Page 1 of 1


Posted: Thu Mar 22, 2012 12:56 pm
by Tim Chamberlain
"Twinkle, twinkle little star ..." how I wonder why you do that?

I've read that the reason Sirius twinkles is simply due to turbulence in our atmosphere, but can anyone tell me why it seems to twinkle in flashes of white, red and green?

Thanks, Tim

Posted: Thu Mar 22, 2012 2:43 pm
by David Frydman
Dear Tim.

Sirius is low in the U.K. It is bright. It is a point source. The colour is normally whitish.
The atmosphere continually moving here through several air masses because of low elevation acts like a crude rapidly variable lens or prism.
Anyway even if steady there would still be atmospheric refraction causing colour.
Horace Dall designed and made a compensating variable prism to counter this, and recently a similar device is commercially available I think from the U.S.
Basically Sirius light is spread out into a rainbow image. In windy or rapidly changing air this spectrum is constantly altered and moved.

Other stars do exactly the same but because they are fainter the eye does not pick up the colours so readily.

I have seen Sirius in Helsinki very low but absolutely steady. I have never seen such steady Seeing in the U.K.

I have always thought that Capella in violent Seeing changes when near the North horizon, where it flashes several times a second can actually momentarily be brighter than in a dark location.

I also suspect these violent changes can momentarily bring faint stars to be detectable when they could not be seen in steady Seeing.
The eye can pick up quite rapid changes and glimpse a faint star.

Another way to see star colours better is to defocus them.

In my case if I don't wear glasses and the stars are expanded discs I see the star's colour much more readily.
The same happens if you deliberately slightly defocus a binocular.

There are photos by Finnish photographers and others where in poor Seeing Sirius has been allowed to trail with a fixed camera and the resulting very long line of different colours is very nice to see.
The photo I remember has many horizontal parallel lines where the random colour pattern is shown well.

I gave up within a minute observing the Sun in the PST this morning as the Seeing was very poor.
Sirius in these conditions would twinkle crazily.

Regards, David

Posted: Thu Mar 22, 2012 9:58 pm
by Tim Chamberlain
Many thanks, David. Fascinating stuff!

So it's just a combination of height and atmosphere causing the prism-like separation effect? Is this the cause of all apparent colours in stars?

I wondered if with some of them it might be due to what their chemical make-up might be, or the stage of their life cycle? Hence Betelgeuse being orange, other stars being white, and so on?

Best regards,


Posted: Thu Mar 22, 2012 10:23 pm
by David Frydman
Dear Tim,

No. The main colours in stars are due to their colour temperature or make up.
Indeed Betelgeuse is orangy. Stars have many hues. There are some red stars.
But all these hues are really quite subtle and some people don't see them well.
Because I had to accurately match plain carpet colours in the days of 27 inch wide Wilton carpet that was hand sewn together to make a carpet, I had to develop a keen colour sense. In fact it is almost impossible to match different batches of carpet. So each batch had a special number.
It may be one can estimate 100,000 or even a million different shades.

But if you look at aircraft navigation lights or traffic lights you can immediately notice how red and green these are compared to star colours.
If a star is dim then it becomes colourless to the eye, so the intensity is a big factor.
They say Mars is red but just compare it to a nearby red aircraft navigation light of the same intensity or magnitude and it is not red at all.

With many double stars people see big colour differences. Some of this is real, but a lot of it is due to the eye comparing the two. in isolation the colours are quite different.
So the eyes colour estimates are really not very accurate.

To make faithful colour estimates really the stars need to be at the zenith where you are looking straight through one atmosphere. I suppose you would have to be in a deck chair not to get false readings from neck strain.
Also some people have various types of colour blindness I think men more than women.

In Finland where the air was exceptionally clean Mercury was nearly always white even though it is never high up.
U.K.astronomers nearly all describe Mercury as pink, as indeed I see it here.
It is only because the air here has so many impurities. In pristine skies Mercury is more or less white not pink.

So with stars it is their make up causing their true colours but our atmosphere in the last say 20 miles the light travels before reaching us that changes these colours.

Perhaps someone else has views as this is a bit rambling.

Regards, David

Posted: Thu Mar 22, 2012 10:50 pm
by Tim Chamberlain
Dear David,

That's very interesting. I've long pondered this one. I did notice some real differentials in colour when looking at some double stars last summer quite high up. And I've read about the apparent colours of certain stars in some books, but I've never really grasped what caused this. So it's interesting to know it is a combination of both composition and atmospheric distortion.

Mars looks a kind of light burnt sienna or milky coffee colour to me. Jupiter is always very white in my bins.

I went to a BSIA star-gazing event in Regents Park last night. It was a lovely clear night and there were plenty of different telescopes to look through. Finally managed to see the rings of Saturn properly, and even managed to see the Cassini division. Also saw one of Mars's polar ice caps. And think I might have managed to catch a glimpse of a very faint Comet Garradd with my 8x25 bins. Hoping to get another clear night soon to see if I can repeat this catch!

All the best,


Posted: Thu Mar 22, 2012 11:03 pm
by David Frydman
Well done Tim,
Sounds like a nice evenings entertainment.
It is good to be able to go to an event where telescopes are set up and you can just look through them.

