Real colours of a galaxy

[Matte]

hello everyone;
i was wandering at which distance the human eye could be able to see the colours of a galaxy like we see them via telescopes in a long exposition picture.
i mean, would it be possible? or the gigantic dimension of a galaxy would let us just see it as a grey nebula?
thanks
Matte

 

[agentgonzo]

You'd see in in just monochrome (grey). As you get closer to a galaxy, it will become brighter, but it will also become larger in your field of view. As the galaxy spreads out, although there are more photons entering your eye, they are spread out over a larger part of your retina and thus more rods (one of the two types of light receptors in your eye). As they spread out, there are still not enough photons striking each rod per second to make it fire and you detect colour.

We are currently in a galaxy (Milky way) and you still can't see that in colour to the naked eye.

 

[Matte]

We are currently in a galaxy (Milky way) and you still can't see that in colour to the naked eye.

yes, you're right!
thanks for the answer, so i will go on appreciating Hubble's photo hoping that the coulours are really those we see vi it's eye!

 

[Artlav]

Quite interestingly, what could be seen by naked eye if one is in the intergalactic space?

Say, you go out of our galaxy just far enough to have it all in your field of view, what will you be able to see with all the lights on the ship turned off?

None of the nice bright galaxyscapes you're shown in starwars-esque movies, i guess.

 

[cjp]

Quite interestingly, what could be seen by naked eye if one is in the intergalactic space?

About the same as what you see on a very dark night on earth, when you look towards the Andromeda Galaxy:

At an apparent magnitude of 4.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects,[9] making it easily visible to the naked eye even when viewed from areas with moderate light pollution. It appears quite small without a telescope because only the central part is bright enough to be visible, but the full angular diameter of the galaxy is seven times that of the full moon.

So, you have some galaxies around you, and you can see them. However, they are very faint, and without optical instruments you can only see the bright core of them, and you can't see much (if any) color.

It must be a bit like looking at the stars from earth, except the galaxies are less bright, and are more like blurry blobs than like points, and there are probably fewer of them.

 

[Urwumpe]

What? Seven times bigger as the full moon? I thought it is only seven times bigger as the perceived spot in the sky?!

Damn, my personal astronomy really needs to be improved.

 

[cjp]

What? Seven times bigger as the full moon? I thought it is only seven times bigger as the perceived spot in the sky?!

Damn, my personal astronomy really needs to be improved.

I got it from Wikipedia, without checking whether it's true.

 

[Urwumpe]

I got it from Wikipedia, without checking whether it's true.

Argh.

But the number seems to be right - 190' or 190 arc-minutes, that is really bigger as the moon, but only along the long axis.

 

[JamesG]

If you could go someplace really dark (a comet or Ortt object at aphelion?) and let your eyes adjust (or you have synthetically enhanced vision), most galaxies and the Milky way would be tinged reddish or orange from the emission line of the hydrogen that is their primary component.

 

[cjp]

If you could go someplace really dark (a comet or Oort object at aphelion?) and let your eyes adjust (or you have synthetically enhanced vision), most galaxies and the Milky way would be tinged reddish or orange from the emission line of the hydrogen that is their primary component.

Maybe the light is red, but you wouldn't be able to see it. In darkness, only rod cells are sensitive enough, and you can't see color with rods. Also, AFAIK rods aren't sensitive to red light, or at least red light doesn't de-sensitize them, which is why e.g. the night light in cockpits is red.

With "synthetically enhanced vision", you can see anything you want to see, so it's hard to tell anything about that option.

 

[agentgonzo]

If you could go someplace really dark (a comet or Ortt object at aphelion?) and let your eyes adjust (or you have synthetically enhanced vision), most galaxies and the Milky way would be tinged reddish or orange from the emission line of the hydrogen that is their primary component.

Hydrogen Alpha is a large intensity spike in the spectrum, but the galaxies aren't dominated by them. This is a real-colour image of Andromeda which presents it as reddish-purple tinged:

Also, AFAIK rods aren't sensitive to red light, or at least red light doesn't de-sensitize them, which is why e.g. the night light in cockpits is red.

