Thank you for your comment! :)
Actually the flashing animation is not voluntary... I think this is due to the fact that the stars are really small and the camera move (and her zoom change).
The star in the background are made with 2 parallax layers with a particule2D on each. And the particule2D are made to make star slowly appear and disappear.
For the black hole, this is a shader that deform the image behind it. The accretion disk is just a star flattened behind the black hole :)