The additional things re. star colours are composition, atmospheric effects, but also the individuals eye's response to colour and with double stars the contrasting colours of the two stars and the actual angular separation. I am pretty sure that with different separation there would be different apparent colour hues. So I presume if you have large changes in magnification you may see different apparent colours with the same double star.
And as I said earlier as soon as you slightly defocus the image the colour leaps out at you.

Best wishes,

Posted: Thu Mar 22, 2012 11:22 pm
by Tim Chamberlain
Oh I see! Yes, of course.

I remember a colour dot optical trick with a blue and a red dot spaced a certain distance apart and you are told to stare at the blue one and after a while the red one seems to vanish! Likewise, I sometimes get an effect where red writing on a black background actually appears to be hovering over the black. But I could never do those "magic eye" picture things which were popular years ago, always hurt my eyes too much.

I do find my eyes get tired sometimes, which is probably connected to being a specs wearer, one eye being slightly more myopic than the other. Staring at a computer screen at work all day doesn't help either (best to take regular breaks if possible), not good when one wants to do some observing in the evening either.

Best wishes,


Posted: Fri Mar 23, 2012 8:55 am
by David Frydman
Dear Tim,
The hovering red on black thing, which I see as stereoscopic images is due to the chromatic aberration of the eye.
It happens with other colours also to a lesser extent.
Some adverts rely on this.

The chromatic aberration of the eye also causes colour shifts.
False colour in telescopes and eyepieces is another problem.

Soviet and Russian also other east european binoculars usually impart a slight yellow cast.

The coatings on an optical instrument changes the colour also.

I have recently had a very well worn binocular renovated. The previous owner rubbed all the coating off the front of one O.G.
There is a very noticeable colour difference now between the two barrels.

One of my eyes used to see grass much greener than the other.
And so it goes on.

I can do those dot stereo images although it took me a while to get the hang of it.

Best wishes, David

Posted: Sat Mar 24, 2012 12:14 am
by Tim Chamberlain
I hadn't thought of coatings on lenses too. Seems obvious now you mention it!

I've never noticed any colour difference between my own eyes, but then I've never really looked for it - I'll have to give it a go on different colours. Right now the things I can see indoors all look the same, but then the yellowish bulbs indoors are themselves are probably contributing towards the shades of colours perceived.

When I lived in Japan I found the majority of lighting there (which tended to be strong bright white fluroescent) too harsh the whole time, but then when I returned to the UK I realised that actually I had got somewhat accustomed to it as I then found the lighting here too dim and very yellow.

Posted: Sat Mar 24, 2012 9:05 am
by dazcaz
Sadly, being colour blind, I rarely get to see any colours of our astronomical wonders. Mars looks a different colour to other "stars" but It's never red.
Same with Betelguese.
I hear people talk of the different colours but I rarely see them.
I can see the colours of the banding on Jupiter, but apparently, I don't see it's full glory. I certainly never see it like I see it in photographs.

I'm OK with larger objects, it's the points of light I have most difficulty with.

So if you are looking at Sirius and seeing it change colour, be thankful. I only see it is as a bright white dot :(

Posted: Sat Mar 24, 2012 9:54 am
by David Frydman
If you see it as a bright white dot you probably have good eyes.
I think most people see it as a flared image brighter in the centre with various extensions depending on astigmatism and other errors in the eye or eyes.

The colour temperature and type of lighting is crucial to the colours people see.
That is why choosing clothes, carpets or anything else is best done in daylight. But daylight colour temperature also varies greatly, and film and movie makers have to check these.
There are special monitors which are colour calibrated.
People with poor sight are often advised to get special bluish bulbs.
Flashguns also vary in colour but are normal biased towards daylight.
On lightbulbs the colour temperature is marked on the box or bulb.
But the spectrum of different lightbulbs is different.
Streetlighting is notoriously bad and variable.

Regards, David

Posted: Mon Mar 26, 2012 8:42 pm
by Tim Chamberlain
After observing Sirius last Wednesday my friend was inspired to photograph a marvellously colouful star trail of it. We then got to discussing the visual phenomenon on another Astronomy forum and someone mentioned the Bayer Matrix or Bayer Filter, which I'd not heard of before.

Posted: Mon Mar 26, 2012 8:51 pm
by David Frydman
Dear Tim,
The photo trail your friend took is similar to the Finnish photo, except he let it go on for ages moving the camera vertically to get numerous parallel lines.

I know the Bayer matrix in regard to the camera sensors, how does it relate to the Sirius photo without my reading the whole of your reference

Venus, the Moon, Jupiter beautiful this evening.

Regards, David

Posted: Mon Mar 26, 2012 10:30 pm
by Tim Chamberlain
Hello David,

Yes, the Venus, Moon and Jupiter group were gorgeous this evening. Caught a lovely view of them in the twilight on my way home in a beautifully vignetted dark blue sky.

Sorry - I should have gone into a bit more detail. I just meant that I'd not heard of the Bayer Matrix until it was mentioned in the other discussion with reference to how digital cameras are designed and how they function, and then looking it up on the link I posted I saw mention that the Bayer Matrix filter in such cameras was meant to mimic the physiology of the human eye. A bit of a by-road to the main discussion above, but I thought it interesting in the same/similar context.

Best wishes,