How many stars and what’s the limiting magnitude?

[SLD]

Here’s a blow up of a picture of a region in the plane of the Milky Way in Cygnus, so not the center of the galaxy.



unfortunately I can’t seem to make it bigger. Just looks like the crop of it. Hmm.

I’ve circled one. What’s its magnitude? I once read that we can’t likely detect stars beyond mag 27 or so. Not sure. Certainly none of these stars are on a star map. But maybe some of the surveys have picked these up, like Hipparchus or Gaia or Sloan, and have analyzed them all.

also why is the sky dark here? If I look at this area of the sky, which is small, I see only darkness at night. But it looks like greater than 50% of the area is covered ina star. But maybe that’s because of pixels blooming? Still there are billions of stars in this picture it seems.

 

[Shadowy Man]

I don’t know if there are “billions” of stars on this image. Don’t know the size of the detector, but since most detectors are listed in *mega*pixels there can’t be more than millions or every pixel would have a star. And likely the filling fraction is substantially less.

The sky is dark to your eye there because of the limitations of your eyes to detect faint light. Effective exposure time and aperture (pupil size) being the primary limiters.

 

[SLD]

I don’t know if there are “billions” of stars on this image. Don’t know the size of the detector, but since most detectors are listed in *mega*pixels there can’t be more than millions or every pixel would have a star. And likely the filling fraction is substantially less.

The sky is dark to your eye there because of the limitations of your eyes to detect faint light. Effective exposure time and aperture (pupil size) being the primary limiters.

But isn’t it one theory that the night sky should be bright if the universe were infinite because eventually every spot in the sky would correspond to a point on the surface of some Star?

im skeptical of that theory though because I think after some point the number of photons coming of that Star would be so diffuse that it wouldn’t matter. It would be dark.

 

[bilby]

But isn’t it one theory that the night sky should be bright if the universe were infinite because eventually every spot in the sky would correspond to a point on the surface of some Star?

That's Olbers' Paradox

The solution is that the observable universe has a finite extent due to having a finite age; You can't see stars that are further away than the distance light can travel in the time that the universe has existed.

You could, of course, see the hot surface of the early universe itself in any direction; and indeed you can - in the microwave band to which that light has been redshifted due to the expansion of the universe. If you look at any 'dark' region of the sky, it's bright in the microwave band - but obviously we can't see microwaves with the naked eye.

The Cosmic Microwave Background is the night sky being bright in every direction; It's just not bright at wavelengths we can see.

 

[Cheerful Charlie]

Some years back, the idea that the observational missing mass of the Universe was to be found in red and brown dwarfs was considered. These distand dwarves could not be directly observable. But radiated in infared and X-rays. An experiment was devised to mask off an empty spot in the field of vision of a large telescope and examine that area for infared radiation. This could not allow observation of an individual dwarf, but amount of radiation would roughly correlate with the red and brown dwarf population.
No large amounts of that radiation were observable. Repeat observations failed to demonstrate large dwarf populations that could account for missing mass.

 

[Shadowy Man]

Some years back, the idea that the observational missing mass of the Universe was to be found in red and brown dwarfs was considered. These distand dwarves could not be directly observable. But radiated in infared and X-rays. An experiment was devised to mask off an empty spot in the field of vision of a large telescope and examine that area for infared radiation. This could not allow observation of an individual dwarf, but amount of radiation would roughly correlate with the red and brown dwarf population.
No large amounts of that radiation were observable. Repeat observations failed to demonstrate large dwarf populations that could account for missing mass.

When I started grad school there were two competing theories: WIMPs and MACHOs. Weakly interacting massive particles and massive compact halo objects. What you are describing would fall into the second category. One of the major observations that ruled out MACHOs was microlensing searches. If there were a lot of massive objects making up the missing matter they would have caused far more microlensing events that were observed.

 

[Shadowy Man]

But isn’t it one theory that the night sky should be bright if the universe were infinite because eventually every spot in the sky would correspond to a point on the surface of some Star?

That's Olbers' Paradox

The solution is that the observable universe has a finite extent due to having a finite age; You can't see stars that are further away than the distance light can travel in the time that the universe has existed.

You could, of course, see the hot surface of the early universe itself in any direction; and indeed you can - in the microwave band to which that light has been redshifted due to the expansion of the universe. If you look at any 'dark' region of the sky, it's bright in the microwave band - but obviously we can't see microwaves with the naked eye.

The Cosmic Microwave Background is the night sky being bright in every direction; It's just not bright at wavelengths we can see.

Yes. Olber’s paradox is solved by the expansion and finite lifetime of the universe.

 

[SLD]

But isn’t it one theory that the night sky should be bright if the universe were infinite because eventually every spot in the sky would correspond to a point on the surface of some Star?

That's Olbers' Paradox

The solution is that the observable universe has a finite extent due to having a finite age; You can't see stars that are further away than the distance light can travel in the time that the universe has existed.

You could, of course, see the hot surface of the early universe itself in any direction; and indeed you can - in the microwave band to which that light has been redshifted due to the expansion of the universe. If you look at any 'dark' region of the sky, it's bright in the microwave band - but obviously we can't see microwaves with the naked eye.

The Cosmic Microwave Background is the night sky being bright in every direction; It's just not bright at wavelengths we can see.

Well, yes , but why is this area, where there are so many stars, not brighter than it seems? What’s the area here that is composed of stars vs. Darkness in this shot? I understand that I shouldn’t see individual stars, but I should see a generally bright area when I look at this area. The entire sky should be brighter.

 

[bilby]

I should see a generally bright area when I look at this area.

Why?

 

[SLD]

I should see a generally bright area when I look at this area.

