Why does Sirius A blink in many colors?

[parshyaa]

When you look at Sirius, it blinks in white/blue/red and even green colors. Why does this happen?
Please answer it with details(as my researched gave me answers like these: it is closer to horizon or closer to Earth's atmosphere,it is brighter , but these didn't helped me, i need a complete detailed answer) thanks in advance.

 

[Jonathan Scott]

This is what is meant by "twinkling". Search for "Why do stars twinkle?" for more information. Sirius is bright enough for this to be very visible.

The effect is primarily due to the fact that the image of a star is so small it cannot be resolved, so all light from it comes via the same path, and is easily affected by atmospheric distortions. In comparison, the light from planets is typically much steadier, as the image is not quite a point and light coming via different paths tends to average out more smoothly.

 

[parshyaa]

Basically this happens due to atmospheric refraction and to every stars but Sirius is the brightest one therefore it is easily observable

 

[Vanadium 50]

Are you answering your own question?

 

[parshyaa]

Are you answering your own question?

No i was just confirming, sorry for not writing it in reply

 

[rootone]

Yes the 'twinkling' effect is entirely down to the light passing through Earth's atmosphere

 

[Chronos]

Sirius is very hot. As spectral type a1 it has twice the surface temperature of the sun and is 25 times more luminous. It naturally emits more light in the blue end of the spectrum than most other stars.

 

[Ken G]

What is still missing is the explanation. Yes it is because of atmospheric motions and density variations, we know that. Yes it is because of refraction. Yes the stellar image has to be small enough and bright enough to see it. But what remains to be answered is: why does it happen?

 

[russ_watters]

What is still missing is the explanation. Yes it is because of atmospheric motions and density variations, we know that. Yes it is because of refraction. Yes the stellar image has to be small enough and bright enough to see it. But what remains to be answered is: why does it happen?

Those were all reasons why it happens. Why doesn't that satisfy you?

 

[Ken G]

Because I know all those reasons, and I still can't explain why it blinks different colors. The explanation must go further, at the very least it must explain why the intensity changes are different for different colors. I can think of several possibilities just off hand, I'm wondering what the right answer is. For example, I can imagine it might be like a focusing effect of refraction, but lenses show relatively small astigmatism. Perhaps that's because they're built to minimize astigmatism, so is that the reason? Or, I can think it might be something biological happening in the eye, about how the eye interprets brightness variations, like how you see colors in darkness after a light goes out. So we can ask, would a spectrometer measure color variations when looking at Sirius, or just an eyeball?

 

[rootone]

I am pretty sure that the perceived twinkling is a real phenomenon and not an artifact of human vision, though I don't know if anyone actually tested that.

 

[jim mcnamara]

@Ken G - here is an answer. https://www.scientificamerican.com/article/why-do-stars-twinkle/

John A. Graham, an astronomer with the Carnegie Institution of Washington, explains.
Have you ever noticed how a coin at the bottom of a swimming pool seems to wobble from side to side? This phenomenon occurs because the water in the pool bends the path of light from the coin. Similarly, stars twinkle because their light has to pass through several miles of Earth's atmosphere before it reaches the eye of an observer. It is as if we are looking up at the universe from the bottom of a swimming pool. Our atmosphere is very turbulent, with streams and eddies forming, churning around and dispersing all the time. These disturbances act like lenses and prisms that shift the incoming light from a star from side to side by minute amounts several times a second. For large objects like the moon, these deviations average out. (Through a telescope with high magnification, however, we see shimmering images.) Stars, in contrast, are so far away that they effectively act as point sources, and the light we see flickers in intensity as the incoming beams bend rapidly from side to side. Planets like ForumsMars, Venus and Jupiter, which appear to us as bright stars, are much closer to Earth and look like measurable discs through a telescope. Again, the twinkling from adjacent areas of the disc averages out, and we see little variation in the total light emanating from the planet.

In outer space, where there is no atmosphere, stars do not twinkle. This is why the Hubble Space Telescope can produce the brilliant and crisp images of the universe that we have come to know. At our Earthbound observatories, we are learning how to compensate for the twinkling effect by adapting the optics of our large telescopes as fast as it occurs. As a result, we should soon be able to produce much sharper images from here on the ground.

Answer originally posted on August 5, 2002.

Which is really what @Jonathan Scott said more succinctly.

