54 replies to this topic

[ilovecomets]

The light escapes when the star gets sufficiently transparent, which happens somewhere before the space vacuum, with the free path. If energy wasn't released, it would just heat up some outer layer, which would release it.

Another way to put it is the internal absorption > emit > absorption cycle is a store of potential energy that slowly makes it way to the surface. It’s essentially at equilibrium with the new energy generated at the core balanced with the amount of energy emitted on the surface. As the energy generated at the core increases the surface temperature increases and this is done through conduction and convection internally. Now I say equilibrium because it’s close enough to energy in = energy out but it does fluctuate every second. Averaging it out it mostly stays the same over a longer period of time.

[Keith Rivich]

Interesting discussion. A couple of points from me.
If photons go through so may interactions, why do we still see point sources in the night sky?

Another is the interior of stars. I think I read somewhere in one of my many books years ago that the gas in a stars deep interior is opaque to visable light due to temperature, pressure and ionization.

It's sheer numbers. Many photons do get scattered by interactions though enough get through which are aimed directly at our eyes allowing us to see the star. You don't see the photons that miss your eye.

 

The person next to you sees a slightly different star. 

[csrlice12]

Ok, Stoopid question time.....if everything else absorbs photons and reemits them, why do we say that the ones that hit our eyes are the end point as it's converted to an electrical impulse.....or, are we remitting photon?  If we also reemits photons, is it possible to photograph or measure this?

[Migwan]

My statement in the mentioned thread, that old photon's just taste better, was metaphoric.   

 

Both AP and visual astronomy use dimension within a medium to intensify information.   AP just intensifies that information further by using artificial sensors over time.   Both can use filters to sort out some of that information, which is kinda like processing.   

 

"Photons and true light..."  What is that?     

 

I kinda think that every "photon" is absorbed and emitted is an assumption.   The mass density of the universe, as we know it, is not fixed.  It appears to both vary and to be expanding as a whole.   So how can we assume that every "photon" is absorbed and emitted evenly while crossing the universe?   That, when we allow the speed of light to vary in the lenses of my ST80.   Nuts.

[Keith Rivich]

Ok, Stoopid question time.....if everything else absorbs photons and reemits them, why do we say that the ones that hit our eyes are the end point as it's converted to an electrical impulse.....or, are we remitting photon?  If we also reemits photons, is it possible to photograph or measure this?

Part of the photon (wave) gets reflected back, our lens is not a perfectly transparent optical window, and part of the remaining energy is converted to an electrical signal and the rest gets turned to heat, or an IR photon if you prefer. 

Edited by Keith Rivich, 22 February 2024 - 11:34 AM.

[Tom Barnacle]

Your spectroscopy answer is here, you just have to read and digest.

https://ocw.mit.edu/..._Background.pdf

Thanks for that (quite a lot to digest), but if the photons entering a spectroscope pointing at a particular star are the product of re-radiation from intervening material and not from the star itself then the resulting spectrum would have little correlation with the spectral properties of the star (as the re-radiated photons could be emitted in any direction and not necessarily at the frequency of the original photon) and only contain information relating to the intercepting medium whether that be gas, dust or organic molecules.

 

So the photons that produce the emission lines in the spectrum of a Wolf–Rayet star for instance would have to be emitted by the star itself and not from some other source which had intercepted the photon en-route, otherwise their presence in the spectrum would be meaningless. 

 

The detection of metallic elements/complex molecules in stellar atmospheres would also require that the photons producing those spectral lines originated from those atoms/molecules within the atmosphere and nowhere else and arrived at the instrument without having been absorbed en-route.

 

Cosmological redshift or doppler shifting of spectral lines obviously occurs - but I guess when they show up in the spectrum as a series (such as the Lyman and Balmer series - or any other element series for that matter) then any shifting can be detected as all the lines are shifted by the same amount.

[AstroVPK]

Interesting discussion. A couple of points from me.
If photons go through so may interactions, why do we still see point sources in the night sky?

Another is the interior of stars. I think I read somewhere in one of my many books years ago that the gas in a stars deep interior is opaque to visable light due to temperature, pressure and ionization.

