Starship Asterisk*

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.

rstevenson wrote: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.

Chris Peterson wrote:... 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. ...

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

Ann wrote: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!

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

rstevenson wrote: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.)

rstevenson wrote: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