars can take a little bit of sharpening, but long before the rest of the image starts looking unacceptable, stars start looking terrible when sharpened.

So first we'll use the magic wand (from Windows->Tools) to select a star:

Take the magic wand and hold it over a fairly bright and round star and click on the left mouse button. This will select the star:

Of course we would like to select most all of the stars so go to Select->Similar:

When you do a Select->Similar hopefully just stars will automatically be selected, but occasionally other items will also be selected that we will need to deselect:

To deselect the parts of the image that aren't stars, move the magic wand over these areas. The cursor will probably turn into a little square when you do so. Hold Alt down and click on the left mouse button and the area selected will become deselected:

Keep deselecting everything that isn't a star:

Now that we only have stars selected we need to inverse the selections so that stars are instead the only things not selected.  To do this we do a Select->Inverse:

 Now that the stars are all deselected from the image, we get to the fun part, the unsharp mask itself:

Move the Amount all the way to the right (500%), the threshhold to 0, and the Radius to 0.3.

Press OK.   I like to fade most operations back a bit: (Edit->Fade):

Press OK and click on the image somewhere to deselect and were're through.

Here is the image after the procedure is finished. Move the mouse over the image to see the image as it was before the procedure:

I've actually exaggerated the amount of sharpening to show the effect more clearly. In practice I recommend using as little sharpening as possible or as processing effects accumulate, the image will end up looking like a drawn cartoon rather then a photograph of a beautiful object in nature which I think is everyone's true goal.