FLPhotoCatcher wrote:I don't mean, looking at it by eye, that it would LOOK significant. In theory though, it could be significant. Take things to the extreme. Imagine a wall of gas 1 million light years thick. Then send two identical stars toward it at an identical speed - say a few dozen km/sec. In front of star #1 would be an empty tunnel 3 light years in diameter. Star #2 would have to make its own path. It would be very counter-intuitive if star #2 made it through the gas a few weeks or even seconds sooner.
When I'm puzzling out something like this I try to reduce it to a simpler form to clarify my thinking. So let's try to visualize the situation this way...
Imagine a steel ball moving through space towards a sheet of soft gelatin. In the first case, imagine a hole in the gelatin large enough for the ball to pass through without touching. The ball will be gravitationally accelerated slightly as it approaches the mass of the gelatin, will pass through unscathed, and will then slow down as the gelatin pulls back on it gravitationally. Net effect: zero.
Now imagine the same situation but without the hole. The ball would again be gravitationally accelerated by the gelatin as it approached. The ball would then hit the gelatin, slowing down but passing through it, while gaining a little gelatinous mass in the process, and would then slow down as the gelatin pulls back on it gravitationally. Net effect: a heavier ball moving slower than before.
(Yes, in case 2 the ball is accelerated and decelerated a tiny bit more than in case 1, because there's a little bit more gelatin mass to pull on it. But the difference cancels out when you consider both approaching and leaving the vicinity of the gelatin. In other words, it isn't just the gelatin directly in front of the ball that pulls on it gravitationally; it's all
of the gelatin.)
The situation you imagined, of a star approaching a wall of gas, is exactly analagous. In the first instance, the hole would allow the star to pass through unimpeded, but the star would still accelerate slightly as it approached the wall and then slow down after it passed through -- both changes in speed due to the gravitational attraction of the gas. In the second instance the same would happen, with the addition of the star slowing down a little as it passes through the gas, while some of the gas gets added to the mass of the star.
Does that help?