What are the conditions under which stars can radiate coherent light?

[ZX.Liang]

Some papers mention the coherent radiation of stars, such as this one:
https://iopscience.iop.org/article/10.1086/519790/pdf

I want to know under what conditions can a star exhibit coherent radiation?

 

[Baluncore]

I want to know under what conditions can a star exhibit coherent radiation?

I would expect it to be when molecular maser or laser activity was present. That would be when a large volume of diffuse cool gas was pumped by radiation from a nearby star.
https://en.wikipedia.org/wiki/Astrophysical_maser#Known_interstellar_species

 

[Devin-M]

Isn’t all starlight coherent? My understanding is to get destructive interference the light has to be coherent. If you view a star through a double slit and narrowband filter you can observe destructive interference, like these pictures I took...

https://www.physicsforums.com/threads/destructive-interference.1048058/

What’s interesting is I still measured almost exactly twice as many photons with the double slit as the single slit despite the presence of destructive interference (within 1%)…

Devin-M's measurements:
https://u.pcloud.link/publink/show?code=kZcjE2VZR1eBfOCgGAYfuGOv9EMgPH3KIR07
(Images 9205 & 9213)

Single Noise R Avg 39.2
Single Light R Avg 128.8
128.8 - 39.2 = 89.6

Double Noise R Avg 37.1
Double Light R Avg 218.5
218.5-37.1 = 181.4

181.4 / 89.6 = 2.02x higher

 

[Baluncore]

Isn’t all starlight coherent? My understanding is to get destructive interference the light has to be coherent.

The light from a star may appear to be coherent when it arrives here, because it is sparse in time, and comes from a very small angular region of space.

That is where VLBI gets interesting. Is it the photons or the waves that are being correlated to synthesise the image. How can two radio telescopes, one on each side of the Earth, receive the same photon at different times?

 

[ZX.Liang]

I would expect it to be when molecular maser or laser activity was present. That would be when a large volume of diffuse cool gas was pumped by radiation from a nearby star.
https://en.wikipedia.org/wiki/Astrophysical_maser#Known_interstellar_species

thank you.

 

[Quarker]

Isn’t the destructive interference that occurs in every airy disc evidence of coherence? Any half decent telescope can show an airy disc, which is just how the telescope presents the visual image of a star.

 

[Drakkith]

What's interesting here is that stars do NOT emit coherent light. But, by the time the light reaches us here on Earth the distance has become so large and the source so small in apparent size that the light becomes spatially coherent. Note that any thermal source does this. You just need the distance to be great enough to make the source become point-like. This is why the Sun, with an apparent diameter of about half a degree, does not appear to emit coherent light, while distant stars do.

Lasers and masers, however, do emit coherent light and don't require that they appear to be point-like in size.

 

[Quarker]

So to answer the OP, any star outside of a certain radius of the observer will emit coherent light, probably depending on the brightness and size of the star as well.

 

[ZX.Liang]

What's interesting here is that stars do NOT emit coherent light. But, by the time the light reaches us here on Earth the distance has become so large and the source so small in apparent size that the light becomes spatially coherent. Note that any thermal source does this. You just need the distance to be great enough to make the source become point-like. This is why the Sun, with an apparent diameter of about half a degree, does not appear to emit coherent light, while distant stars do.

Lasers and masers, however, do emit coherent light and don't require that they appear to be point-like in size.

One of the characteristics of coherent light is polarization. Will distance cause polarization?

 

[ZX.Liang]

So to answer the OP, any star outside of a certain radius of the observer will emit coherent light, probably depending on the brightness and size of the star as well.

The viewpoint you presented seems different from the one here.
https://iopscience.iop.org/article/10.1086/519790/pdf

 

[Drakkith]

One of the characteristics of coherent light is polarization. Will distance cause polarization?

I think it only matters that the light be the same polarization at the same time. That is, two points on the same wavefront have the same phase and polarization. Even if both the phase and polarization changes randomly over time (because the light is unpolarized) this change should be identical for every point along the same wavefront but not between different wavefronts. Someone correct me if I'm wrong, please.

The viewpoint you presented seems different from the one here.
https://iopscience.iop.org/article/10.1086/519790/pdf

That paper is specifically about radio emissions, not visible light. The coherent radio emissions are generated by a cyclotron effect on electrons, while visible light is generated thermally and is incoherent upon emission.

 

[Baluncore]

A molecular maser has a known wavelength. That peak can be observed against the noise background, and measurements of Doppler shift can be made. It is the emission peak that characterises astronomical maser radiation, not coherency, nor polarisation.

