For this one I actually took a "celestial" type background I found by googling images, recolored it, smudged a cloud brush across the background on "dodge/highlights", and then painted the stars on in a slightly lighter color.
She's right that's the brush I was using for my stars in other sigs but it's not the one I used for that sig, I don't think I have a link to it but it was an 83 point brush I found somewhere (might even be in the default PS3 brushes) that looked like a four point star. However, if you download the sparkle/star brush Ammeg linked (it's in my favorites on my deviantart) it makes some very nice stars!