I read a disturbing thread on DPReview that the Sony A7 series of cameras eat stars when you use Bulb exposures. You're OK up to 30 second times but if you use Bulb it applies a filter to the RAW image to remove hot pixels presumably and it eats stars like Nikon used to before their algorithim got updated by Mariane.
http://www.dpreview.com/forums/post/55841466
Greg.