When I tried using CG the effect didn't look realistic enough. Are there any tips for better CG? I'm using the free trial. By CG I mean keyframing, light flares, compositing, ect.
This heavily depends on exactly what you want to do. Best would be if you had a screenshot of something not looking "quite right", so we can pinpoint for you how to make you shots more convincing - otherwise at least try to explain some specific scenarios you have trouble with! Your question like this is way to broad to be answered adequately.
For example in one scene a spaceship is being sucked backward into a black hole, however it doesn't look that realistic.
Can you upload a screenshot or even better a video from what you've got so far? This way we can give more detailed feedback on what you can try.
In this case I would generally start by animating the spaceships motion backwards towards the black hole, simultaneously getting smaller. Make sure the starting keyframes for the motion are set to smooth interpolation, the ending keyframes to linear, this way the motion will accelerate towards the end.
Now the actual disappearing can be stylized in a lot of ways, for example using Twirl or Vortex Displacement effecs and opacity animation. The specifics come down to tweaking parameters in a trial-and-error process until it looks right. If you're going after a scientifically correct depiction it will be less dramatic, I think I recall an article somewhere where this was described that at the so called event horizon of the black hole, the spaceship would seem to come to a stop for an outside spectator, and then just fading away in a red tint, or something along those lines.
This tweaking gets easier if you know what exactly the different controls do, posting example videos here can get you more detailed feedback on what direction to go to make it more realistic.
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