 

[Ken G]

Actually, that's not an answer, it doesn't say a single thing about color changes. Even the analogy they use is not associated with any color changes at all!

 

[parshyaa]

Because I know all those reasons, and I still can't explain why it blinks different colors. The explanation must go further, at the very least it must explain why the intensity changes are different for different colors. I can think of several possibilities just off hand, I'm wondering what the right answer is. For example, I can imagine it might be like a focusing effect of refraction, but lenses show relatively small astigmatism. Perhaps that's because they're built to minimize astigmatism, so is that the reason? Or, I can think it might be something biological happening in the eye, about how the eye interprets brightness variations, like how you see colors in darkness after a light goes out. So we can ask, would a spectrometer measure color variations when looking at Sirius, or just an eyeball?

I think Change in colour can also be explained by atmospheric refraction, when white light passes through prism it scatters into 7 different colours and this is due to Refraction just like this when light passes through rarer to denser medium(Atmosphere) or vice versa, it refracts as well as scatters into different colours

 

[Ken G]

What's more, most stars don't show color changes, they just show intensity changes, perhaps even a sense of jittering around in location. But they stay white. It's only a few stars, perhaps the brightest ones, that really go through all the colors, and very clearly so. So is this because the colors are really varying in intensity, and our eyes only detect it for the brighter stars, or is it something about how our eyes are responding to that level of brightness that produces an illusion of color change? If refraction is all that is going on, one might expect the refraction at different wavelengths to show much smaller variance than the refraction average, as happens for a prism or any kind of lens. So how could the lensing be so much different for different wavelengths, it would be like putting something under a magnifying glass and only having one color get magnified.

 

[Ken G]

I think Change in colour can also be explained by atmospheric refraction, when white light passes through prism it scatters into 7 different colours and this is due to Refraction just like this when light passes through rarer to denser medium(Atmosphere) or vice versa, it refracts as well as scatters into different colours

Sure, but when you look at an unresolved picture of a rainbow, you're going to see something white, not something flickering different colors. So that's why it is hard for refraction to do it, though I don't know if it is the key or not. You also mention scattering, so that's the issue right there-- we don't even know if the effect is due to scattering or refraction! Given that, how can we claim the question has been answered? All I'm hearing is, "the air does it somehow, or maybe the eye does, but I think it's the air." That's not much of an answer.

 

[parshyaa]

So is this because the colors are really varying in intensity, and our eyes only detect it for the brighter stars,

Yes i think this happens to every stars but sirius is the brightest one therefore it is easily observable

 

[Ken G]

Maybe that's true, but do we know it's true? And is there an explanation here anywhere? I think your question was a good one, and has still gone unanswered. For one thing, I still don't know the answer. What bothers me about it is that the explanations don't seem to even realize they aren't explaining it, beyond what we've seen already-- which says nothing about why the colors seem to change. When I stand in front of an oddly shaped mirror, I expect my image to distort in shape and total intensity, but I do not expect to see one color. Lenses have more aberration than mirrors, but still, to see one color, that's a heck of a lot of aberration. I suspect there has to be a lot of how the eye works in the explanation there. If not, it might be due to what is known as a "caustic", where you get a kind of perfect storm of refraction that gives a big magnification that might only be achieved at one color, and you only see colored caustics with refraction, because reflection has almost no aberration. and the caustics I've seen were either from reflection or from gravitational lensing (neither of which exhibits chromatic aberration).

 

[Ken G]

Googling caustics gives me images like this:
https://en.wikipedia.org/wiki/Caustic_(optics)#/media/File:Kaustik.jpg
but nothing about color changes. You can see the caustics in that image are white, yet they are from refraction. Maybe variations in surface shape give less aberration than variations in density, but I'm still not seeing how the color effects are being explained here. Maybe the answer is that different wavelengths tend to pick out density variations on the scale of that wavelength, so the different scales of variations might produce different (and time varying) caustic patterns. That might at least explain it, but I still can't rule out that intensity variations at a given level could by itself appear to the eye like color changes.

 

[Drakkith]

I'd guess that the effect is being magnified by the small diameter of the light beam entering your eye and the long distance between your eye and the region of the atmosphere undergoing most of the turbulence. At a distance of a foot or so you won't see much of a rainbow with a glass of water because the different colors can't separate much in such a short distance.

