Since childhood, I’ve always heard that the Milky Way is 100,000 lightyears across. That’s an impressive number, of course (and I’m not easy to impress). However, during a Google image search for depictions of the Milky Way (which I intend to use to map out a galactic civilization for a space opera story I am writing, if you must know), I came across a few sites: This one which indicates that the Milky Way is 150,000 lightyears in diameter (scroll about 2/5ths of the way down, to the large box marked “The Milky Way Galaxy”). Then I also came across a SETI site which lists the diameter as 120,000 lightyears.
Now, I understand that the galaxy isn’t perfectly round… but a discrepancy of 20,000 (and 50,000!) lightyears is nothing to sneeze at. Can someone enlighten this poor, dumbfounded SPOOFE? Are they just upper and lower estimates? Or are the larger sizes due to globular clusters and the like which extend beyond the diameter of the galaxy proper?
And I have another, only-slightly-related question which I’ve been meaning to ask for a while… aside from time and speed constraints, is there any reason why a starship wouldn’t be able to leave the galaxy? Any theories about radiation or strong gravity pulls that would make it extremely difficult to leave the galactic disk?
To borrow an analogy from the late Mr. Adams, you might think it’s a long way down the street to the 7-11, but that’s just -peanuts- compared to the galaxy.
Well there’s no real distinct border to the Milky Way, much as there’s no real distinct border to our solar system. So different sources may include different fringe objects. Here’s a good point from the site you gave.
The halo is probably more ovalish then round since it is tugged by certain extra galactic structures like our smaller orbiting galaxies like the Large and Small Magallenic Clouds.
Probably it would be useful to redefine the diameter of spiral galaxies a little better. Perhaps as the point at which the circle described would include 99% of all matter bound by gravity to said galaxy?
Because it depends on the direction they use. It’s not an estimate but rather a complete answer. For instance if you look up into the southern skies you can see two large splotches of stars which are small galaxies called the Large and Small Magellenic Clouds (LMC & SMC). They orbit the Milky Way. Eventually they’ll be absorbed but for the moment they still orbit. Anyway the parts of our galaxy closest to these clouds get pulled toward them stretching out our galaxy. Something most pronounced in the halo. This effect doesn’t not happen in other directions. So the distance from the galactic center to the edge of the halo is further in the direction for the LMC and SMC then they are in the opposite direction.
To the OP…good question. I agree with the others here that are saying that part of the problem is that the boundary is non-distinct. I’d also add that it is difficult to take such measurements from the inside of the galaxy. Our view is necessarily obscured, especially in the direction toward the center (since there is more stuff that way).
I believe intergalactic space has even less matter per cubic whatever than interstellar. If you use a ramjet system, this would be a minus… that much harder to accelerate and decelerate. On the other hand, it’s that much less stuff to have to shield against in one way or another.