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Seeing Red! UX Draco

May 1,2008 - 4 am - T5, S3 8-inch LX90

Take your average nuclear explosion, mix in a lot of carbon to absorb the blue light, and what you get is one unequivocally red star.

The one I was looking at this morning is called UX Draco and it's a rarity as stars go. Oh, there are several well known stars that are described as "red." Aldebaran, Betelgeuse, and Antares all come quickly to mind. But if we're honest about what we see when we look at these, we should describe them as "orange," ot maybe "reddish," and we shouldn't say that they were "colored" orange, we would say they have an orange "tint."

Truth is, this business of star color can be quite frustrating, especially for beginners - but I know veteran observers - very good, veteran observers - who say they just don't see the colors that others report. And even when I show people a very "colorful" close pair, such as Iota Cancri, and ask them the color of the two stars without any prejudicial advice in advance, perhaps one in three, will report the stars by their textbook description as "blue and yellow." What's more, read any reports of the observations of double stars by veteran observers and you will get all sorts of colorful descriptions of the same two or three stars - descriptions that frequently do not agree with one another.

In my early days of observing I simply could not see the color in stars, except when confronted with an exceptional pair, such as the popular "blue and gold" Albireo. In those days I was a pretty careful observer, sending in variable star reports to the AAVSO where I had estimated the brightness of a long-period variable to within one tenth of a magnitude. Such stars are almost always "red," but I seldom saw them that way. Generally, I just took star colors as wildly imaginative.

Now I feel differently. Now I can't look at Arcturis in the scope without thinking immediately of how beautifully orange it is, and to my naked eye Spica always jumps out as the iciest of blue. Interesting choice of adjectives, I know, since it's rich blue color reveals it as an exceptionally hot star - not icy. But hey, I'm human and in my human experience I think of blue as being cold. But then, in my role as a somewhat normal human I think of stars as tiny little lights - not as roaring, tumultuous, hydrogen bombs in continuous explosion, though that's what they are.

The difference isn't accidental. I didn't suddenly go from being color blind to color sensitive. Nothing to do with age, or equipment, or seeing conditions, though all these play a role, I'm sure, in how we see color. What it has to do with is about five years ago I simply made up my mind I was going to study examples of bright, colored stars - both naked eye and in the telescope. That's when I started to understand that "color" really isn't the proper word to describe what I saw - :"tint" was. The problem is, color differences are indeed subtle and can be exaggerated, or otherwise compromised, when two stars are very close together, such as a telescopic double. But once you become accustomed to noting color differences, you find it hard to believe others don't see them. To me, every time I look at bright, colored stars, their colors now scream at me. Arcturus is Orange - Antares I think I would call red, but just. Betelegeuse and Aldebaran are more orange than red. Rigel is blue and Sirius blue/white, while Capella has an unmistakeable yellow tint. Vega is more blue than white, but certainly not as blue as Spica. Feel free to disagree, of course, but I'm just telling you how I see these now and how I can understand when people give them brilliant color labels that are exagerated - in the end they are tints to me and if I were to paint them I would use pastels or water colors - and with great restraint.

Hmmm. . . come to think of it, maybe that's the difference between my perception of star color in my early days of observing and my perception now. In the interim i had taken up painting and most of my painting was done with pastels and when you have a case of a couple hundred pastels you begin to appreciate the subtleties of different tints. To do the stars I would choose one yellow, one blue, one red - all from among the palest of the groups.

Except when I was trying to capture a carbon star - then I would take deep red, going towards brown but with bits of orange sparks flying from it.

UX Draco and, I suspect, similar "carbon" stars are the exception to the "tint" rule., They are red, not simply to my subjective view, but to the the more objective view of the spectroscope and camera. But don't take my word for it, try it for yourself - and if you do, I'd love to hear your report and I'll add it to this posting. Just email me at gstone@umassd.edu.

There are other carbon stars such as UX Draco, but what I like about this one is it's only 13 degrees from Polaris, so most people should be able to see it year-round. There is a wildcard in this otherwise stacked deck, however - UX is also a variable, changing magnitude from 5.9 - 7.1 over about six months. Does it also change in redness? Apparently. How much, I don't know, nor do I know whether it's redder when at its brigtest or faintest. What I can tell you is when I looked at it about 4 am today I instantly knew which star it was, even though my field of view was quite large and there were several stars in it. UX simply jumped out as the one odd ball in the group. Everything else was distinctly blue or white - it was -well, red.

It was the "Celestial Sampler" of Sue French that got me started on UX Draconis. She featured it in her article on Draco, and that's where I got its color index and variability range.

Here's a couple of finder charts from Starry Nights. Since it's circumpolar, just twirl the large-scale chart around until the Little Dipper matches the orientation of the moment. That circle is 5-degrees, a good binocular field. (Hmm... have to look for this with binoculars.) The second chart has a much smaller circle - about one degree - and I did it in black and white so as not to indicate too strongly which one is UX. When I first saw it, it was not near the center of my field, but it did leap out as different. See if that happens with you.



BTW - want more objective indicator of color?. Consider the "color index" that astronomers use for stars. It goes roughly like this:

-0.33 O5 Blue
-0.17 B5 Blue-white
0.15 A5 White with bluish tinge
0.44 F5 Yellow-White
0.68 G5 Yellow
1.15 K5 Orange
1.64 M5 Red

The middle letter/number is spectral type. Our Sun is yellow with a color index of 0.65. Spica is off the blue end of the above chart with a color index of -0.13. Antares goes off the chart on the red end with an index of 1.8. And UX is really out there on the red side with an index of 2.7.

So if you're seeing red when you look at UX Draconis, it's not your imagination - this one really is red.

Posted by Greg Stone at May 1, 2008 07:51 AM Comments? Please email me: gstone@umassd.edu

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