The colours of Albireo

[rstevenson]

In the latest Recent Submissions thread was posted this lovely image...

Albireo: Double Star in Cygnus
Copyright: Rich Hammar

I was intrigued by the colours of the two stars so I blew it up to have a look, and noticed something which seems odd -- to me at least. The reddish looking star is displaying colours ranging from pink to purple to brown, with a general tone of what I would call bronze. Here's the blowup...

I've added some colour patches picked out of the image with my colour picker tool and dumped into small rectangles so you can see the colours better. All the patches in the top-right were picked from the spikes of the reddish star. The bottom ones are from the blue star and they run the gamut all the way from blue to, uh, blue.

I'm wondering about the evident bronze tones in the red star and whether these are "real" colours or simply the result of applying particular processing to the image.

Rob

[owlice]

Rob, I'll email the submitter.

Lovely image, isn't it? It was a pleasure to post!

[Ann]

Rob just sent me a PM about Albireo, and I wrote this as part of my response to him:

When I look at Rich Hammar's picture, I think that the colors of the two components of Albireo look very "true". When I looked at Albireo through a telescope, it looked just like this. I had expected something very colorful, and I was so disappointed at Albireo! I had seen stars that were much yellower than the primary of Albireo, and I had seen stars that were considerably bluer than the secondary of Albireo.

The magnitude of the primary of Albireo is 3.2. Its B-V index is 1.088 ± 0.002. Since Albireo is a K3II giant, with an absolute luminosity of around 900 times solar, a color index of around 1.1 is almost "unacceptably pale". Compare the color index of the primary of Albireo with the color index of Arcturus. Arcturus is a K2III giant, meaning that it is a little warmer than the primary of Albireo, which ought to make it a little bluer (or rather, a little less yellow) than the primary of Albireo. So if Albireo has a color index of around 1.1, then Arcturus, a slightly warmer star, should have a color index of around 1.0. But that is not the case. Arcturus has a color index of 1.239 ± 0.02.

Is it Albireo that is "too pale", or is it Arcturus that is "too yellow"? It is Albireo that is "wrong". Compare Albireo with Pollux, a rather small giant of spectral class K0III. Pollux has a color index of 0.991 ± 0.005. That means that the difference in color index between a giant of spectral class K0III and a giant of spectral class K3II is only about 0.1! That's far too little. But there is nothing wrong with the color of Pollux.

There is one more thing to keep in mind here. The bigger the star, the redder it will be. This is true of stars of all spectral classes. An O-type supergiant of, say, spectral class O9 will be redder (or less blue) than a main sequence star of spectral type O9. An F-type supergiant will be redder than an F-type main sequence star of "the same temperature". A K-type large giant should definitely be redder than a much smaller K-type giant "of the same temperature". Consider Pollux and Albireo. Pollux is about 30 times brighter than the Sun. Albireo is about 900 times brighter than the Sun, making it about 30 times brighter than Pollux. Albireo is definitely cooler than Pollux. Yet Albireo is barely yellower than Pollux!

Well, there is a reason for all this. Bright Star catalog says that the primary component of Albireo is itself double. I'm not talking about the visual blue component, the one that everyone can see through a telescope. I'm talking about a component in a very tight orbit, so that its light is usually inseparable from the light of the K3II giant. And this component in a tight orbit is, according to Bright Star catalog, of spectral class B0.5V, making it a very hot, blue star.

So that is why the primary component of Albireo is so pale. Its own yellow-orange light has been diluted by the blue light of its hot companion. (I should add, by the way, that Bright Star catalog claims that Albireo has two more components in tight orbits, although nothing is said about their spectral classes.)

However, if Albireo has a companion of spectral class B0.5V, and the yellow giant itself is of spectral class K3II, then I think that the primary component of Albireo, including its tree companions in tight orbits, is underluminous. A star of spectral class B0.5V can itself be almost 900 times as bright as the Sun in visual light, or at least it is not uncommon for stars of spectral class B0V to be that bright. But the combined light of "Albireo A" is yellow enough that the yellow component dominates over its companions, meaning that the B0.5V component can't be more than, say, 200-300 times the Sun in visual light. I can't help wondering if, perhaps, the yellow giant is losing mass to one or more of its companions, slowly turning this system into another version of Algol.

