If a spaceship were traversing a galaxy, which trajectory would require less energy: cut straight through from one spiral arm to the next, or keep curving within the spiral arms themselves. I guess what I'm wondering about here, is whether there is a denser field of gravity within the arms dragging on the vessel, or not?
Thanks for your insight!
There are about as many stars between the arms as in the arms! The spiral arms are visible because that's where stars are formed. Larger stars are brighter and burn their fuel faster, therefor large bright stars are necessarily young and are only found near where they were formed. Older stars just orbit straight through the arms. The spiral shape is some kind of standing wave phenomena that concentrates the cold gas out of which stars are formed into regions of such a shape. And that's where I leave the answer to astronomers.
Since there's much more normal matter mass in the shape of gas than in the shape of stars, I suppose that spiral arms could have slightly higher mass density than the gaps, because cold gas somehow tends to concentrate there. But since many generations of short lived stars in the bright arms have ended up as star remnants (black holes, neutron stars, white dwarfs) which do not remain in the arms, I would think that the current short lived population of young large bright stars makes up only a small fraction of a galaxy's mass. And dark matter in some kind of spherical halo in and around the galaxy has several times the mass of all gas and stars anyway.