There will not be just one maser operating in a gas cloud. The entire cloud will be continuously radiating, in all directions, with varying phase and polarisation.

If the observed signals were coherent, that coherency would be lost when observing from Earth, as the EM waves must pass through the atmosphere to be observed.

Variations in magnetic fields will result in dispersion of polarisation. It is probable that some of the polarisation observed will be due to the path taken by the EM waves to reach Earth through space, and then the ionosphere, think magnetic rotation, Pockels effect, Kerr cells and Brewster angle.

 

[ZX.Liang]

A molecular maser has a known wavelength. That peak can be observed against the noise background, and measurements of Doppler shift can be made. It is the emission peak that characterises astronomical maser radiation, not coherency, nor polarisation.

There will not be just one maser operating in a gas cloud. The entire cloud will be continuously radiating, in all directions, with varying phase and polarisation.

If the observed signals were coherent, that coherency would be lost when observing from Earth, as the EM waves must pass through the atmosphere to be observed.

Variations in magnetic fields will result in dispersion of polarisation. It is probable that some of the polarisation observed will be due to the path taken by the EM waves to reach Earth through space, and then the ionosphere, think magnetic rotation, Pockels effect, Kerr cells and Brewster angle.

What method do you think can be used to observe the coherent light in this paper?

https://iopscience.iop.org/article/10.1086/519790/pdf

 

[ZX.Liang]

I think it only matters that the light be the same polarization at the same time. That is, two points on the same wavefront have the same phase and polarization. Even if both the phase and polarization changes randomly over time (because the light is unpolarized) this change should be identical for every point along the same wavefront but not between different wavefronts. Someone correct me if I'm wrong, please.That paper is specifically about radio emissions, not visible light. The coherent radio emissions are generated by a cyclotron effect on electrons, while visible light is generated thermally and is incoherent upon emission.

If coherent radio emission is generated by the cyclotron effect on the electron, how does the cyclotron effect make the electron emit coherent radio waves of the same frequency?

I can understand that the cyclotron effect generates light radiation, but I cannot understand the principle of generating coherent radiation.

 

[Vanadium 50]

First. this isn't light. It's radio. Wavelenthes are millions of times larger so the astrophysics is totally different.

Second, you should look up the Hanbury-Brown and Twiss effect. It describes a phenomenon much more like coherent reception than coherent emission.

 

[Baluncore]

What method do you think can be used to observe the coherent light in this paper?

It is circularly polarised microwave radiation at 4.88 and 8.44 GHz.

A crossed dipole at the focus of a parabolic reflector can resolve polarisation of H, V, LC and RC, by combining the two dipole signals.

The CP signals are resolved by introducing a λ/4 delay into one of the channels before summing the signals.

Note that the reflection from a parabolic reflector will reverse the sense of the CP signal. LCP↔RCP; just like a bathroom mirror does to a helix.

 

[DaveE]

A single wavefront can interfere with itself if the path lengths are essentially equal in all respects. Coherence is often described in terms of a coherence length (or time). This refers to the path length difference after the beams are split. The double slit usually works for an incoherent point source because the slits are close together and identical. Starlight is a great point source, there is only one path to get to the slits. What won't work for starlight is a larger interferometer with different path lengths.

So, no, I don't think stars emit coherent light. That doesn't mean you can't see interference effects. Other things like Newton's Rings or thin film interference (like oil on water) will also demonstrate interference effects on incoherent light.

 

[ZX.Liang]

A single wavefront can interfere with itself if the path lengths are essentially equal in all respects. Coherence is often described in terms of a coherence length (or time). This refers to the path length difference after the beams are split. The double slit usually works for an incoherent point source because the slits are close together and identical. Starlight is a great point source, there is only one path to get to the slits. What won't work for starlight is a larger interferometer with different path lengths.

So, no, I don't think stars emit coherent light. That doesn't mean you can see interference effects. Other things like Newton's Rings or thin film interference (like oil on water) will also demonstrate interference effects on incoherent light.

thank you.
I still need to study relevant knowledge well.

 

[ZX.Liang]

It is circularly polarised microwave radiation at 4.88 and 8.44 GHz.

A crossed dipole at the focus of a parabolic reflector can resolve polarisation of H, V, LC and RC, by combining the two dipole signals.

The CP signals are resolved by introducing a λ/4 delay into one of the channels before summing the signals.

Note that the reflection from a parabolic reflector will reverse the sense of the CP signal. LCP↔RCP; just like a bathroom mirror does to a helix.

I want to know how to judge the coherent radiation on the star from the data received by the radio telescope.
Does it mean that if polarized light is received, it can be determined that coherent radiation has appeared on the star?