That's (mostly) correct. The rods in your eyes are sensitive to a large spectrum of light, but much more sensitive to deep blue/purple light than other wavelengths, explaining why dark rooms/night lights are red and we use blue lights in theatre/film to indicate darkness.

The night vision that we actually see is all in this purple colour, but our brains filter that out to leave us with a black-and-white image rather than the black-and-purple image the retina sends out.

 

[Urwumpe]

Maybe the light is red, but you wouldn't be able to see it. In darkness, only rod cells are sensitive enough, and you can't see color with rods. Also, AFAIK rods aren't sensitive to red light, or at least red light doesn't de-sensitize them, which is why e.g. the night light in cockpits is red.

With "synthetically enhanced vision", you can see anything you want to see, so it's hard to tell anything about that option.

You also use red lights for moving more or less unseen over ground, as red light does not reflect from the ground as good as other wavelengths.

 

[Artlav]

Hm.
Just how light-sensitive could the non-theoretical cameras be in real-time?

I've just taken a 30-seconds exposure frame of the clear night sky outside, yet all that could be seen is some bright stars on bluish-gray background.

I guess you'll need much more amplification to see the galaxy in real-time image from outside of it.

 

[agentgonzo]

With a much larger aperture (so more photons hitting the sensor) and more sensitive sensors (think higher ISO) you will get a 'brighter' image with a shorter exposure time. No idea what you'd need to see it in realtime though. Even through a large telescope, nebulae and galaxies are still too dim to be in colour. They don't let you stick your eyeball to the really massive telescopes though - they get in the way of the sensors.

 

[RisingFury]

I wouldn't set the ISO too high.

I've tried a couple of shots on my cam, with 30 seconds exposure, aperture as low as it goes (2.8, I think) and high ISO... and although you see a lot of stars - way more then you can see with the naked eye - there's a lot of jitter in the image.


The best thing would be to leave them cam for 5 minutes or so, on a stand that rotates.

 

[cjp]

Just how light-sensitive could the non-theoretical cameras be in real-time?

The theoretical limit is detecting individual photons of course. Modern CCDs come quite close to that, and BTW, so do the rods in the human eye.

So, at that level it comes down to how many photons you can catch. A bigger lens or mirror will help(*), and a longer exposure time will help too.

You said "real-time". Does this mean "25 frames per second", so "exposure time 1/25 second"?

That leaves one variable: the size of the camera. How big do you want it to be?

Changing ISO level just changes an amplification factor. A multiplication factor where a single electron of extra sensor charge is just guaranteed to result in an increased (digital) output value is the maximum useful, but remember that an ordinary 8-bit sensor will be saturated at 256 electrons(**) at this level, and you need to detect several thousands of photons(**) if you don't want a really noisy image. So, at really high ISO levels your picture will be either noisy or saturated.

(*) Assuming there are no diaphragms in the optical path. Using diaphragms doesn't make real sense anyway when photographing galaxies. Just focus on infinitive and open the diaphragm to maximum.
(**) I sometimes say photons and sometimes electrons, deliberately. In digital image sensors, capturing of photons results in moving electrons through a potential barrier. in the ideal case, capturing one photon results in a charge build-up of one electron.

 

[JamesG]

Maybe the light is red, but you wouldn't be able to see it. In darkness, only rod cells are sensitive enough, and you can't see color with rods. Also, AFAIK rods aren't sensitive to red light, or at least red light doesn't de-sensitize them, which is why e.g. the night light in cockpits is red.

Unless the light is intense enough or your color sensing cones of your eyes become sensitive enough to build a picture.

With "synthetically enhanced vision", you can see anything you want to see, so it's hard to tell anything about that option.

Well, I meant a "true color" as the human eye sees it in keeping with this thread. You can use anything from a simple telescope to genetic engineering to boost low-light vision, etc...