Star Wars is Science Fiction. Completely science fiction.
Fantasy includes things such as Lord the Rings, Dungeons and Dragons, Dragon Lance, Warhammer, the early Final Fantasy's etc
Pure fantasy is set in medieval worlds which have very primate technology (at most steam engines), mythical monsters, usually magic, often gods are in there. The genre can also include things like Harry Potter given its about magic and monsters, but this would be classified as Alternative Fantasy not Pure Fantasy.
Science Fiction includes things like Star Trek, Star Wars, Star Gate, Asimov's Robot books, World of the Worlds, Warhammer 40K, Back to the Future, the Matrix etc
Its based (usually in the future) in worlds with high levels of technology (whether its possible or not), and other scientific issues such as aliens. For example, time travel is most likely impossible, it does not make having it in you're novel turns it into a fantasy novel with magic and monsters in it. The chances of Aliens ever visiting earth given the energy needed to travel through space is unlikely, however that does not turn Independence Day into a fantasy novel, if it had wizards and magic in it it would be fantasy.
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It must be built on solid scientific theory.
That’s completely untrue. That’s why its called science fiction, and not science fact. Because its made up. It doesn't matter if hyper drive and every alien species you meet looking completely human except for their ears is scientifically impossible, its fiction, but has scientific elements. I've read tons of science-fiction novels, very few of them base themselves on technology which is possible now, or may be possible in the future (that said, we have no idea what will be possible in the future).
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but they need to be more constrained and stringent because they must be consistent and believable
That’s nonsense. Science-fiction is not limited to the believable, because of the simply fact its FICTION. A space-ship which can travel the galaxy in a matter of minutes might not be believable, but 1. how do you know it one day won't be possible? 2. who cares, its FICTION!
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Elements are added for convenience or plot setting or simply because they are cool. What is most vital, plot element wise, is the story involved. The only real constriction for setting and detail is does it fit with and make sense for the story. That is fantasy.
... there is absolutely nothing you have said there that proves it's fantasy. Sci-fi novels are, believe it or not, capable of having settings which are 'cool' such an alien planet or space ship. Sci-fi stories believe it or not, can have plots and story, in fact a lot of sci-fi novels have very detailed ones.
The only issue in Star Wars which even hints at fantasy is the Sith and Jedi's powers, and this is explained to us in a very science-fictionish (if rubbish) explanation of single celled life forms in our body producing extra energy which we can harness and use (of note there are actually mitrocondria in all our body's cells which do produce energy, and aren't actually part of us since they are individuals creatures who just live inside us, however these aren't the ones in star wars since star wars calls them Midiclorians). Also its not much unlike chi and other forms of bodily energy in modern Buddhist and Taoist religions, but I don't see anyone claiming the Orient is currently a living in a fantasy novel, no more than they claim the west is because Christians actually believe some nonsense about flying people in the sky who made the world and are at war with demons underground.