Iris Nebula 105 minutes LRGB

[Xiga]

The basic gist of trying to tame the stars during stretching in PS, is as follows:

1. Make a copy of the background layer. Then copy the image (CTRL-A, CTRL-C), create a Layer Mask, select the Layer Mask (ALT-Click) and paste it into it (CTRL-V). Then invert the mask (CTRL-I). You should now have something like this:

Use a Levels adjustment to bring the White and Black points in, and then add a slight Gaussian Blur. Start out with something small, like 0.5 pixels. 

Then select the Background copy layer and apply your first stretch. I usually use Arcsinh 10 stretches myself. 

Flatten and repeat as many times as is needed until you're happy it's been stretched as far as you want. Remembering also that each time as the stars get a little bigger, you will need to gradually increase the pixel radius of the Gaussian Blur too. This is where some experimentation will be needed. For example, i sometimes find it's a good idea to not mask every single stretch, and let the odd one or two small stretches go unmasked. The payoff of slightly bigger stars is usually worth it in the end, as they are more likely to come out a bit more natural, and in any case you can always just use a bit of star reduction to compensate. 

Now, depending on how well it goes, you will almost definitely still get at least some harsh edges to the outer core of the larger and medium-sized stars. It's really hard to eliminate this entirely, so if the no. of worst offenders are not too high, you can just use the Blur tool and manually soften them up a bit. 

Here's an example of how this method looks compared to a normal DDP stretch. This is a 200% crop of a small area of your image. You can see that the really large star (which is over-exposed) has not fared quite so well, this is one that would need a little cosmetic touching up, but the improvement in the rest is pretty apparent to see.

HTH.