This tutorial is 6 steps and it has a difficulty level of Intermediate. This tutorial will show you from start to finish how to make a nebula over a star field quickly and easily.

     1. Follow my star field tutorial to first create a star field. After that is done open your star field and reset the colors by pressing D, and then X. Create a new layer by pressing Shift + Ctrl + N or go to Layer > New > Layer. Name the new layer nebula.

     2. Now, select the nebula layer from the layers pallet which you can bring up by going to Window > Layers. Just click the layer called nebula to select it. Now that the nebula layer is selected go to Filter > Render > Clouds You should now have some clouds, which will eventually become our nebula.

     3. Now color the clouds now by pressing Ctrl + U, or go to Image > Adjustments > Hue/Saturation. Now press O, or just check the colorize box to change the color of the clouds. Duplicate your nebula layer by going to Layer > Duplicate Layer. Play around with those settings until you get something good. I used Hue: 50, Saturation: 30, and Lightness: 0 Now repeat this step for the other nebula layer. Note: using a different color for each layer can give you some interesting results when we're all done.

     4. Select the lasso tool and set the feather on it to 15, feather should be below Select at the very top of Photoshop. Just to let you know, you will have to do this to both of the nebula layers, so select a lighter area and delete the lighter areas, and with that selection still there click the other layer and delete.
Step 4

     5. You have some areas still around the edges that weren't deleted so now go back and reselect them and delete the fuzzy spots around the edges you don't want. Your image should now look likr the one below.
Step 5

     6. Now click the bottom nebula layer and set it's layer style to screen. Layer style is next to Opacity on the Layers pallet. You should now be able to see the stars behind it and it should look ok, but, it's not good enough so now select the nebula layer above it and set that one's style to Overlay. The first image is with 1 nebula screened, and the other hidden, the second image is with 1 nebula screened and the other overlayed.
Step 6a

Step 6b