When I stand in front of an oddly shaped mirror, I expect my image to distort in shape and total intensity, but I do not expect to see one color. Lenses have more aberration than mirrors, but still, to see one color, that's a heck of a lot of aberration.

You won't see any chromatic aberration with mirrors, as they have no dispersion.

 

[andrew s 1905]

To see colour changes the twinkling star must be bright enough to engage the cones in the eye (i.e. the colour receptors) if not only the rods respond and you only see intensity variations not colour.

Regards Andrew

 

[Ken G]

Yes, I agree that chromatic aberration could be very different with refraction. But note that aberration by itself is not enough, because we are not talking about separating the colors like a rainbow does. If you look at a rainbow with insufficient spatial resolution, it doesn't look like one color, it looks white. We are talking about a "rainbow" that flickers between different colors, not one where the different colors appear along different lines of sight. That's the problem, that's why an explanation is missing. All I can think of is, it's either an illusion of the eye that a spectrometer would not get, or it is a remarkable effect of colored caustics.

 

[Ken G]

To see colour changes the twinkling star must be bright enough to engage the cones in the eye (i.e. the colour receptors) if not only the rods respond and you only see intensity variations not colour.

Regards Andrew

Agreed-- but that only explains why you don't see the color changes in dimmer stars. It does not explain why one does not see color changes in stars that clearly do look yellow, so the cones are being engaged.

 

[andrew s 1905]

Agreed-- but that only explains why you don't see the color changes in dimmer stars. It does not explain why one does not see color changes in stars that clearly do look yellow, so the cones are being engaged.

It is a combination of the factors already discussed I propose you put some effort into working it out yourself

Regards Andrew.

 

[Ken G]

In other words, you want me to answer the question myself? I mean, of course it's a combination of the factors that have been mentioned so far, because we've talked about the fact that there is an atmosphere, and there are eyes involved. So yes, it's a combination of air and eyes.

 

[Ken G]

Many sites suggest that chromatic aberration is indeed the culprit, meaning that when stars are low to the horizon, there is so much refraction that different colors can be refracted not only to different image locations, but also to receive different degrees of lensing. So by chance, one color follows a path that magnifies the image, while another follows a path that diminishes it. This is indeed essentially colored caustics. This means that if a yellow star like Capella were to be seen low to the horizon, it would only flicker red and yellow, without the blue. I've never seen Capella low to the horizon, but this is the prediction that would be made by this explanation. Also, it requires that dimmer stars don't engage the color receptors, even though a spectrometer would see the color changes. Finally, there are implications of this explanation for the art of adaptive optics, which can keep the image stable but cannot keep it from scintillating its colors. This also means adaptive optics cannot maintain a fixed brightness, so it really can't correct for twinkling, but it can keep the image sharp, it just has to average the intensities over an integration time longer than the twinkling variations.

 

[Drakkith]

Agreed-- but that only explains why you don't see the color changes in dimmer stars. It does not explain why one does not see color changes in stars that clearly do look yellow, so the cones are being engaged.

Not sure. Perhaps because dispersion is much less prominent between yellow and red vs blue and red?

Many sites suggest that chromatic aberration is indeed the culprit, meaning that when stars are low to the horizon, there is so much refraction that different colors can be refracted not only to different image locations, but also to receive different degrees of lensing. So by chance, one color follows a path that magnifies the image, while another follows a path that diminishes it.

That makes sense to me.

 

[Ken G]

It sounds like when bright stars are near the zenith, there is only enough refraction to variably lens the entire image, without enough chromatic aberration to separate the colors enough to probe different lensing paths. But near the horizon, there is not only the variable lensing, there is also enough separation of the colors that they traverse different enough optical paths that they experience independently varying degrees of magnification. If so, then it's a real effect, happening to all stars near the horizon but we only see it for the brightest ones.

 

[andrew s 1905]

It sounds like when bright stars are near the zenith, there is only enough refraction to variably lens the entire image, without enough chromatic aberration to separate the colors enough to probe different lensing paths. But near the horizon, there is not only the variable lensing, there is also enough separation of the colors that they traverse different enough optical paths that they experience independently varying degrees of magnification. If so, then it's a real effect, happening to all stars near the horizon but we only see it for the brightest ones.

Sorry if I was too abrupt yesterday Ken. I think you have it down well. For me there are 3 key components.

