Star Colors

So, What Color is it – Really?

Impressions of star colors can vary significantly, sometimes wildly, from one observer to the next.  Greg’s comments on Cor Caroli – here – are an example of this, and left me in deep thought about the reasons behind it.

There are a myriad of factors at work which influence how we perceive colors in stars.  Usually, red is red, and bluish-white is bluish-white, but beyond that, things get tricky — to say the least — and there are times when you wonder if your eyes are deceiving you.

Several things are at work from what I’ve seen, and I’ll list them here:

1. The observer, of course, which may not necessarily be the most significant factor.  And “observer” also includes experience, not merely sensitivity to color.

2. Transparency – it’s surprising how much transparency can influence color.  I spent a night hunting down several doubles that I had never seen before and was definitely not the least impressed with their color.  And I didn’t think the transparency was all that bad – I rated it as average that session.   A few nights later, I looked at the same stars again – when the transparency was obviously better – and I was stunned by the color in several of those same pairs.   A big effect on transparency, of course, is elevation above the horizon.  Rho (ρ) Ophiuchi, for example, is yellow, but can easily be seen as red because of it’s low position on the southern horizon.  More on this below.

3. Aperture – which Greg has touched on in his comments.  Aperture affects color not only because of the amount of light gathered – or not gathered – but I think some of today’s lens coatings also can contribute either a warm or a cool tint.  But there again, the amount of that effect varies from one person to the next.

4. Eyepieces – Lens coatings definitely play a part here, not only with regard to warm and cool tints, but also in how much light is transmitted.  I don’t have any of them now, but my first impression when I looked through an Ethos was how vibrant the colors were.  There is a distinctive red star in the Double Cluster in Perseus that absolutely sprang to life for me with a 13mm Ethos – I have never seen it that RED in another eyepiece.   Others have commented that Radians impart a warm tone, which I have not noticed, but on the other hand, I have noticed that the 5, 7, and 9mm Naglers add a cool tone to an object.

Star Temperatures and Colors

Greg has pointed out that the spectral classification of both components of Rho Ophiuchi should result in a pair of white stars, not yellow.  So, consulting the Haas book, I found them described there as yellow, matching my observation.  But – hold on –  the Night Sky Observers’ Guide describes them as white!  So, what color are they – really?

More than likely the low altitude of Rho from my latitude (45.7 degrees North) results in the yellow color.  But Greg brings up a good point, and this would be a good place to review spectral classifications – not that they’re a sure guide to what colors you are going to see, as should be obvious by now,  and for the reasons explained below.

Chart adapted from Wiki link referenced at the end of the paragraph on the right. Click on the chart for a larger view.

In the chart shown here, the star classification is listed in the first column, followed by the temperature.  The star’s conventional color (pay close attention to this term!), which is the third column,  is directly related to its temperature.  If you study the chart carefully, you’ll see the hottest stars are blue and blue-white.  As the temperatures become more cool, the colors make a gradual transition from yellow to red.   It’s important to understand that the conventional color as shown on the chart is not necessarily what you will see when you look up at the sky – although it’s close.  Actually — not that you would ever do this  —  the conventional color is what would be seen if the star was put under high magnification and projected onto a white sheet of paper in the daylight.  The last column, the apparent color, is what is seen by the naked eye (no optical aid) under a dark sky.    As you go down the last column, you can see some loss of color to the unaided eye as the star temperatures become cooler.  For a very detailed discussion of stellar classification, take a look at this Wikipedia link, or better yet, read through this piece by Greg for the same information that is shorter and easier to digest.

So we have two designations of color here:  conventional — and — apparent.   And they’re not quite the same.  And that’s a good part of the reason that a star’s spectral classification doesn’t necessarily match what is visible to the naked eye – and based on my experience, also what is sometimes viewed in the eyepiece.

HOWEVER – at the eyepiece, our eyes are also easily fooled by intensity, brightness, and proximity.  So if you aren’t totally and absolutely confused by now, what follows will probably take care of that.

When a star viewed through a telescope is very bright or intense, our eyes tend to see more white than is actually there.  That would explain why Cor Caroli appeared white when I observed it in a six inch refractor, even though it was clearly yellow in a 60mm scope.  On the other hand, when a star is very faint, our eyes have a difficult time seeing color (as is most obvious in nebulae and galaxies), and the result is faint companions often appear blue to us, or even gray – as in no detectable color.  In connection with that, contrasting colors will also cause a faint companion to appear more white or blue than it would if it could be seen apart from the much brighter primary, and the effect is more pronounced the closer the stars are to each other.

One more thing needs to be added to all of this: the sky background.  More than I would like, I make double star observations under a sky filled with a considerable amount of moonlight.  In the summer, when the moon is low in the southern sky, I can get away from a large percentage of the glow it adds to the sky by staying in the northern sky.  In the winter, when the moon is much higher in the sky – frequently almost directly overhead from my latitude – the only place you can go to get away from the glow is back inside the house.  And there aren’t any stars in my house.  At least not yet.  😉

Under a dark sky, star colors in a telescope are much more intense.  I really notice that on a star like Eta Persei, which I first observed under a bright moonlit sky.  It tends toward the faded red side in that situation – nice, but not particularly eye-catching.  But under a black sky, it is really a magnificent sight at about 100x in a 90mm scope or larger – a very rich orange-red or red-orange color.  That black background really enhances star colors, and up to a point, an increase in magnification will also do a lot to emphasize the color.   (Too much magnification will cut down on the amount of light entering the eyepiece and make the star too dim).

The moral of this long story, then, is don’t feel like you’ve missed something or have poor color vision because your impressions of color don’t match what other people see.  As you should realize after reading all of this, there are a tremendous number of variables and influences at work.   There is only one solution: pull up a good chair,  sit down behind the eyepiece,  and just enjoy the view!