 

[DaveE]

Does it mean that if polarized light is received, it can be determined that coherent radiation has appeared on the star?

No. Those are different things. Light can be polarized but not coherent, like what you see with polarized sunglasses on or at a 3D movie.

Lasers are usually polarized to make them work better, and to be more useful. The restrictive conditions on achieving laser/maser amplification make it likely that only one sort of light (frequency, polarization, etc.) makes it workout.

I want to know how to judge the coherent radiation on the star from the data received by the radio telescope.

Nope. It's just not coherent radiation. I'm not even sure what coherent radiation means for a constant (CW) wide bandwidth source. You can have maser like amplification from interstellar gas/dust as @Baluncore said.

Anyway, you would need to build some sort if interferometer and measure coherence with different optical path lengths. i.e. measure interference with varying path lengths for a split beam.

 

[Devin-M]

I'm not even sure what coherent radiation means for a constant (CW) wide bandwidth source.

My setup used a narrowband Hydrogen Alpha filter to only let 1 wavelength hit the sensor (656nm).

 

[Ken G]

The problem here is that there are actually three very different meanings of "coherent" that are getting conflated, and it is unfortunate that the term is used in so many different ways, all related to the concepts of "phase" and "interference."

If you consider a single photon, then we can use a wavefunction to describe its behavior. That wavefunction, as mentioned above, will have a "coherence length" (which is infinite for a plane wave of definite frequency, but actual photons have wavefunctions that depend on how they were created, and have a finite coherence length). The coherence length of the wavefunction has to do with the time t it took to create the photon, and then ct is essentially the coherence length. Within that length, the photon wavefunction can interfere with itself, and produce the slit patterns you are talking about. This would be true even if you are only observing a single photon.

A second meaning has to do with radio detection, where you have so many photons that you don't actually detect the photons at all, you detect the classical field they produce. In fact, you think of the creation of the field directly, so you never think of photons at all, you just solve the classical electrodynamics equations. This ends up being very like the wavefunction, because the amplitude of an electromagnetic field has similar properties, including a coherence length. So radio detection amounts to correlating fields at different places in the detector, so you are detecting phase differences due to different propagation lengths, i.e., you are detecting an interference pattern in the field amplitudes, not a photon flux (though you could reconstruct a photon flux if you wanted to, you just have no reason to).

But the meaning in the OP is quite different from either of those, it relates to the photon distribution and its occupation numbers. What is meant by "coherent radiation" here is that the brightness temperature is very high because the occupation numbers in the photon modes you are observing are significantly above unity, i.e., it strongly matters that photons are bosons. These photons are "coherent" because they are all in the same state, so have the same phase not just the same energy.

Indeed, you can associate a thermal spectrum with a brightness temperature by looking at what photon energy do the photon modes have occupation numbers above unity, and associate that photon energy with kT, where T is the brightness temperature, and it will be similar to the local temperature for a blackbody radiation from a stellar photosphere. But lasers pick out special states and put lots of photons into those individual states, driving the brightness temperature way above the local temperature. This is why lasers are good at cutting things-- you can get a lot of energy into the photons because of the constructive interference that comes from adding amplitudes in a coherent way (which is what puts the photons into the same states). In effect, what you have is a source of amplitudes, not a source of photons per se, and when you add the amplitudes coherently (because you are selecting the same photon state), they add up to very large amplitudes, which in turn causes there to be a large number of photons (you get a high brightness temperature and high energy efficiency from a laser).

The way to tell if this is happening in a star is to look at the processes that create the photons. Photons are created by a feedback between how the creation of a photon produces a field that resonates with the process creating the photon. "Coherent" processes mean that the stimulating field is due to photons that already exist, called "stimulated emission", whereas a more typical process is when the stimulating field is from a virtual photon that does not yet exist, called "spontaneous emission." So what is meant by coherent radiation from stars is when you have most of the light you are talking about coming from stimulated rather than spontaneous emission. But stimulated emission is also associated with scattering and absorption, which reduce the brightness temperature, so the trick to getting "coherent" radiation is to maximize the stimulated emission and minimize the scattering and absorption, which requires "population inversion." This means you have a higher population in the upper level of the atomic or molecular transition that is creating the photon, which gives you "amplification" (the "A" in LASER and MASER) because it gives you more stimulated emission than scattering or absorption.

In short, to have coherent emission of the type implied in the OP, you need to "pump" the upper levels of the transitions, so they act like a laser. This can be done by radiation at other wavelengths, as mentioned above.

