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koendb
2001-Nov-09, 06:33 PM
How is it possible that it's dark on earth at night? Is it because the stars are too far away and the light waves aren't visible for the human eye anymore (due the redshift), is it because of the large dust clouds, or are there not enough stars to fill the whole sky (although that sounds rather stupid to me)?
thanks
kdb

<font size=-1>[ This Message was edited by: koendb on 2001-11-09 13:39 ]</font>

<font size=-1>[ This Message was edited by: koendb on 2001-11-09 13:49 ]</font>

ToSeek
2001-Nov-09, 06:48 PM
The official name for your question is "Olber's Paradox," and it's actually one of the pieces of evidence for the Big Bang theory.

I think the answer is really all of the above. In particular, the Big Bang theory requires a finite amount of matter, therefore the sky is not in fact filled with stars.

dgavin
2001-Nov-09, 07:26 PM
Actually at night, at a dark sky location, (such as my fathers place) you'd be amazed at how many stars there are visible. And thier light actually does reach earth. Not enough to read by like when there is a full moon out, but significant enough for some to see by.

Also eyesight is a factor. Some people have more receptors then others in thier eyes. I am one that has excelent night vision. In winter, when there is snow on the ground and clouds in the sky's I can see in almost full color (in a city, or just outside of one) from the reflected light. This little talent when I discovered it, kept me on night guard duty almost every winter when i was in service.

I like to take my scope to my dad's occasionally, as there is no light polution there. With no moon out, can actually see by the light of the milky way. The house even has a distiguisable shadow from blocking the light.

My wife on the other hand, has very poor night vision. She can barely see during a full moon.

There are some tests avaiable to determine ones light sensitivity. But a good rule of thumb is this, if you monitor is at 10-33% of full brightness and you can see well, but others complain it's dark. Your probably light sensitive.

On the other hand, if you have your monitor from 65% or even higher on brightness, and people comment it's too bright, your probably light impaired. Avereage people have monitors set from 40 to 60% Brightness.

Unless your in the military, the actual test for light aquity can be expensive, but could be worthwhile to have done if you are in the service.

Hat Monster
2001-Nov-09, 07:42 PM
Redshift would actually make stars brighter. The most powerful stars, such as Rigel, Naos, Deneb, etc. put out far more energy in the ultra violet than they do in visible. Redshift would shift the UV into the visible spectrum.
The main reason the night sky is dark is because there are multiple orders of magnitude greater amounts of dim stars than there are of the brighter ones. Out of all the stars you can see, the top few hundred are more bright than the other few hundred thousand combined.
Also, dust and gas in space blocks out much of the distant light. Cassiopeia is home to a vast cloud, and so is Orion, both constellations being dwarfed by their molecular clouds. The Orion molecular cloud spans most of Orion, Taurus, Gemini and Canis Major.

CJSF
2001-Nov-09, 07:48 PM
Cassiopeia is home to a vast cloud, and so is Orion, both constellations being dwarfed by their molecular clouds. The Orion molecular cloud spans most of Orion, Taurus, Gemini and Canis Major.


This is a little unclear. Orion is a grouping of stars at greatly varying distances from Earth, and I suspect Cassiopea is the same. I know some constellations are actually made up of stars that are loosely bound together - Ursa Major is one, I believe.

Where in relation to these constellations' stars are these clouds?

CJSF

The Bad Astronomer
2001-Nov-09, 08:54 PM
On 2001-11-09 13:48, ToSeek wrote:
The official name for your question is "Olber's Paradox,"

OK, pet peeve time: it's Olbers' paradox; the man's name was Olbers.

Anyway, I have a web page about this: http://www.badastronomy.com/mad/1996/olbers.html

Hale_Bopp
2001-Nov-09, 09:25 PM
Great Question. Stars farther away are fainter, but there are more of them per area, so the effects cancel out. The night sky should be blazingly bright, but it isn't. Hence the paradox.

However, this assumes the universe is infinitely old, which it isn't. When the paraodx was first proposed, it was thought that the universe was eternal and unchaning. We know that isn't true now.

