How dark is intergalactic space?

[farmerben]

I heard the claim that intergalactic space is so dark, that with the naked eye you could see nothing.  

If you were halfway between the Milky Way and Andromeda you would see two points of light, but not much more.  If you were halfway to the nearest other galactic cluster it would be so dark, you could not see any lights with the naked eye.

Does this sound valid?

 

[JoeSchmuckatelli]

We can see other galaxies from Earth. 

They used to call them nebula 

 

https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question15.html

https://www.pbs.org/seeinginthedark/astronomy-topics/andromeda-galaxy.html

 

 

Edited May 11, 2023 by JoeSchmuckatelli

[tater]

8 minutes ago, farmerben said:

Does this sound valid?

No.

In sufficiently dark skies, Andromeda is a naked eye object on Earth.

Minus the atmosphere, it would always be naked eye, as would the Milky Way.

From Earth Andromeda is ~3° wide (~6 Moon dia), so both would be larger halfway between—about twice that for Andromeda, and about the same for the Milky Way (which is ~1/2 the dia).

Course not very bright, just fuzzy patches. So the cores would be most of the angular size you'd resolve (and then just fuzzy patches).

Edited May 11, 2023 by tater

[farmerben]

You can see Andromeda with the naked eye from the distance of the Milky Way.  Out between galactic clusters it is much darker than that.

[Brotoro]

At a point midway between Andromeda and the Milky Way, you could see both of those galaxies and the M33 galaxy as fuzzy patches of light, with both Andromeda and Milky Way galaxies being brighter than Andromeda is as seen fro Earth. But in the voids between superclusters, you should be far enough away from all galaxies that no galaxy would be visible to the naked eye.

[cubinator]

Just look at the apparent magnitude of the various galaxies in the sky. If you imagined the Milky Way disappeared and all else remained in the sky, you'd see Andromeda fairly clearly, and could probably pick out Triangulum. Wikipedia lists out the galaxies that are bright enough to be seen by naked eye: https://en.wikipedia.org/wiki/List_of_galaxies

It's a very short list. As they start getting into magnitudes 8+ and on they are most likely not visible to humans.

So you wouldn't be able to see the web-like galactic superstructures, and most of the sky would be dark, but there would be a few galaxies visible as faint blurs.

[kerbiloid]

I can't see even Milky Way from here.

Living in a ciy is like flying between the galaxy clusters...

[Nuke]

2 hours ago, kerbiloid said:

I can't see even Milky Way from here.

Living in a ciy is like flying between the galaxy clusters...

i used to visit my sister out in the sticks periodically. space is more interesting when you can see it. i once sat on the roof all night on a warm summer day, much to the appreciation of the local mosquitoes giant bloodsucking helicopters. 

Edited May 12, 2023 by Nuke

[Codraroll]

5 hours ago, kerbiloid said:

I can't see even Milky Way from here.

Living in a ciy is like flying between the galaxy clusters...

Nah, you can see a little bit of it. The bit you're standing on.

[JoeSchmuckatelli]

11 hours ago, Brotoro said:

you should be far enough away from all galaxies that no galaxy would be visible to the naked eye.

That does not seem correct. You might not see it as a galaxy shape but light does travel through the intergalactic voids - so they probably look like  stars. 

(just spitballing here - but even if a galaxy were far enough away that it's apparent diameter was tiny you should see it.  Only way I can see not perceiving it would be if the redshift had taken it down below human visible spectra) 

[tater]

2 hours ago, JoeSchmuckatelli said:

That does not seem correct. You might not see it as a galaxy shape but light does travel through the intergalactic voids - so they probably look like  stars. 

(just spitballing here - but even if a galaxy were far enough away that it's apparent diameter was tiny you should see it.  Only way I can see not perceiving it would be if the redshift had taken it down below human visible spectra) 

On Earth in super dark skies maybe magnitude 7 might be visible (usually you see 6.5 quoted). Minus the atmosphere... dunno. 7?