 

That's just geometry. In the absence of interactions, only the photons emitted directly at you should enter your eyes - the rest will never be seen by you. Now imagine an interaction that scatters a photon away the line connecting the source to your eyes. That photon is now lost and will never reach your eyes, dimming the source slightly. Let's say that the photon scatters a second time, and coincidentally, that it happens to scatter back towards your eye. Now you'll see a second source appear, slightly offset from the first source,  but considerably dimmer since only a very small fraction of the original photons will scatter twice in such a way as to end up coming back towards your eyes. Hence most point sources stay point sources. However, if the original source is enveloped in a cloud of dust and gas, e.g. the stars of the Pleiades, then the original source can end up looking non-stellar or stellar+other stuff. Again, it's all dependent on the geometry, opacity, transmissibility, and emissivity of the stuff around and between the original source and us. 

[Phil Cowell]

Every one is absorbed and emitted on its journey out of the star.

My statement in the mentioned thread, that old photon's just taste better, was metaphoric.   

 

Both AP and visual astronomy use dimension within a medium to intensify information.   AP just intensifies that information further by using artificial sensors over time.   Both can use filters to sort out some of that information, which is kinda like processing.   

 

"Photons and true light..."  What is that?     

 

I kinda think that every "photon" is absorbed and emitted is an assumption.   The mass density of the universe, as we know it, is not fixed.  It appears to both vary and to be expanding as a whole.   So how can we assume that every "photon" is absorbed and emitted evenly while crossing the universe?   That, when we allow the speed of light to vary in the lenses of my ST80.   Nuts.

[Inkswitch]

Thanks for that (quite a lot to digest), but if the photons entering a spectroscope pointing at a particular star are the product of re-radiation from intervening material and not from the star itself then the resulting spectrum would have little correlation with the spectral properties of the star (as the re-radiated photons could be emitted in any direction and not necessarily at the frequency of the original photon) and only contain information relating to the intercepting medium whether that be gas, dust or organic molecules.

 

I believe it goes more like this.  There are bunch (understatement) of photons emitted from a star.  There is an intervening "cold" dust cloud.  Most of the photons pass through the cloud without interaction, but enough of them get intercepted that you can detect both the star and the cloud.  The intercepted photons will re-emit at the same frequency (photons come in discreet energy units and the amount going into the absorption must be the same coming out), but in a random direction.  This causes the dark lines seen in the rainbow spectrum and are called "absorption lines" because many of the photons at this frequency were absorbed and re-emited but not toward the detector.  The discreet energy units of photons and the geometry of atoms conspire to cause certain atoms to absorb and re-emit certain frequencies of light.  If the intervening dust cloud was "hot", then you would see the spectrum of the star, plus, some bright lines that correspond to the glow from the hot cloud, these are known as emission lines.

 

This is not to be construed as spectroscopic advice but may be used for general purposes.

[Tom Barnacle]

Basically, when you observe a star your photoreceptors are registering photons that originated in the photosphere of that star - you are seeing starlight. 

[Phil Cowell]

But there are also radio waves, X-Rays, UV, IR all filtered by either the atmosphere or outside the eyes abysmal capture ability at night.

 

Basically, when you observe a star your photoreceptors are registering photons that originated in the photosphere of that star - you are seeing starlight. 

[AstroVPK]

I believe it goes more like this. There are bunch (understatement) of photons emitted from a star. There is an intervening "cold" dust cloud. Most of the photons pass through the cloud without interaction, but enough of them get intercepted that you can detect both the star and the cloud. The intercepted photons will re-emit at the same frequency (photons come in discreet energy units and the amount going into the absorption must be the same coming out), but in a random direction. This causes the dark lines seen in the rainbow spectrum and are called "absorption lines" because many of the photons at this frequency were absorbed and re-emited but not toward the detector. The discreet energy units of photons and the geometry of atoms conspire to cause certain atoms to absorb and re-emit certain frequencies of light. If the intervening dust cloud was "hot", then you would see the spectrum of the star, plus, some bright lines that correspond to the glow from the hot cloud, these are known as emission lines.

This is not to be construed as spectroscopic advice but may be used for general purposes.

They don't have to be emitted at the original energy. There are lots of processes that bleed energy from excited states into other frequencies. Thermalization is the predominant process when the density of the intervening matter is high and acts in the interior of stars (amongst other places) to turn the well defined energies of the gamma ray photons produced by the thermonuclear reactions inside the star into the blackbody spectrum observed by us.

Edited by AstroVPK, 24 February 2024 - 02:55 AM.

[Migwan]

I believe it goes more like this.  There are bunch (understatement) of photons emitted from a star.  There is an intervening "cold" dust cloud.  Most of the photons pass through the cloud without interaction, but enough of them get intercepted that you can detect both the star and the cloud.  