Light from the really distant stars hasn't had time to reach us. Hence, not all the light from all stars contribute to the light of the night sky.

There was an aritcle in Astronomy or Sky&Telescope this month on this question...I will find it when i get home tonight and post more (star party night, so don't expect an early post from me /phpBB/images/smiles/icon_smile.gif

Rob

koendb
2001-Nov-11, 11:34 AM
Thanks all, but still i don't know the real answer; is it because of the dust (or other dark objects), or are there not enough stars in the universe or are some to far away (their light hasn't reached us yet).
I understand that all of this arguments are true, but which one is the most important?
kdb

Trish
2001-Nov-11, 12:35 PM
I would think the effect to be cumulative. That no one thing takes any precedence over the others, but that all combine to produce this result.

Also, aren't we currently at the edge of an arm of just between arms of the galaxy. So wouldn't proximity to the vast number of stars also affect this?

Donnie B.
2001-Nov-11, 01:09 PM
On 2001-11-11 06:34, koendb wrote:
Thanks all, but still i don't know the real answer; is it because of the dust (or other dark objects), or are there not enough stars in the universe or are some to far away (their light hasn't reached us yet).
I understand that all of this arguments are true, but which one is the most important?
kdb


I'm not a professional astronomer, but I sometimes try to play one on this board. /phpBB/images/smiles/icon_biggrin.gif

The most important answer to the Olbers' paradox (happy, BA?) is that the universe is NOT infinite in either time or extent. That means that the sight lines from Earth do not all terminate at a star (that's not technically rigorous... we don't "look out" from Earth, the light from out there comes to us. Rather, the phrase is one used in computer graphics, and it's appropriate here just to help us think about this issue.)

The dust and gas issue is secondary. It dims the distant stars a little (not much, in most directions, but quite a bit in specific places). However, if the universe truly were infinite in size and time, and there truly were an infinite number of stars, all the gas and dust would eventually heat to incandescence, and would not dim the sky to any appreciable degree.

In other words, if the universe were truly infinite in size, age, and number of stars, life as we know it would be impossible... we'd be toast (or rather, we'd make toast seem ice-cold by comparison).

David Simmons
2001-Nov-11, 05:12 PM
On 2001-11-11 08:09, Donnie B. wrote:


On 2001-11-11 06:34, koendb wrote:
Thanks all, but still i don't know the real answer; is it because of the dust (or other dark objects), or are there not enough stars in the universe or are some to far away (their light hasn't reached us yet).


I'm not a professional astronomer, but I sometimes try to play one on this board. /phpBB/images/smiles/icon_biggrin.gif

The most important answer to the Olbers' paradox (happy, BA?) is that the universe is NOT infinite in either time or extent.

In addition, the stars are not uniformly distributed and the number of is not infinite. If they were, the night sky would be as bright as the individual stars are.

If the stars' distribution were uniform then there would be an equal density of stars in any thin, spherical shell of the space surrounding the earth.

The area of the surface of a sphere is 4*pi*r^2 and the thickness of the thin shell is delta(r}. So the volume of the thin shell is proportional to the square of the radius and with equal density of stars, the number of stars would increase as the square of the radius to the shell in question.

The light intensity falls off as the square of the radius so all shells would be uniformly bright. And with random, uniform distribution there would be a star at every point in the heavens which would then be bright.

Ergo (as we experts love to say /phpBB/images/smiles/icon_wink.gif), the stars cannot be uniformly distributed.

PS - Dear Donnie B. (nit-pick of the day). The BA might be delighted but the English teacher won't be. If you write, "the" in front of "Olbers' paradox," drop the possessive on "Olbers".(Why isn't there a smiley for "full of trivia?")


<font size=-1>[ This Message was edited by: David Simmons on 2001-11-11 18:59 ]</font>

Donnie B.
2001-Nov-11, 11:45 PM
PS - Dear Donnie B. (nit-pick of the day). The BA might be delighted but the English teacher won't be. If you write, "the" in front of "Olbers' paradox," drop the possessive on "Olbers".(Why isn't there a smiley for "full of trivia?")