There are not many visible from these parts, slim pickins. ~4 of the 200 brightest. If we can gain a magnitude of visibility (whatever the human eye limit is) we can add a few more.

https://www.icc.dur.ac.uk/~tt/Lectures/Galaxies/LocalGroup/Back/galax200.html

If you're in intergalactic space, hope for some supernovae to make life slightly more visually interesting?

 

[sevenperforce]

2 hours ago, JoeSchmuckatelli said:

14 hours ago, Brotoro said:

in the voids between superclusters, you should be far enough away from all galaxies that no galaxy would be visible to the naked eye.

That does not seem correct. You might not see it as a galaxy shape but light does travel through the intergalactic voids - so they probably look like  stars. 

(just spitballing here - but even if a galaxy were far enough away that it's apparent diameter was tiny you should see it.  Only way I can see not perceiving it would be if the redshift had taken it down below human visible spectra) 

Light travels through the intergalactic voids, but it's still subject to the inverse-square law.

Most intergalactic voids are 30 to 300 million lightyears in diameter, so you can imagine being in the center of the smallest of these voids and being 15 million lightyears away from the nearest galaxy.  That's six times as far as we are from Andromeda or Triangulum, meaning that an equivalently-bright galaxy would be 36 times dimmer than they are, thanks to the inverse square law.

The human eye can't take long exposures like a telescope; our maximum "shutter speed" is about 1/50th of a second. And the lens of a human eye is only about 1 cm in diameter, so together we get a total light collection area of 1.57 cm². Our retinas need to be getting a certain minimum number of photons per second or we simply won't see anything. The theoretical limit for the naked-eye visibility in a perfectly dark sky, ignoring atmospheric extinction, is around +8^m, or 69.2 times dimmer than Andromeda's +3.4^m and 8.3 times dimmer than Triangulum's +5.7^m (apparent magnitude is an inverse logarithmic scale). So Andromeda would be readily visible from the center of the smallest intergalactic voids, but Triangulum would not.

The most massive elliptical galaxies, like M87, have absolute magnitudes as high as -22, making them naked-eye visible to a distance of 10 million parsecs or 32 million lightyears. The active blazars in certain galactic nuclei can reach brightnesses of over -32, which would be naked-eye visible from as far as 32 billion lightyears, but only if they are pointed directly at the observer (and even then, the brightness is variable).

So while the largest intergalactic voids are vast enough to extinguish the light of the brightest galaxies, there is still a chance of catching a glimmer of relativistic fire through the darkness.

And of course a moderately large telescope like JWST with a decent exposure time would still be able to see all the way to the edge of the visible universe.

[tater]

Thinking about this while driving, I should be clear that what I think might change is not the human limit in visibility, but the apparent magnitude of galaxies outside the atmosphere. So some on that list appear slightly brighter. Extinction is ~0.8 mag.

So nothing really changes on that list. Anything above ~7.3 is not visible to the naked eye.

So much farther away than halfway between the MW and Andromeda, the sky would become even more black than just 2-5 visible objects. Easily possible to see nothing at all naked eye I think, if you pick the location right.

Edited May 12, 2023 by tater

[AtomicTech]

19 hours ago, farmerben said:

I heard the claim that intergalactic space is so dark, that with the naked eye you could see nothing.  

If you were halfway between the Milky Way and Andromeda you would see two points of light, but not much more.  If you were halfway to the nearest other galactic cluster it would be so dark, you could not see any lights with the naked eye.

Does this sound valid?

 

In a supervoid, you couldn't see anything.

[tomf]

My searching suggested that the human retina could register a single photon, so in that sense you could potentially see light anywhere if only as occasional flashes. However for the signal to register with consciousness 5-9 photons within 100ms

Edit:

Further searching has turned up a study where participants could detect single photons with better than pure chance. Apparently it was less like seeing than having a gut feeling that there was light. So I guess anywhere in the universe you would have a feeling that galaxies are out there somewhere.