Why?

Well, I mean, look at the picture. It’s so full of stars that almost every point is covered by the surface of some Star. It’s almost as if Olbers paradox is being disproven here.

 

[Tharmas]

Why is the sky dark at night? There’s actually been a book written on the subject, now apparently out of print: Darkness at Night, by the late astronomer Edward Harrison. I read it over thirty years ago and can’t remember his conclusion exactly, but the Wiki article I linked to gives an overview.

In googling the book I noticed there are a number of articles in various reputable periodicals that deal with the question (Olbers' paradox).

 

[Elixir]

The sky is dark wherever the light is insufficient for our senses to detect. In fact the sky has photons coming at us from every direction. The famous Hubble deep space photo was taken looking at one of the darkest tiny patches of sky, and reveals thousands of galaxies. Nothing that can be seen with the naked eye from earth’s surface is outside the Milky Way galaxy we live in.

 

[Shadowy Man]

I should see a generally bright area when I look at this area.

Why?

Well, I mean, look at the picture. It’s so full of stars that almost every point is covered by the surface of some Star. It’s almost as if Olbers paradox is being disproven here.

You are being limited by the resolution of the camera. If you calculate the angular size of these distant stars they will be virtual points

And often the images of stars will be stretched so that you’re not just looking at the core of the point spread function but down into the wings a little and it will look even bigger.


When you see the Milky Way you are seeing just a band of diffuse flow from a ton of unresolved stars. The overall •surface brightness• of the MW is low even if the radiance of each star is very high (don’t look directly at the sun). This surface brightness is usually lower than the sky brightness in urban environments, where the sky is scattering lights of our civilization.

So, the idea of Olber’s paradox is that in an infinite universe every sightline will eventually land on the surface of a star so the sky’s surface brightness should be the same as an average star (so something like looking at the sun but in all directions). The paradox is solved by two things: the universe’s finite age means that you can’t keep looking infinitely far (eventually every sight line lands on the so-called “surface of last scattering” in the early universe and additionally the expansion of the universe means that the distant starlight gets redshifted to wavelengths we can’t see. It’s this latter effect that keeps the cosmic background radiation itself from blinding us.

 

[steve_bank]

The Hubble Deep Field picture.

Discoveries - Hubble's Deep Fields

NASA.gov brings you the latest images, videos and news from America's space agency. Get the latest updates on NASA missions, watch NASA TV live, and learn about our quest to reveal the unknown and benefit all humankind.

www.nasa.gov

Hubble imaged a small dark area in space essentially adding photons on successive passes until images of distant objects emerged.

The lght from a distant star to us is like an isiotropic radiatorradiating equally in all directions. An expending sphere. By the time it gets to our eyes the watts/m^2 are low, low numer of photions.

I'd say dark is the limit of our ability to detect photons.

 

[Loren Pechtel]

I don’t know if there are “billions” of stars on this image. Don’t know the size of the detector, but since most detectors are listed in *mega*pixels there can’t be more than millions or every pixel would have a star. And likely the filling fraction is substantially less.

The sky is dark to your eye there because of the limitations of your eyes to detect faint light. Effective exposure time and aperture (pupil size) being the primary limiters.

But isn’t it one theory that the night sky should be bright if the universe were infinite because eventually every spot in the sky would correspond to a point on the surface of some Star?

im skeptical of that theory though because I think after some point the number of photons coming of that Star would be so diffuse that it wouldn’t matter. It would be dark.

It doesn't matter how far away it is for Olber's Paradox, although if it's redshifted sufficiently (which would have to be extreme) you could avoid it. There was an answer on StackOverflow a while back that showed that some ridiciously tiny portion of the sky actually is stars--small enough you would use scientific notation to represent the number.

 

[Loren Pechtel]

I should see a generally bright area when I look at this area.

Why?

Well, I mean, look at the picture. It’s so full of stars that almost every point is covered by the surface of some Star. It’s almost as if Olbers paradox is being disproven here.

Stars don't fill a pixel.

 

[Shadowy Man]

Think about it this way. Even from the distance of the closest star (1.3 parsecs) the Sun would only subtend an angle of 3.5e-8 radians, which is about 7 milliarcseconds. The human eye resolves about 1 arc minute, which is about 8500 times bigger.

So, since most stars are much much farther away than the closest one and most stars are the size of the Sun or smaller then you can see why individual stars would fill so very little of our eye’s resolution.

 

[bilby]

Think about it this way. Even from the distance of the closest star (1.3 parsecs) the Sun would only subtend an angle of 3.5e-8 radians, which is about 7 milliarcseconds. The human eye resolves about 1 arc minute, which is about 8500 times bigger.

So, since most stars are much much farther away than the closest one and most stars are the size of the Sun or smaller then you can see why individual stars would fill so very little of our eye’s resolution.

Point of order: The nearest star is about 0.000004861 parsecs away, and at that distance the Sun would subtend an angle of about 0.0087 radians.

 

[Bomb#20]

Nothing that can be seen with the naked eye from earth’s surface is outside the Milky Way galaxy we live in.

The Magellanic Clouds and the Andromeda Galaxy are naked-eye objects.

 

[Shadowy Man]

Nothing that can be seen with the naked eye from earth’s surface is outside the Milky Way galaxy we live in.

The Magellanic Clouds and the Andromeda Galaxy are naked-eye objects.

Yes, indeed. These can be seen with the naked eye given dark skies. When I spent some time in South African Astronomical Observatory location in Sutherland, it was dark enough to see the Magellanic Clouds. I could even make out the bar of the Large one. It was also my first time (and only so far) seeing the zodiacal light (sunlight scattered off of interplanetary dust).