1) The chromatic effect of a steady unperturbed atmosphere. This goes from zero at the local zenith to a maximum at the horizon. Under steady seeing this makes the star at the horizon look like small rainbows and has to be accounted for when doing spectroscopy.

2) The effect of turbulence in the atmosphere. This both causes apparent motion of the star and adds additional chromatic aberration as the changes in air density cause wavelength dependent shifts in the wave front. Given the entrance pupil of the eye is small compared to the size of most density fluctuations the fluctuations cause shits in the image. (In telescopes it shifts from rapid motion of the image to a general blurring of the star at about 300mm diameter.)

3) The response of the eye. Only bright white stars have enough illumination across the main part of the visual range to show coloured twinkling. If the star look normal yellow or red it is because they don't generally have enough energy in the other wave bands to trigger your colour vision so they remain yellow or red. This is enhanced but the blue being preferentially scattered in the atmosphere.

I am sure the brain has an effect as well but I don't know anything about that aspect.

Regards Andrew

 

[Ken G]

Sorry if I was too abrupt yesterday Ken. I think you have it down well. For me there are 3 key components.

1) The chromatic effect of a steady unperturbed atmosphere. This goes from zero at the local zenith to a maximum at the horizon. Under steady seeing this makes the star at the horizon look like small rainbows and has to be accounted for when doing spectroscopy.

Right, and for our eyes that don't have the spatial resolution, we wouldn't see this at all, and the stars would not vary in their appearance.

2) The effect of turbulence in the atmosphere. This both causes apparent motion of the star and adds additional chromatic aberration as the changes in air density cause wavelength dependent shifts in the wave front. Given the entrance pupil of the eye is small compared to the size of most density fluctuations the fluctuations cause shits in the image. (In telescopes it shifts from rapid motion of the image to a general blurring of the star at about 300mm diameter.)

Right, this would cause some flittering about of the image, and if we also include variable magnification and de-magnification, it would cause intensity changes too, though still no differences for different colors as our eyes cannot resolve the degree of chromatic aberration.

3) The response of the eye. Only bright white stars have enough illumination across the main part of the visual range to show coloured twinkling. If the star look normal yellow or red it is because they don't generally have enough energy in the other wave bands to trigger your colour vision so they remain yellow or red. This is enhanced but the blue being preferentially scattered in the atmosphere.

Yes, so this gives the consistent colors we see, but not changes in color.

So to get the changes in color as the star twinkles, we have to add a fourth key point, and this is for me what was missing in the previous answers. There has to be enough refraction and chromatic aberration such that the different colored images are so separated from each other that they sample independent optical paths, meaning that the degree of random magnification or de-magnification they receive has to be independent of the other colors. Our eyes never resolve the different colored images, instead we average them all together but they are independent intensities due to the variable lensing effects. Then, if bright enough for our eyes to detect it, the star will twinkle whatever color happens by chance to receive the largest magnification at that moment. So the variable independent lensing effects are crucial, not just that there be refraction and chromatic aberration. It still surprises me that the lensing effects could be so variable as to produce colored twinkling, like a rainbow where suddenly the blue part grows much wider, and then shrinks and the red part grows wider, but it's hard to argue with what we see.

 

[sophiecentaur]

Googling caustics gives me images like this:
https://en.wikipedia.org/wiki/Caustic_(optics)#/media/File:Kaustik.jpg
but nothing about color changes

The shape of the caustic curve depends on wavelength. The light from Sirius contains wavelengths right across the visual range so the caustic effect is working on a bigger range of wavelengths (a whole octave). Where the equivalent effect on a cooler star will be mostly on the Rs and Gs - producing a range of reds and yellows, mainly, the effect on the light from Sirius can be to eliminate the Rs and just leave the Gs and Bs - not a usual thing to see and remarkable to the eye. But remember our very idiosyncratic perception of colour. It's hard to predict or explain accurately our subjective appreciation of such a relatively rare object under unusual (dark) conditions. A camera with high IR sensitivity could perhaps 'see' more significant 'colour' changes for a cool star.

 

[andrew s 1905]

There has to be enough refraction and chromatic aberration such that the different colored images are so separated from each other that they sample independent optical paths,

Yes I agree I was assuming in points 1 and 2 and should have made it explicit. Andrew