Question about gravitational revolution..

[Magical Realist]

Why do moons revolve around planets, and planets around stars, and stars around the center of the galaxy, instead of just being pulled straight into them?

[Syne]

Some do get pulled straight in, if they are on direct impact trajectories. It's a matter of their path when they meet. If the attracted body is at the right tangent to the gravity well, it's straight path is bent into an orbit.

Per the first law of motion, an object in motion will remain in motion, at the same speed and direction, unless acted upon. The influence of a larger gravity well bends it's path, but its inertia is still trying to follow a straight path (technically it still is, but spacetime is bent), meaning it's motion is always trying to escape a gravity well, but the gravity is strong enough to hold it in orbit, like having it on a leash.

[confused2]

Why do moons revolve around planets, and planets around stars, and stars around the center of the galaxy, instead of just being pulled straight into them?

Space is big. The chances of a thing whizzing past never to be seen again are vastly higher than the chances of an impact or capture. The few things we've noticed coming from outside the Solar system are going so fast they'll shoot straight out the other side. So how did the Solar system even get started.. sigh.. really a question for another day but.

I'll start with a cloud of gas. Maybe something passing through the cloud of gas gets slowed down .. maybe enough that it doesn't just fly out the other side of the cloud. Next the something sees something else in the cloud and they are attracted to each other enough that they start going round each other. Collide maybe .. but most likely not. The gas clears and the sun shines on 9 8 planets, umpteen moons and a lot of bits left over.
And that's how galaxies, stars, planets and moons are born. Possibly.

[confused2]

My definition of "a straight line" is "the shortest distance between two points",
So what does "The Moon travels in a straight line round the Earth" mean?

Newton’s first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.

This was before spacetime had been invented and 'gravity' was thought of as a force which curved the path of (say) the Moon round the Earth and it all made perfect sense.

With 'gravity' now considered to be a property of spacetime (not a force) the Moon goes round the Earth without being acted on by a force and so (by Newton's definition) it travels in a straight line. Makes sense (or not)?

[Syne]

For most practical purposes, it really doesn't matter if we view it as a straight line through curved spacetime or a curved line through flat spacetime. In most terrestrial circumstances, we treat gravity as a force.