MargaritaMc wrote:
I was going to post and ask Ann about the ¿PURPLE? tinged star, but decided to make a puzzle of this delightfully named asterism - SO BRILLIANTLY DEDUCED BY BEYOND - before asking
- DO purple stars exist?? HOW??
Margarita
No, Margarita, purple stars do not exist.
The star that was described as purple is the fainter component of the binary star Delta Corvi. The brighter component of Delta Corvi is described by my software as being of spectral class B9.5V, and its Johnson B-V index is given as -0.012. The fact that Delta Corvi A has a negative B-V index is certain proof that it is blue, which is normal for B-type stars, even for late B-type stars like Delta Corvi.
But the fainter component is supposed to be purple. How is that possible? The answer is that it isn't.
Robert Burnham, Jr., wrote about Delta Corvi in Burnham's Celestial Handbook, volume two:
Name - ALGORAB. Mag. 2.95; spectrum A0 V or B9.5 V. Position 12273s1614. Delta Corvi is an easy double star for the small telescope, first observed by J. Herschel and J. South in 1823. There has been no change in either the separation or the PA of the pair in the last 140 years, although the physical association is proven by the common proper motion of 0.26" per year in PA 235 degrees. Most observing lists have called the colors "yellowish and pale lilac"; occasionally "white and orange". The small star, magnitude 8.4, has a spectral type of dK2 and an actual luminosity of about half that of the Sun.
The spectral class of the fainter star is described by Burnham as dK2, by which he means that this is a main sequence K-type star. Such stars are
never purple. On the contrary, they are yellow, yellower than the Sun. So why would anyone think that this one might be purple?
I think it is obvious that the first influential observers of double stars had a great impact on the way these double stars were perceived, because they described the colors of the double stars they observed in a way that seemed convincing and inspiring to others. One such early observer was
William Henry Smyth. You should read the entire wikipedia entry about him, because he seems like a fascinating man. Just consider the fate of his wives and daughters - they either died early and childless, or else they procreated as if they were trying to populate the Earth all by themselves!
But now we should discuss him as an observer of double stars.
http://en.wikipedia.org/wiki/William_He ... other_work wrote:
The first volume of this work was on general astronomy, but the second volume became known as the Bedford Catalogue[5] and contained Smyth's observations of 1,604 double stars and nebulae. It served as a standard reference work for many years afterward; no astronomer had previously made as extensive a catalogue of dim objects such as this. It was reprinted in 1986, and in the Foreword to that edition George Lovi (astronomer and writer)[6] writes, "What makes it so special is that it is the first true celestial Baedeker and not just another "cold" catalogue of mere numbers and data. Like the original Baedeker travel guidebooks of the last century,
this work is full of colorful commentary on the highlights of the heavenly scene and heavily influenced several subsequent works of its type, even to the present day. ... It is in the descriptive material that Smyth is a delight. He not only describes what the user of a small telescope will see, but also includes much fascinating astronomical, mythological, and historical lore. Many of these descriptions are especially valuable for the novice and user of small telescopes of a size similar to Smyth's."
Burnham of Burnham's Celestial Handbook often quoted Admiral Smyth's description of the colors of various binaries. When I first read about these color descriptions in Burnham's Celestial Handbook, I often thought the colors described sounded totally improbable. I didn't believe in his colors at all. So where did he get them from?
Well, frankly, I think he imagined them! I think he imagined them in the same way as
Percival Lowell imagined that he saw canals on Mars. I think Admiral Smyth imagined at least some of the colors he reported.
Very often, Admiral Smyth reported seeing the brighter component of a double star as yellowish and the fainter component as bluish. I think you will agree with me that purple is a bluish color rather than a yellowish one. It is quite possible that his eyes worked like that, so that he almost always saw the brighter component as yellower than the fainter one.
There are, of course, several double stars where the primary component is much yellower than the secondary one. Albireo is the archetypal example, but there is also Gamma Andromedae and several others, for example 30 and 31 Cygni.
I have written here at Starship Asterisk* on another occasion that for a long time, blue stars were considered impossible. Stars were regarded as either white, yellow, orange or red. With the invention of the telescope, blue stars were discovered, particularly the fainter component of Albireo, of course.
David Malin wrote in his book, A View of the Universe:
After centuries of sky-watching with the unaided eye, no-one had ever described the stars as other than white, yellow or reddish-orange. Some blue stars were known from telescope observations, but they were always members of strongly coloured pairs of visual double stars and were therefore a curiosity.
So according to David Malin, blue stars were long believed to exist only in visual double stars. If blue color in stars was believed to exist only in double stars, then this blue color could be considered "a miracle". And if it was the the binary nature of some stars that conferred this wondrous blue color onto one of the components in a binary, why shouldn't there be other amazing colors in double stars, too? Why not lilac? Why not green?
I think Admiral Smyth and a few other pioneers like him mostly noted color in pairs like Albireo, where the primary is yellow and the secondary is blue. I think that because of this, they always expected this pattern to hold. So when they observed a star like Delta Corvi, where the primary is blue-white and the secondary yellowish, their brains "adjusted" the colors of the stars so that the primary appeared to be yellower than it really is and the secondary bluer. Remember that the primary of Delta Corvi is a blue-white star of spectral class B9.5V, while the secondary is a yellowish K-type dwarf. Yet someone, likely Admiral Smyth, described the primary as yellowish and the secondary as pale lilac. But others, far more correctly, described the pair as white and orange!
So to summarize, Margarita, there are no purple stars. But some authorities are incredibly influential, and if they said that they saw purple stars, then so will many of their followers. Because sometimes colors are all in your mind.
Ann
Thank you, Colour Commentator! I KNEW that you would come to the rescue!
