Do spaceships travel in a straight line? I thought their trajectories are heavily curved in really weird ways due to the garvities of stars and planets around them. Please enlighten me!
It depends on the frame of reference, which can be somewhat arbitrary.
A ship can sail a straight line from Falmouth to Boston, but that motion can be described as an arc because of the curvature of the Earth. Add in the motion of Earth's orbit, and the shape of that motion changes again. Add in the Solar System's motion relative to the Galactic Centre, and it's yet another shape.
The straight line from Falmouth to Boston is only straight on a 2d map, because of particular transformation from 3d surface to 2d plane. Nothing to do with reference frame.
You indeed can change a curve into a straight line by manipulating the frame of reference, but it would need to be very contrived - basically you follow the object movement and add constant translation and some linear movement on top of that (one example - camera is in on a kite tied to the ship, and the rope is progressively loosened).
I think you overestimate the density of stars in the universe. If I remember correctly there are typically a few lightyears between each star (on average).
There would still be a curved orbit around the center of the galaxy. That is a bit like arguing that a mission from Earth to Mars would travel in a straight line because there isn't anything in between them.
A ship can sail a straight line from Falmouth to Boston, but that motion can be described as an arc because of the curvature of the Earth. Add in the motion of Earth's orbit, and the shape of that motion changes again. Add in the Solar System's motion relative to the Galactic Centre, and it's yet another shape.
It's all a matter of perspective :-)