What would the Milky Way look like from just outside it?

[dracoi]

@Peter_Morris

Well, we actually live in the Milky Way, and for most people it’s too faint to see most of the time. My guess is that if you go outside then it would be even fainter. If you see it edge on, then it would be pretty much as we see it, a streak of light against the sky. And if you see it flat on, then it would be a faint disk.

As I understand it, the Milky Way would be brighter seen from the top compared to from the side (or from inside). There’s a lot of interstellar dust that blocks light, so the view from the top would have less dust in the way.

Still, I think you’re right. The Milky Way is not going to get brighter from 100,000 LY farther away than we are now.

[Chronos]

Interestingly, an extremely bright supernova in M81 would be (barely) visible to the naked eye. This would be the farthest possible naked-eye object, by a factor of about five. I wouldn’t count on its ever being possible during anyone reading’s lifetime, though.

There was a recent gamma-ray burst whose optical counterpart would have been barely visible to the naked eye (had anyone known to look for it during those few minutes) which I’m pretty sure was further away than that.

Back to the original question, the “surface brightness density” of an object will be the same no matter what distance you are away from it, and no matter what passive optics you use to observe it. If you’re further away, it’ll be dimmer because it’s taking up less area of the sky. A big light-bucket of a telescope will gather more photons, but those photons will go into making it take up a greater area in your field of view, not into making the surface brightness greater (basically the same as being closer to it).

So if something is invisible to the naked eye, the only ways of making it visible are if, a, the surface brightness is high enough but it takes up an area of the sky smaller than the resolution of the human eye, in which case magnifying it through a telescope will help, or b, if you’re using some sensor other than the eye (such as photographic film or an electronic detector) which you can set to gather light for a long time and add it all together. This is one of the reasons why real astronomy is never done by just sticking your eyeball up against the end of a telescope.

So a galaxy seen from the outside will have about the same brightness density as one seen from the inside, but it’ll take up a different part of your field of view. From somewhere straight “above” the disk, it’d probably be pretty spectacular, due to its shape and it filling a big chunk of the sky), but you’d still need a dark sky (comparable to what you need on Earth to see the Milky Way) to see it at all.