 

[DaveE]

The way to tell if this is happening in a star is to look at the processes that create the photons. Photons are created by a feedback between how the creation of a photon produces a field that resonates with the process creating the photon. "Coherent" processes mean that the stimulating field is due to photons that already exist, called "stimulated emission", whereas a more typical process is when the stimulating field is from a virtual photon that does not yet exist, called "spontaneous emission." So what is meant by coherent radiation from stars is when you have most of the light you are talking about coming from stimulated rather than spontaneous emission. But stimulated emission is also associated with scattering and absorption, which reduce the brightness temperature, so the trick to getting "coherent" radiation is to maximize the stimulated emission and minimize the scattering and absorption, which requires "population inversion." This means you have a higher population in the upper level of the atomic or molecular transition that is creating the photon, which gives you "amplification" (the "A" in LASER and MASER) because it gives you more stimulated emission than scattering or absorption.

Does this actually happen to a significant degree (i.e. measurable) in any stars?

 

[Ken G]

Does this actually happen to a significant degree (i.e. measurable) in any stars?

Yes, there are "stellar masers", typically requiring special circumstances and generally not at the surface of the star, but rather at a large distance from the star (which is pumped by the light of the star). This is because the density in the photosphere is usually too high, since collisions act to depopulate the upper levels and remove the population inversion, returning the situation to what corresponds to the local temperature. But much farther out the density is much lower, and so collisions don't happen as often, and the light from the star can act to populate quasi stable upper levels in molecular lines, causing a population inversion and masering (often in the microwave regime, which is the "m"). This can sometimes encode special information not otherwise available. A random example is https://www.cfa.harvard.edu/news/hydrogen-masers-reveal-new-secrets-massive-star.

 

[Baluncore]

The closest thing to "coherent light" that I know of, is summed up well here with plenty of references.
https://en.wikipedia.org/wiki/Megamaser

 

[DaveE]

at a large distance from the star (which is pumped by the light of the star).

OK, I knew about those. But, that's not the star, IMO. Still, I suppose it could look like the star to a telescope pointed that way.

generally not at the surface of the star,

"generally...". So has this ever been observed? Can it happen?

 

[hutchphd]

So, no, I don't think stars emit coherent light. That doesn't mean you can't see interference effects. Other things like Newton's Rings or thin film interference (like oil on water) will also demonstrate interference effects on incoherent light.

Some papers mention the coherent radiation of stars, such as this one:
https://iopscience.iop.org/article/10.1086/519790/pdf

I want to know under what conditions can a star exhibit coherent radiation?

I think the term coherence is often defined by the context without really specifying what is meant. Usually within a given subspecialty the "'colloquial" definition is used sometimes unwittingly but usually without repercussion. As a working definition, it often is used to mean that one can measure interference effects temporally or spatially.. That covers a pretty broad range of situations.
For a rainbow, for instance, a very narrow range of incident geometries are selected in a color-specific way. Stars subtend such a small angle that the different possible paths to the detector do not really depend upon the source. See van Cittert–Zernike Theorem as to why ordinary stars rely on their incoherent emission to produce a reliably coherent source. Generally I think one needs to be quite careful to specify the explicit correlations one is interested in. I get very confused very quickly here.

 

[Baluncore]

"generally...". So has this ever been observed? Can it happen?

The Hallinan et al 2007 paper hypothesises, bursts of highly beamed emission, generated by the electron cyclotron maser instability, from compact sources located at the magnetic polar regions.
Is that close enough to qualify as the surface of the star?

If a maser is involved, then coherence is part of the generation process, as is narrow bandwidth. The paper shows plots of power against time, but no spectrum of the emission, so I find it hard to determine if it is maser generated or broadband.

The magnetron in your kitchen microwave oven is not a maser, but is narrowband. Does it produce coherent radiation?

 

[hutchphd]

In your oven it is coupled to a resonant cavity and produces standing waves. The turntable belies the coherence, but I dont know if the Magnetron itself does it.

 

[ZX.Liang]

I think the term coherence is often defined by the context without really specifying what is meant. Usually within a given subspecialty the "'colloquial" definition is used sometimes unwittingly but usually without repercussion. As a working definition, it often is used to mean that one can measure interference effects temporally or spatially.. That covers a pretty broad range of situations.
For a rainbow, for instance, a very narrow range of incident geometries are selected in a color-specific way. Stars subtend such a small angle that the different possible paths to the detector do not really depend upon the source. See van Cittert–Zernike Theorem as to why ordinary stars rely on their incoherent emission to produce a reliably coherent source. Generally I think one needs to be quite careful to specify the explicit correlations one is interested in. I get very confused very quickly here.

Thank you.