I knew that... /phpBB/images/smiles/icon_biggrin.gif

Actually, I changed the wording of that line before posting and left in the "the" just so you'd have a nit to pick.

David Simmons
2001-Nov-11, 11:59 PM
On 2001-11-11 18:45, Donnie B. wrote:

Actually, I changed the wording of that line before posting and left in the "the" just so you'd have a nit to pick.


You're all heart, Donnie B.

Sam5
2004-Mar-10, 02:49 AM
The official name for your question is "Olber's Paradox," and it's actually one of the pieces of evidence for the Big Bang theory.

I think the answer is really all of the above. In particular, the Big Bang theory requires a finite amount of matter, therefore the sky is not in fact filled with stars.

I think I’ve figured out a possible solution to Olbers’ Paradox. If photons move in straight lines, then the most distant galaxies we can see are emitting photons that follow the inverse square law, and, thus, the photons (or beams) or “wave trains” of photons, are separating as they move through space. By the time they reach us from the most distant galaxies, they have separated so much, we are looking in between the photons of the galaxies. Individual photons from the galaxies are zipping past our eyes and are hitting our face, our chin, and our forehead. Occasionally one goes into our eye, but it’s too weak, as a single photon, for us to notice. This would also tend to suggest that the universe is not “infinite” in size.

The HST has a diameter much larger than our eyes, so it is gathering photons from a wide circle of incoming photons, and it focuses them into the small electronic camera. Also, notice that the NASA/Hubble article of today (3/9/04) says the Hubble was receiving as few as one photon per minute from the galaxies, and they had to accumulate enough of them to show up in this photo. The article says this photo was exposed over a period of about a million seconds, or about 11.3 days of total exposure time. Not all at one time, not continuously, but many exposures over a period of several months. Only a computer controlled space telescope could do that, because of the registration problem encountered when adding a lot of multiple photos together so that they could accumulate enough light for these galaxies to show up.

LATEST HUBBLE PHOTO (http://www.spaceflightnow.com/news/n0403/09deepfield/)

Back when it was assumed that light traveled as “spherical waves”, something like the way sound travels, then they couldn’t imagine any “space” in-between the light. I think this is one thing that caused Olbers’ Paradox to be suggested in the first place. That, plus the possibility of an "infinite" universe. But, if light travels as individual “side by side” photons, and if they gradually separate further and further apart as they move through space, then we are looking in between the incoming photons from the most distant galaxies, and that could be why the night sky is not bright all the time.

Kaptain K
2004-Mar-10, 12:37 PM
The article says this photo was exposed over a period of about a million seconds, or about 11.3 days of total exposure time. Not all at one time, not continuously, but many exposures over a period of several months.
Sorry Sam,
It was one, long, continuous, 11+ day exposure! :o

Spaceman Spiff
2004-Mar-10, 03:05 PM
How is it possible that it's dark on earth at night? Is it because the stars are too far away and the light waves aren't visible for the human eye anymore (due the redshift), is it because of the large dust clouds, or are there not enough stars to fill the whole sky (although that sounds rather stupid to me)?
thanks
kdb

&lt;font size=-1>[ This Message was edited by: koendb on 2001-11-09 13:39 ]&lt;/font>

&lt;font size=-1>[ This Message was edited by: koendb on 2001-11-09 13:49 ]&lt;/font>

Have a look at a piece I wrote up for the Kalamazoo Astronomical Society, on the dark sky (http://homepages.wmich.edu/%7Ekorista/bigbang-darksky.html)(aka "Olbers' Paradox").