 

  

 

The intercepted photons will re-emit at the same frequency (photons come in discreet energy units and the amount going into the absorption must be the same coming out), but in a random direction.  This causes the dark lines seen in the rainbow spectrum and are called "absorption lines" because many of the photons at this frequency were absorbed and re-emited but not toward the detector.

 

 Wouldn't those absorption lines be present in any direction? 

 

The discreet energy units of photons and the geometry of atoms conspire to cause certain atoms to absorb and re-emit certain frequencies of light.  If the intervening dust cloud was "hot", then you would see the spectrum of the star, plus, some bright lines that correspond to the glow from the hot cloud, these are known as emission lines.

 

This is not to be construed as spectroscopic advice but may be used for general purposes.

Like each sort of atom prefers a certain flavor of energy and spits the rest out.  Or maybe it dislikes that flavor and changes that wavelength into something more to its liking.

Edited by Migwan, 24 February 2024 - 09:04 AM.

[Migwan]

Every one is absorbed and emitted on its journey out of the star.

Not what I was looking at, but how does that work in a plasma?    

[12BH7]

But there are also radio waves, X-Rays, UV, IR all filtered by either the atmosphere or outside the eyes abysmal capture ability at night.

The visual spectrum on only a fraction of the photon wavelength. 

[Tom Barnacle]

Not what I was looking at, but how does that work in a plasma?    

In the radiative zone (talking about the Sun here) temperatures are so high that the gas is a plasma and photons are scattered through collision with other particles on their way out from the core - once they enter the convective zone where the lower temperatures allow some metallic atoms to retain their electrons then absorption/re-emission can take place. Both processes contribute the photons Random Walk outwards from the core.

[Inkswitch]

Like each sort of atom prefers a certain flavor of energy and spits the rest out.  Or maybe it dislikes that flavor and changes that wavelength into something more to its liking.

Because of the discreet energy units of photons and the geometry of atoms, they do not spit out the ones they don't like, they never absorb the ones they don't like.

[Phil Cowell]

I posted how that happens earlier. Have a read.

 

Not what I was looking at, but how does that work in a plasma?    

[Migwan]

Because of the discreet energy units of photons and the geometry of atoms, they do not spit out the ones they don't like, they never absorb the ones they don't like.

OK.  But how then, is that flavor absent in the spectrogram?    Maybe if they weren't called absorption lines.

 

Always hear about discreet energy.  Never  about the additional space that would be within a discreet orbit. 

 

Like "geometry of atoms".

Edited by Migwan, 24 February 2024 - 03:29 PM.

[Inkswitch]

OK.  But how then, is that flavor absent in the spectrogram?    Maybe if they weren't called absorption lines.

 

Always hear about discreet energy.  Never  about the additional space that would be within a discreet orbit. 

 

Like "geometry of atoms".

Full disclosure, I'm pretty far from an expert.

 

That flavor isn't completely absent.  Recall that they get "re-emited" in a random direction, so most of them will not reach the detector by virtue of there being more directions available than the one that leads to the detector.  Some of that flavor will reach the detector but it will appear darker due to less photons arriving than the rest of the spectrum and therefore be visible.  Hope this made some sense.

 

Regarding the "geometry of atoms", the real answer is mathematical and can only be analogized outside of math.  Certain discreet energies, when "passing through" the atom, tend to hit electrons that are in certain configurations.

[PKDfan]

If your real lucky you get to see some/many undisturbed several million year old photons straight from the Andromeda 'nebula' full extent unmolested, right from the horses mouth so to speak, then immediately into your cerebral cortex lighting up a tiny dark spot in your memory and turning it on and thereby crafting a very precious memory and all that from some tiny mischievous intergalactic interlopers.

Gosh!!





Clearest unperturbed Skies

[yuzameh]

Predominantly the photons impacting the retina are the very photons that left the surface of the star.  Some may have gotten "tired" along the way, read up on interstellar extinction, however that kind of mostly ish blocks the blue end, or more properly scatters it away from the line of sight from us to the star, thus artificially reddenning the light, but some of the photons from the star will be the same ones with just a bit less energy, and not enough to change the "colour" as far as the eyeball is concerned.  Now, if you are looking at 3C 273 through a telescope cosmological red shift will mean the photon has lost energy and become reddened compared to when born.  The atmosphere does this to a more marked degree, especially as you get nearer the horizon (looking through not only more air but also continuously denser air than just looking straight up).

