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Author Topic: light from a distant star  (Read 5597 times)

Offline barneyboy

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light from a distant star
« on: 23/10/2014 23:27:40 »
do we see distant stars as photons enter our eyes? if so how does a light source create so many? in every direction at any point of space, mm by mm, you can see the star. that's a lot of photons containing energy and an infinite number of degree's/directions for them to travel.


 

Offline chiralSPO

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Re: light from a distant star
« Reply #1 on: 24/10/2014 01:12:19 »
well, none of the stars we can see are infinitely far away. The flux of photons from a star a given distance away is proportional to the output of the star divided by the distance squared.

photons per mm2 per second at the observer = photons emitted by the star per second / (distance between the star and observer in mm)2

Betelgeuse is 650 lightyears away (6.15x1021mm), and puts out 4.15x1031 watts, which, if 20% of that is visible light would be 2.9x1049 photons per second, so where we are there is a photon flux of 770,000 visible photons per second per mm2 coming from Betelgeuse.
 

Offline PmbPhy

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Re: light from a distant star
« Reply #2 on: 24/10/2014 04:43:43 »
do we see distant stars as photons enter our eyes? if so how does a light source create so many? in every direction at any point of space, mm by mm, you can see the star. that's a lot of photons containing energy and an infinite number of degree's/directions for them to travel.
It's even more fascinating than that. We can even see galaxies which are near the edge of the visible universe. Think about how far those photons traveled and how many photons must be generated by the galaxy in order for it to be seen here! Wow!
 

Offline lightarrow

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Re: light from a distant star
« Reply #3 on: 24/10/2014 08:09:36 »
do we see distant stars as photons enter our eyes? if so how does a light source create so many? in every direction at any point of space, mm by mm, you can see the star. that's a lot of photons containing energy and an infinite number of degree's/directions for them to travel.
Because it's not as you think, that the distant star emits a fixed number of little corpuscols spatially localized: every photon is "dispersed" in the entire spherical wave around the star, and increases its dimensions according to the distance. The photon become localized only when it is detected by your retina.
So, whatever the distance, you will always be able to see photons, just that you will have to wait more time to see (at least) one of them.

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Offline barneyboy

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Re: light from a distant star
« Reply #4 on: 25/10/2014 13:59:05 »
thanks for that but what I meant is that, lets say from our star, we can see every mm from every (infinite) direction and every (infinite) angle so every mm on the surface (as we can only see the surface) is emitting umpteen to the umpteenth amount of photons, giving them the energy to move and to power up our, and every other observers optic nerve. and non of the photons from mm 1, travelling in every (infinite) direction, interfere with photons from mm 2 of equal energy travelling in every (infinite) direction.
if it is different photons lighting up our optic nerve then no two people see exactly the same thing as they will have had different photons hitting them and they will have been created at (fractionally) different times and places.
« Last Edit: 25/10/2014 14:02:42 by barneyboy »
 

Offline Bill S

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Re: light from a distant star
« Reply #5 on: 25/10/2014 15:33:48 »
If I understand correctly what you are saying, this applies to everything we see.  When a photon strikes your retina it ceases to exist as a photon.  No photon can be seen twice, or by two observers. 

A few minutes ago my wife and I were both looking at our dog.  We could see him because of the photons bouncing off his coat etc.  Neither of us saw the same photons, but we both saw the same dog.  OK I canít prove that, but itís my story, and Iím sticking to it.  :D
 

Offline lightarrow

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Re: light from a distant star
« Reply #6 on: 25/10/2014 18:15:43 »
So, you can say: "The Dog is in the eye of the beholder"
 :)

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Offline evan_au

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Re: light from a distant star
« Reply #7 on: 25/10/2014 23:38:07 »
Quote
none of the photons from mm2 1, travelling in every direction, interfere with photons from mm2 2 of equal energy travelling in every direction
It is true that in a perfect vacuum, photons will not interfere with each other, and can travel millions of light-years without loss.

However, they can interfere with each other at a detector - for example at your retina, or the CCD of a telescope. The ability to distinguish photons from one part of a star and photons from another part of the star is determined by the resolving power of a telescope: bigger is better! That is why astronomers regularly request budgets to build "The [insert superlative here] Large Telescope".