Edited May 12, 2023 by tomf

[sevenperforce]

1 hour ago, tomf said:

My searching suggested that the human retina could register a single photon, so in that sense you could potentially see light anywhere if only as occasional flashes. However for the signal to register with consciousness 5-9 photons within 100ms

Edit:

Further searching has turned up a study where participants could detect single photons with better than pure chance. Apparently it was less like seeing than having a gut feeling that there was light. So I guess anywhere in the universe you would have a feeling that galaxies are out there somewhere.

In that study, the participants only identified a single-photon exposure correctly 51.6% of the time -- enough to be statistically significant under the test conditions, but certainly not enough to identify anything we would consider visible.

The minimum light intensity that would register as a meaningful point of light depends on the amount of energy necessary to fully interrupt the flow of sodium cations in a photoreceptor cell, triggering the nerve signal which is registered by the brain as a persistent point of light. The necessary light intensity, then, depends not only on the number of photons but also on their frequency, which corresponds to the amount of energy they carry. It also depends on the actual wavelength-dependent sensitivity function V(λ) which peaks at 555 nm. 

Finally, not all light that enters the lens actually reaches the retina. Although studies from the 1940s proved that you only needed 5-7 photons to reach the retina in order to register a visible flash of light, you need far more to actually hit the lens (since 90% of the light which enters the human eye ends up scattered).

[JoeSchmuckatelli]

https://www.npr.org/2020/11/18/936219170/scientists-discover-outer-space-isnt-pitch-black-after-all

 

[kerbiloid]

https://astrobackyard.com/brightest-galaxies-in-the-sky/

Quote

List of Brightest Galaxies

Below is a list of the brightest galaxies in the night sky based on available apparent magnitude values. Magellanic Cloud types and dwarf galaxies have not been included in this list. Any of these objects would make great deep-sky targets.  

1. Andromeda galaxy (Mag 3.1)
2. Triangulum galaxy (Mag 5.72)
3. Whirlpool galaxy (Mag 5.8)
4. Centaurus A (Mag 6.84)
5. Bode’s galaxy (Mag 6.94)
6. Southern Pinwheel galaxy (Mag 7.6)
7. Pinwheel galaxy (Mag 7.9)
8. Sombrero galaxy (Mag 8)
9. Sculptor galaxy (Mag 8)
10. Cigar galaxy (Mag 8.41)
11. Leo Triplet (Mag 8.9)

These things are outside of the Milky Way, and will stay visible if remove its stars from the sky.

As a human with normal vision can see up to 6, there should be two bright fuzzy spots  the sky, midway to Andromeda, and a ten more if use theatre binoculars.

[NFUN]

I have seen the fluorescence of a single atom before, so you don't need much

[Shpaget]

3 minutes ago, NFUN said:

I have seen the fluorescence of a single atom before, so you don't need much

Can you elaborate?

[kerbiloid]

16 minutes ago, NFUN said:

I have seen the fluorescence of a single atom before, so you don't need much

Cosmonauts see even single protons.

[SunlitZelkova]

9 hours ago, kerbiloid said:

Cosmonauts see even single protons.

But unfortunately, due to the cancellation of the L1, never got to fly on them.

[NFUN]

11 hours ago, Shpaget said:

Can you elaborate?

got an ion trap and a dark room and one (1) ion and a tube (that has magnification but it's only 20x that's basically nothing)

[cubinator]

One thing I'm curious about is the combined light from the multiple galaxies, clusters, etc. Within the Milky Way we'd say that most of the stars are much too far away for us to see, yet we see their combined light in the galactic plane because there are enough of them together. Could the galaxies be packed visually close enough that the clusters would be at least a little visible?

[sevenperforce]

2 hours ago, cubinator said:

One thing I'm curious about is the combined light from the multiple galaxies, clusters, etc. Within the Milky Way we'd say that most of the stars are much too far away for us to see, yet we see their combined light in the galactic plane because there are enough of them together. Could the galaxies be packed visually close enough that the clusters would be at least a little visible?

The cross-sectional area collapses as you move away, so yeah. But it's gonna come down to photon counts. Those will stack.