 

As for coated lenses in eyepieces, I'm never quite sure what they do, never read up on it, although I very much doubt that they are passive image intensifiers as no wiring exists from them to your optical nerves.

 

On the sideways hand, the light from 3C 273 is not a billion years old.  It will be of some age and some people probably know how to calculate that, because the limit is the speed of light in vacuo, and space isn't empty so the photons are nearly at the speed of late, thus relatively time hasn't passed much at all for them as far as their own reference frame is concerned (time dilation).

 

Something like that anyway, give or take a yard, ish, valid except for where I've got it wrong.

 

Personally there's an aesthetic to the view of something through a bit of glass relative to a screen.  Sometimes after examining an object I'll just stick in a low power eyepiece and enjoy the field of tight crisp points of stars revealed against a blacker looking sky (sky can go a bit not quite black at high powers).  A screen can no more emulate this than a photograph of a silver or gold object glistening or looking at a torch bulb directly, they just don't have the logarithmic range of t'eyeball.

[yuzameh]

If your real lucky you get to see some/many undisturbed several million year old photons straight from the Andromeda 'nebula' full extent unmolested, right from the horses mouth so to speak, then immediately into your cerebral cortex lighting up a tiny dark spot in your memory and turning it on and thereby crafting a very precious memory and all that from some tiny mischievous intergalactic interlopers.

Gosh!!





Clearest unperturbed Skies

Nope, there is no absolute time and space so the photons are not around million years old photons, they are actually quite young (ignoring the time before they reached the photosphere of their source star/object and only counting the post-photosphere time).  We may be seeing the Andromeda Galaxy as it was millions of years ago but the photons aren't that old.

[Keith Rivich]

Predominantly the photons impacting the retina are the very photons that left the surface of the star.  Some may have gotten "tired" along the way, read up on interstellar extinction, however that kind of mostly ish blocks the blue end, or more properly scatters it away from the line of sight from us to the star, thus artificially reddenning the light, but some of the photons from the star will be the same ones with just a bit less energy, and not enough to change the "colour" as far as the eyeball is concerned.  Now, if you are looking at 3C 273 through a telescope cosmological red shift will mean the photon has lost energy and become reddened compared to when born.  The atmosphere does this to a more marked degree, especially as you get nearer the horizon (looking through not only more air but also continuously denser air than just looking straight up).

 

As for coated lenses in eyepieces, I'm never quite sure what they do, never read up on it, although I very much doubt that they are passive image intensifiers as no wiring exists from them to your optical nerves.

 

On the sideways hand, the light from 3C 273 is not a billion years old.  It will be of some age and some people probably know how to calculate that, because the limit is the speed of light in vacuo, and space isn't empty so the photons are nearly at the speed of late, thus relatively time hasn't passed much at all for them as far as their own reference frame is concerned (time dilation).

 

Something like that anyway, give or take a yard, ish, valid except for where I've got it wrong.

 

Personally there's an aesthetic to the view of something through a bit of glass relative to a screen.  Sometimes after examining an object I'll just stick in a low power eyepiece and enjoy the field of tight crisp points of stars revealed against a blacker looking sky (sky can go a bit not quite black at high powers).  A screen can no more emulate this than a photograph of a silver or gold object glistening or looking at a torch bulb directly, they just don't have the logarithmic range of t'eyeball.

Hopefully we aren't invoking the "tired light theory" here. It was debunked long ago...

[Tom Barnacle]

If your real lucky you get to see some/many undisturbed several million year old photons straight from the Andromeda 'nebula' full extent unmolested, right from the horses mouth so to speak, then immediately into your cerebral cortex lighting up a tiny dark spot in your memory and turning it on and thereby crafting a very precious memory and all that from some tiny mischievous intergalactic interlopers.







 

Not only that, but (according to Quantum Entanglement) if some of those photons were created as entangled pairs, then when they strike the photoreceptors in your eyes, and their wave function collapses then the wave function of their entangled partners will also collapse – and these partners could have left Andromeda heading in the opposite direction to Earth. So you registering photons from Andromeda with your retina is instantly having an effect on other photons potentially 4 million light years away.

 

Puts the phrase 'at one with the universe' in perspective.

 

Travelling at the speed of light (in a vacuum) massless particles such as photons do not experience time - the result of time dilation in Special Relativity. But to a stationary observer (us) it still takes light from Andromeda 2 million years to get here. 

Edited by Tom Barnacle, 25 February 2024 - 12:08 PM.