Create a new document in Photoshop and fill the canvas with black. Then add a new layer and fill this one with black too. Next click filter > noise > add noise and use these settings: Amount- 15%, Gaussian, Monochromatic. This will make the background stars.
Now click Image > Adjust > Brightness and Contrast and use these settings
Now duplicate the stars layer and click Image > Adjust > Brightness and Contrast and use these settings: Brightness- 85 and Contrast- 50. Now Edit > Transform > Scale this layer to about twice its size.
Now inverse the layer by clicking control-I and then bring up the contrast of these stars by clicking image>adjust>levels and use these settings: 230, 1.00, 250.
Now inverse the layer again to set it back to the proper color (Control-I)
Next step is to set this layer blend mode to screen, click the eraser tool and pick a nice fuzzy brush. I use a bristle brush and made it nice and large. Then I started erasing random areas of each layer.
Now flatten the image, click on the rubber stamp tool, set the blending option for the tool to Linear Dodge and start cloning different starry patches to create some clusters.
Here's the image so far
Now click image > adjust > color balance, click highlights, and add some subtle blue.
Next, add create a new layer, fill it with black and set the layer blending mode to Linear Dodge. Now use a feathered paintbrush and set the color to light blue, almost white and start painting over the clusters to give them some pop.
Now add another new layer, fill it with black and set the layer blending mode to Linear Dodge. Now click Filter > Render > Lens Flare and use 105mm prime. This will make a star. Repeat this step a couple times and add more. Don't shy away from making a cluster of these as well. Also, be sure to use different sizes.
Now combine all the lens flare layers by shift-clicking all the layer in that contain them in the layer palette and click control-E to combine them. Now click image > adjust > color balance, select highlights, and add some more blue.
Now I selected the brush tool and used the chalk brush within the brush options and made a very large brush size set to opacity 20%. Now create a new layer and set the mode to Linear Dodge and start brushing in a color of your choice. I'm still using the cosmic bright blue.
That's it! There's many ways to do this so experiment and come up with what works best for you.