What about the optical blue component of Algol? Oh, it's a B8V star, nothing special. Okay, it is admittedly bright for a B8V star, 111.8 ± 6.9 times the Sun. That is admittedly bright for a main sequence star that is no hotter than spectral class B8. Compare it with Alkaid, a main sequence star as hot as spectral class B3V, yet its V luminosity is only 153.7 ± 2.4 times the Sun, less than 30% brighter than the B8V component of Albireo. Compare it with Vega, too, an A0V star. The V luminosity of Vega is about 48 times that of the Sun, only about 43% the V luminosity of the B8V component of Albireo.

The color index of the B8V component of Albireo is 0.095 ± 0.009. That is "okay blue" for B8V star, but nothing special or truly exciting.

Rob, let me draw a conclusion from my description of Albireo. Since the yellow component is made up of perhaps four and certainly at least two stars of very different temperatures and colors, the color of Albireo A (the yellow-looking component" is not the same "all over". What your pixellated version shows is that Albireo A is made up of one hot and one cool companion.

Star images "bleed" so that stars look bigger in photographs than they really are. They are really point sources as seen from the Earth. But in the case of Albireo A, this "bleeding point source" is not the same "all over its own point", and Rob, your pixellating has found that the color of Albireo A "varies over the face of it"!

Excellent job, Rob!

And that's a very fine job too, Rich Hammar!

Ann

[rstevenson]

Thanks Ann! I've always known that different stars are different colours and that they progress through an aging sequence, but I never knew the details. I learn a lot from your discussions of stars and their colours.

And to find out that what we call Albireo is really as many as 5 stars is a bonus.

Rob

[Chris Peterson]

I'm wondering about the evident bronze tones in the red star and whether these are "real" colours or simply the result of applying particular processing to the image.

The effect you see in the blowup is the result of artifacts. For the most part, the colors are not real.

An RGB (or any multiple wavelength channel) image can only approximate the color space of our eyes. In this case, there is first an encoding error, and then a display error. Additional problems are introduced in that a star is a point source optically, and at the resolution of our eyes we perceive it that way. But a star image, made at higher resolution, is broadened by diffraction and other optical effects. The result is a profile spread out across multiple pixels, which because it varies in intensity also varies in color (intensity is a component of color).

This image is overexposed... if the goal was to capture the most accurate color possible for the stars with the available equipment, the exposure (or exposures) needed to be shorter. As it is, the stars are saturated, so much of the color information has been lost. Only on the wings, where the intensity is lower, does the star go out of saturation and hues start appearing. However, not all the color channels desaturate at the same time, so the range of colors is extreme, and unphysical. Even some impossible colors, like green, are present. And of course, there is the choice of the imager in how to represent the colors as well.

Visually, I don't find this image comes all that close to the view of these stars through the eyepiece. The blue component is too saturated, and the yellow component isn't yellow enough. (It's still a pretty picture; I don't personally think an astroimage "fails" if it doesn't perfectly represent colors as seen by the eye. In most cases, the image shows more than the eye can detect... which is a primary reason for making the image in the first place!)

[rstevenson]

... As it is, the stars are saturated, so much of the color information has been lost. Only on the wings, where the intensity is lower, does the star go out of saturation and hues start appearing. However, not all the color channels desaturate at the same time, so the range of colors is extreme, and unphysical. Even some impossible colors, like green, are present. ...

Ah, yes. That's what I was wondering about. It seemed peculiar that I could pick out both an olive-green tone and a purple tone from the same star, though I could see there was overlapping saturation from boths stars in the central part of the image.

Here's an image I found on Hubblesite which is less saturated. It looks to me to be a little too far to the red end of the spectrum, as the blue star has definite purple tones. It's interesting what happens in my image editor when I start to play with the colours. For example I tried reducing the Green Brightness by itself -- which in theory should have no effect on star colours -- and the colours changed dramatically. It would seem that getting accurate images of star colours is quite difficult!

