[I realize my question/confirmation here could have been made way way back, but the conversation got intense]
So if I’m in an area of (intergalactic, non Earthbound) space in which the size of my field of view – tell me the word for this again? – would allow the whole spiral image to be observed, it’s just too dim to see anything like the Hubble, but at best the way the Milky Way does to us now in an area without light “pollution.”
And color is right out (as the Brits say).
Rats.
Huh. Thanks. This is an official Cool Fact. Just the other day I was posed the “number of grains of sand on the beach vs. stars in the sky” puzzler.
But: Can you clarify “half the sky?”
I was similarly engaged by ftg’s observation, so did a follow-up. The 2 to 4K refers to the total number of stars that can be observed with the naked eye from anywhere on Earth, but since you can only see (at best) half the sky, it cuts the number one can see from any point on Earth at any time (presumably, when it is dark) by half.
To possibly clarify a bit more, “the sky” is not a dome; it’s a complete sphere. But half of that sphere is underneath the Earth, so you can only see half of it, a dome, at once.
Another factor is that the human eye has a limited integration time (think of it as a fixed, relatively fast shutter speed), whereas an electronic sensor can have a very long integration time (until eventually limited by leakage), and so can collect orders of magnitude more photons per pixel than the human eye can.
One interesting factoid is that how large an object looks in the sky (its angular diameter) depends on the (proper) distance when the light was emitted. As the Universe is expanding this means that galaxies appear larger in the sky than their current proper distance would naively suggest. For nearby galaxies the effect is slight, but beyond about 5 billion light years the size of galaxies in the sky actually increases with proper distance. The very furthest galaxy observed is about 30 billion light years away, but is the same size in the sky as a galaxy of the same dimensions that is 3 billion light years away.
Correct. And I think “field of view” is a correct term.
If you were near an emission nebula like the Orion Nebula (M42), you would see a hint of color - a pale green. The vivid red nebulae you see in photos - and the red patches you see in photos of other galaxies - are H-alpha (656.3 nm). The human eye isn’t very sensitive at this wavelength. For reasons we discussed earlier in the thread, there is no optical aid that would allow us to see those red nebulae in color with our eyes.
OK, so is the most accurate way to depict a naked eye view of these objects a black and white photo?
Hmmm…I agree with everything scr4 said above. I guess it’s just that when you look up at the milky way on a really dark night with a dark adapted eye, it seems really bright. Much brighter than you’d expect, to me anyway. Of course that galaxy observed from intergalactic space won’t actually be bright, but you’d be able to see it clearly. I agree it would only be as bright as the milky way, but that would seem bright-ish to you as you floated there thousands of light years from any star contemplating your own mortality.
Largely. There are some objects that you can see some color on. Mars is distinctly reddish, as is Antares (hence it’s name). But most are too dim to show colors. Here’s a good rule of thumb: any colored astronomical picture should be assumed to be false color unless it says explicitly that it’s true color.