B What is the black that we see in the night sky?

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Summary
Can black means many thing?
Summary: Can black means many thing?

The black we see in night sky has to mean many things
First of all, from point of views of human eyes then the photons can't reach our eyes so it appears black.
Secondly, the objects appear smaller as we go farther from that object. So, at some point object won't be detected. And for the observer or the detector lying beyond that point sees nothing an it appears as black.
Or, what is black ? Is it something we can't detect? or there exist nothing where we see black? or, is it something else?
 
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Will the body won't be visible at some point in space if the object continues to appear smaller at increasing distance?
 
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Will the body won't be visible at some point in space if the object continues to appear smaller at increasing distance?
Obviously if an object is far enough away we won't be able to see it, yes.
 
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Moderator's note: Thread level changed to "B".
 

256bits

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The black we see in night sky
Can we objectively SEE black?
If I put you in a dark room with visible light below the threshold that your eyes could detect .........

Yet, there are photons traversing that space on either side of the visible spectrum that devices other than our eyes can detect. Describing something as being "black" could considered as being relative to the sensitivity of the sensor to particular wavelengths of light.
 

mathman

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That's why astronomers use telescopes.
 
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No object can ever be black. All bodies are leaking radiation. You might not be able to detect it with your eyes (the number of photons may be too low or the energies can be too high/lower) but it's always radiating. Even black holes have a very slight glow of Hawking radiation. Even if that weren't true, the universe itself has an afterglow from the big bang. It's in every direction and mostly homogenous, lookup the cosmic microwave background. The night sky isn't black, it's faintly glowing in microwave.
 

russ_watters

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Will the body won't be visible at some point in space if the object continues to appear smaller at increasing distance?
Not exactly. All of the objects we see in space are too small to be seen with the naked eye except two or three. Everything else we see as just points of light. So all of these other objects become invisible to the naked eye when they are so far away that they become too dim to see.
 

pinball1970

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Summary: Can black means many thing?

The black we see in night sky has to mean many things
First of all, from point of views of human eyes then the photons can't reach our eyes so it appears black.
Secondly, the objects appear smaller as we go farther from that object. So, at some point object won't be detected. And for the observer or the detector lying beyond that point sees nothing an it appears as black.
Or, what is black ? Is it something we can't detect? or there exist nothing where we see black? or, is it something else?
Black in a scene snap shot forms an image in your mind so it is a "thing" in this sense.
Black tells us something about a scene and is important from an evolutionary point of view.
An astronomer will conclude, black hole nothing there or I need a more powerful telescope .
 

kimbyd

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Will the body won't be visible at some point in space if the object continues to appear smaller at increasing distance?
The apparent size is less important. It's all about how much light it's belting out, and what the wavelength of the light is. We can definitely see objects that are far too small to be resolved by our eyes, as long as they are sufficiently bright compared to their surroundings. This is the case with all stars beyond our Sun: they're far, far too small for our eyes to resolve. To the extent that they appear to have any shape at all, it's because of optics which spread out their images in our eyes.

The wavelength is important as well because we can only observe light in a narrow band of wavelengths. If the wavelengths are too short or too long, we won't be able to see them with our eyes. Gamma ray bursts are an example of events which are simply too high-energy for us to observe (short wavelength = high energy). There are also a number of sources that put out x-rays.

Similarly, there are objects putting out radiation whose wavelength is too low, such as the supermassive black hole at the center of our galaxy (Sagittarius A*): the matter around this black hole emits radio waves that we also can't see. We can build sensors to observe the sky at every wavelength, and every wavelength that we observe shows us entirely new things.

For brightness, our eyes are only so sensitive. And the further away an object is, the dimmer it gets. Worse, if there are lights around us on Earth (such as street lights), our eyes won't adjust and we'll be unable to see the dimmer objects in the sky we could see from a darker location. If you've never seen the expanse of the Milky Way spread across the night sky, I definitely recommend taking a trip away from civilization to view it.

With telescopes, we can deal with the visibility of far-away objects by viewing the same portion of sky for a long period of time. It also helps to get away from the atmosphere, as with the Hubble Telescope, as the atmosphere obscures observation. Larger telescopes also help, as there's a larger light-collecting area to work with.
 
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Can black means many thing?
Yes. All the things mentioned in the above replies plus:
Stars that are so far away that their light has not reached us yet (since they began their fusion process), or anything beyond the distance of the visible universe (about 13.8 billion light years) -- that is before anything began to emit light.
Secondly, the objects appear smaller as we go farther from that object. So, at some point object won't be detected.
As mentioned in other replies only a few things can be resolved to a size. There is one galaxy that you can actually see with you naked eye, the Andromeda Galaxy, which has a resolvable size. But even with a big telescope, only a few stars have been resolved to a size. See: https://en.wikipedia.org/wiki/List_of_stars_with_resolved_images

Also, dark nebulae are a type of interstellar cloud that can obscure the light from objects behind them. You can see these dark patches in the Milky Way when there is no light pollution in the night sky.
 
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Also...

The reason why we don't we a black sky during the day is because light from the Sun hits small molecules of nitrogen and oxygen in the atmosphere and gets scattered. This is called Rayleigh scattering. This is why the sky is blue during the day and black at night (when the atmosphere is in the Earth's shadow.)

Where there is no atmosphere, like on the moon, the sky is always black.
 
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sophiecentaur

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All of the objects we see in space are too small to be seen with the naked eye except two or three.
I don't think you meant that. Two or three hundred, on an only moderately good night are visible and resolvable by the naked eye. I think it would be a good idea to replace "small" with "subtend extremely small angles". Apart from a few Solar System objects, all the rest are 'point sources' (effectively). The optics of the eye tend to make the brighter stars look larger.

'Black' is a very loose term because it really means 'below the threshold of the detecting instrument'. A telescope with more gathering power will see things in a 'black' region.
 

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