You can also increase the resolving power by having several small telescopes which are well-separated. This technique is commonly used for radio astronomy, but is harder to apply for optical astronomy, because the positioning tolerances are much tighter.

At this point in time, we only routinely take photos of the surface of the Sun - all other stars are beyond the resolving power of a single telescope, and appear as a point of light. However, there have been attempts to image supergiant stars and planet-forming disks around other stars using speckle interferometry.
« Last Edit: 28/10/2014 08:53:41 by evan_au »
 

Offline barneyboy

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Re: light from a distant star
« Reply #8 on: 31/10/2014 22:24:41 »
why is it that they do not interfere with each other in a vacuum?
do they also not interfere with each other in a non vacuum? ie our atmosphere. allowing individual photons to strike my retina's.
if two photons of equal value and equal make up collide, why would they not join and become twice as energetic. how does a photons energy stay coherent enough to pass through another photon and not allow its energy to be corrupted and yet the instant it strikes my retina it transforms to an energy state that we can process. 
 

Offline dlorde

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Re: light from a distant star
« Reply #9 on: 31/10/2014 23:08:09 »
So, you can say: "The Dog is in the eye of the beholder"
 :)

Exactly - both beauty and the beast are in the eye of the beholder  [8D]
 

Offline jeffreyH

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Re: light from a distant star
« Reply #10 on: 31/10/2014 23:43:33 »
thanks for that but what I meant is that, lets say from our star, we can see every mm from every (infinite) direction and every (infinite) angle so every mm on the surface (as we can only see the surface) is emitting umpteen to the umpteenth amount of photons, giving them the energy to move and to power up our, and every other observers optic nerve. and non of the photons from mm 1, travelling in every (infinite) direction, interfere with photons from mm 2 of equal energy travelling in every (infinite) direction.
if it is different photons lighting up our optic nerve then no two people see exactly the same thing as they will have had different photons hitting them and they will have been created at (fractionally) different times and places.

I understand EXACTLY what you mean. Yes it is a puzzle. It is the same with any type of field. This is where uncertainty is invoked. I dislike the uncertainty principle. Good post.
 

Offline CliffordK

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Re: light from a distant star
« Reply #11 on: 01/11/2014 09:44:27 »
The intensity of radiation decreases with the square of the distance.

So, say you have a star, and observers at 1 unit away, 2 units, 3 units, and 4 units distant.  Then each observer will see (1, 1/4, 1/9, and 1/16) of the intensity of the first observer (independent of the actual units of measurement).  Thus, some of the more distant objects get mighty dim.  Those visible to the "naked eye" are only a relatively small number of the closest stars, mostly in our own Galaxy.

One can increase the chance of picking up a wayward photon coming this way by two methods, either increasing the time of observation, or increasing the collector size (size of the telescope).

As far as the most distant objects viewed...  At least one of the most distant objects was attributed to a single star.  Well, not an ordinary star, but a supernova with the light output essentially equivalent to an entire galaxy.
 

Offline jeffreyH

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Re: light from a distant star
« Reply #12 on: 01/11/2014 14:37:30 »
The intensity of radiation decreases with the square of the distance.

So, say you have a star, and observers at 1 unit away, 2 units, 3 units, and 4 units distant.  Then each observer will see (1, 1/4, 1/9, and 1/16) of the intensity of the first observer (independent of the actual units of measurement).  Thus, some of the more distant objects get mighty dim.  Those visible to the "naked eye" are only a relatively small number of the closest stars, mostly in our own Galaxy.

One can increase the chance of picking up a wayward photon coming this way by two methods, either increasing the time of observation, or increasing the collector size (size of the telescope).

As far as the most distant objects viewed...  At least one of the most distant objects was attributed to a single star.  Well, not an ordinary star, but a supernova with the light output essentially equivalent to an entire galaxy.

At last some clarity and I learned something.
 

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Re: light from a distant star
« Reply #12 on: 01/11/2014 14:37:30 »

 

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