I’m going to say something that totally goes against what everyone else here has told you.  Don’t worry about the damn stars, how your spaceships work, or if plastic eating aliens are waiting at the edges of our solar system.

Write the story first.  Write the story first.

Because, basically, you can justify almost any event, plot point, creature or location in science fiction with enough semi-imaginary words (thank you Star Trek) and tight story-telling.  But you can’t do shit if you don’t have the story.

A first draft of a story should have almost no research in it.  It should be your characters, your plot, and the progress of events.  It’s your second (and third) drafts that you spend time looking up current string theory science.  You don’t know the name of the planet your crew has crash-landed on?  Put ‘##” there and go back later.  Don’t know how the aliens communicate but know what they’re going to say?  Put in quotes and worry about how it got there, actually, later.

If you look around the ‘net, you’ll find thousands of people who are ‘oy, I’m building a whole new universe, how do I do it?’ and most of them … they never get to the story that inspired the months and months of research.

Research is a killer for writing stories.  Do it later, not first, or it will both eat up your time and put you in a nit-picky, editorial mind-set that isn’t good for writing the actual story.

The only time this type of story first work shouldn’t be done is if you are a hard SF author – and you are specifically writing about a point of alien ecology, future technology or planetary science.  These types of hard SF authors are often scientists themselves, exploring how a point of physics they understand intimately would work in a fictional setting.  OR, if you are a gaming writer, as I am, and you have both a short turn-around for your work and the game itself needs to stand up to the curious minds of hundreds of gamers wondering how people fly from planet to planet and won't take 'they just do it' for an answer.

Some examples of science fiction that didn’t use much in the way of science fact:

Start Trek
Star Wars
BladeRunner
Outland
Stargate etc
BattleStar Galactica
You may or may not like those films or shows but any ‘hard science’ (like the reams of ‘Star Trek Tech’ books, and same for Star Wars) came after the shows gained popularity.