Manchurian Taikonaut
2004-Mar-10, 04:22 PM
With so many Billions of stars in the galaxies extending light years away, why is the night sky largely dark and black? Look at the Olbers’ problem the layers way, think of the sky at night as onion layers each with its own number of stars. The further out the distance of a layer the greater the number of stars in the layer coming, the idea is that this compensates for the reduced brightness of the more distant stars. So the bigger distance does not decrease the brightness, but also there is the problem of dust in the universe absorbing the light from distant stars, dulling down the sky like a painter using darker colors on his painting so they background can not be seen clearly. If the Earth was inside a dark Nebulae, would we be able to see anything?, but even in day-light some of the night-sky objects can be seen...Venus, Sirius ( alphacanis majoris ), Jupiter, Vega and our Blue-Moon of course! Our universe is a big growing room, expanding outwards and universe provides insufficient energy to brighten the room or the night sky.

Sam5
2004-Mar-10, 04:51 PM
Sorry Sam,
It was one, long, continuous, 11+ day exposure! :o

The Hubble Space Telescope can’t do a continuous 11 day exposure while orbiting around the earth. Read the article again.

LATEST HUBBLE PHOTO (http://www.spaceflightnow.com/news/n0403/09deepfield/)

”The Hubble telescope's Advanced Camera for Surveys' wide-field camera snapped 800 exposures, which equals two exposures per orbit. The exposures were taken over four months, from Sept. 24, 2003 to Jan. 16, 2004.

The 800 exposures amounted to about 1 million seconds or 11.3 days of viewing time. The average exposure time was 21 minutes.”

milli360
2004-Mar-10, 05:01 PM
The article says this photo was exposed over a period of about a million seconds, or about 11.3 days of total exposure time. Not all at one time, not continuously, but many exposures over a period of several months.
Sorry Sam,
It was one, long, continuous, 11+ day exposure!
No, of course it wasn't. The Hubble orbits the Earth a few times each day, Fornax is not always visible. The picture (http://www10.ksc.nasa.gov/mirrors/stsci/hubbledev/newscenter/newsdesk/archive/releases/2004/07/text/index.html) took four months.

Sam5
The thing about the photons as particles doesn't affect the Olbers Paradox because the intensity doesn't change. It doesn't matter if you think of waves as spread out, or particles as spread out.

Sam5
2004-Mar-10, 05:15 PM
Sam5
The thing about the photons as particles doesn't affect the Olbers Paradox because the intensity doesn't change. It doesn't matter if you think of waves as spread out, or particles as spread out.


Could you explain that a little more, please.

The intensity doesn’t change, but the intensity is not unlimited and it spreads out due to the inverse square law. It is concentrated into billions of photon wave-trains or “beams”, and if we are far enough away, we can be looking in between the beams because they are spread out so much at our distance. At great distances, what we get of the “intensity” is just one or two photons a minute, as the article said. Of course, if we are right up close to the galaxy we would receive many more per minute, billions.

Kaptain K
2004-Mar-10, 07:42 PM
Sam5 and mille360,
#-o You are, of course, correct. :oops: I realized my error after I went to work last night, but it was too late to delete my post.

milli360
2004-Mar-10, 09:10 PM
Kaptain K, is your name Orenthal? 'Cause you're OK


Could you explain that a little more, please.

The intensity doesn’t change, but the intensity is not unlimited and it spreads out due to the inverse square law. It is concentrated into billions of photon wave-trains or “beams”, and if we are far enough away, we can be looking in between the beams because they are spread out so much at our distance. At great distances, what we get of the “intensity” is just one or two photons a minute, as the article said. Of course, if we are right up close to the galaxy we would receive many more per minute, billions.
What I was responding to was the suggestion that that was a solution to Olbers Paradox, which depends upon that inverse square fall off. Whether the fall off results in a decrease in the number of particles or the intensity of the wave, the conclusion of Olbers Paradox shouldn't change, I think.

Sam5
2004-Mar-11, 12:46 AM
What I was responding to was the suggestion that that was a solution to Olbers Paradox, which depends upon that inverse square fall off. Whether the fall off results in a decrease in the number of particles or the intensity of the wave, the conclusion of Olbers Paradox shouldn't change, I think.



Hmm, I think it is the, uhh, not necessarily the fall off in the intensity, but the separation of the photons. When the distant galaxies are far enough away, we are in between their photons as they pass by us. This has an effect of a fall off in intensity, but only because there are fewer side by side photons at our distance.