Rob

[Chris Peterson]

It would seem that getting accurate images of star colours is quite difficult!

Very difficult, indeed. And even if you capture the raw data accurately, the technology to display it isn't very good.

[Ann]

Rob, that Hubblesite picture of Albireo is far too red. As I said, Albireo A is a pale yellow, but in that picture is looks yellow-orange, almost bordering on brick red. No way Albireo A looks like that!

Ann

[Chris Peterson]

Rob, that Hubblesite picture of Albireo is far too red. As I said, Albireo A is a pale yellow, but in that picture is looks yellow-orange, almost bordering on brick red. No way Albireo A looks like that!

On my monitor (which I regularly calibrate), there is no evidence red (or brick red). The hues of the two stars look about right to me, although they are presented with much higher saturation than we see visually.

Of course, this just goes back to the display issue. Everybody is viewing these images on different devices, with different characteristics. Those of us who actively calibrate our displays are going to see generally similar images, but even those aren't truly realistic, given that the color space of typical displays doesn't match our eyes, and isn't all that well suited to most astronomical objects.

[Ann]

Well, I disagree with the hue of Albireo. To me, this kind of yellow-orange is what Betelgeuse looks like.

I was not the least surprised to find out that Albireo A is more like the color of pale Pollux than the color of much yellower Arcturus, because it was obvious to me when I saw this star through a telescope that it is nowhere near as yellow as a bright K-type giant ought to be.

Ann

[rstevenson]

Of course, this just goes back to the display issue. Everybody is viewing these images on different devices, with different characteristics. Those of us who actively calibrate our displays are going to see generally similar images, but even those aren't truly realistic, given that the color space of typical displays doesn't match our eyes, and isn't all that well suited to most astronomical objects.

Indeed. I calibrate my display as well, but even different applications will give slightly different results for some images.

I'm running Mac OS X and can run Windows XP in a virtual machine simultaneously. Viewing images in Firefox in Mac OS X and then immediately in FF in Win XP gives noticeably different results. All Windows machines, virtual or otherwise, seem pale and indistinct to me in comparison to the righly saturated colours common on Macs. (There are historical reasons for this which are irrelevant to this discussion. I just wanted to reiterate Chris' point about monitors and viewing environments. They make quite a difference.)

Rob

[Chris Peterson]

Well, I disagree with the hue of Albireo. To me, this kind of yellow-orange is what Betelgeuse looks like.

I think you may have missed my point. You have no grounds to disagree. Or to agree. We are viewing on different displays. We can really only discuss the finer points of the color if we sit down together and do so in front of the same monitor (although we can do better if we have similar hardware which has been similarly calibrated... which I don't think is the case here).

[Chris Peterson]

Indeed. I calibrate my display as well, but even different applications will give slightly different results for some images.

I'm running Mac OS X and can run Windows XP in a virtual machine simultaneously. Viewing images in Firefox in Mac OS X and then immediately in FF in Win XP gives noticeably different results. All Windows machines, virtual or otherwise, seem pale and indistinct to me in comparison to the righly saturated colours common on Macs. (There are historical reasons for this which are irrelevant to this discussion. I just wanted to reiterate Chris' point about monitors and viewing environments. They make quite a difference.)

Yes, the gamma differences used by default with different operating systems can be a problem. It's why I never use a Mac for any of the ads I prepare for publication, and have moved a couple of service agencies I consult for away from using them as well.

Hardware matters. Software matters. Calibration and color profiles matter. The nature of the source data matter. Our eyes matter. It's amazing we ever see anything on computers similar to what others see!

[Beyond]

Thanks Ann! I've always known that different stars are different colours and that they progress through an aging sequence, but I never knew the details. I learn a lot from your discussions of stars and their colours.

And to find out that what we call Albireo is really as many as 5 stars is a bonus.

Rob

So then, Albireo is really a Star of stars. A Light of many lights. If you were to associate it with the song "You light up my life", you would have to change a couple of words in the Title and the verses. "You light up my life" would become-->Youse lights up my life. You would have to pluralize "You" and "light".