Like, say, 100 archers are suspended from a rope and they fire their volley of arrows in all directions, up, down, left, right, front, back, etc. If we are close to them, the more chances we have of being hit by two or three of their arrows. But if we are some distance away, the arrows are more likely to zoom past us without hitting us.

If the universe were infinite, with an infinite number of galaxies, then I think the sky should glow at night. Like with an infinite number of archers. Assuming their arrows can travel in a straight line forever, we would eventually be hit by a large number of arrows, no matter how far away the infinite number of archers are. So I think the dark night sky indicates a finite number of galaxies.

Celestial Mechanic
2004-Mar-11, 04:52 AM
[Snip!]If the universe were infinite, with an infinite number of galaxies, then I think the sky should glow at night. Like with an infinite number of archers. Assuming their arrows can travel in a straight line forever, we would eventually be hit by a large number of arrows, no matter how far away the infinite number of archers are. So I think the dark night sky indicates a finite number of galaxies.
Or that the archers only started firing away 13.8 billion years ago. 8)

freddo
2004-Mar-11, 05:20 AM
That's what I thought. There are an infinite number of archers, but some, or many even, of the arrows are still on the way.

Isn't that where the idea for the heat death of the universe comes from? Or part of it?

SAMU
2004-Mar-11, 08:15 AM
If the universe were infinite then there would be infinite light coming from it to us but there would also be infinite mass pulling it back.

The farther you go from here the closer you get to there (infinite mass). At some finite plane between here and there a star will be so much closer to the infinite mass its light could not reach here because the gravitational pull on it would keep it from reaching us. Sort of like the situation of an outside in black hole. Rather than a sphere of the event horizon of a black hole from which light can't escape it is a shell around us from which light can not enter.

eburacum45
2004-Mar-11, 10:10 AM
In particular, the Big Bang theory requires a finite amount of matter, therefore the sky is not in fact filled with stars.

Is that true? Does the Big Bang theory imply a finite universe in extent as well as in age?

Spaceman Spiff
2004-Mar-11, 02:38 PM
In particular, the Big Bang theory requires a finite amount of matter, therefore the sky is not in fact filled with stars.

Is that true? Does the Big Bang theory imply a finite universe in extent as well as in age?

In short, no -- it implies no such thing.
The universe may be infinite or in any case MUCH larger in extent than the observable universe. The key to understanding Olbers' so-called paradox is what was mentioned a couple of other times in this thread, most recently by Celestial Mechanic (http://www.badastronomy.com/phpBB/viewtopic.php?p=221491#221491).

milli360
2004-Mar-11, 04:52 PM
In particular, the Big Bang theory requires a finite amount of matter, therefore the sky is not in fact filled with stars.

Is that true? Does the Big Bang theory imply a finite universe in extent as well as in age?

In short, no -- it implies no such thing.
The universe may be infinite or in any case MUCH larger in extent than the observable universe. The key to understanding Olbers' so-called paradox is what was mentioned a couple of other times in this thread, most recently by Celestial Mechanic (http://www.badastronomy.com/phpBB/viewtopic.php?p=221491#221491).
What implies? The Big Bang, or Olbers Paradox?

Spaceman Spiff
2004-Mar-11, 09:02 PM
In particular, the Big Bang theory requires a finite amount of matter, therefore the sky is not in fact filled with stars.

Is that true? Does the Big Bang theory imply a finite universe in extent as well as in age?

In short, no -- it implies no such thing.
The universe may be infinite or in any case MUCH larger in extent than the observable universe. The key to understanding Olbers' so-called paradox is what was mentioned a couple of other times in this thread, most recently by Celestial Mechanic (http://www.badastronomy.com/phpBB/viewtopic.php?p=221491#221491).
What implies? The Big Bang, or Olbers Paradox?

The question pertained to the Big Bang Theory. My reply was that the BBT does not require the universe to be finite in size. Olbers' paradox does not require it, either. A finite age of the phenomenon called stars and galaxies is sufficient.