The Other Sirius Mystery: Red or White

Today Sirius, the brightest star in the night sky is an unmistakable blue-white in colour although it does twinkle a lot and can appear to change colour. However no one would think of describing it as red.It is thus a source of confusion that many ancient writers describe the star as red or  ruddy. Assuming that colour blindness wasn’t rampant in the distant past this discrepancy is a puzzle that has exercised the minds of modern astronomers and other interested parties, including myself in recent weeks.

I have come to the conclusion that Sirius was never red, based on

(a) what I consider to be unreliable use of the ancient sources,

(b) the evidence from Chinese records that unfailingly refer to the star as white and

(c) the fact that if we accept that Sirius was red roughly 2000 years ago then we have to seriously question modern accepted thinking on the life cycle of star

Image of Ptolemaeus

Claudius Ptolemy probably didn't look much like this depiction from 1400 years after his death (image credit: via wikimedia.org)

 

One of the most oft quoted sources of this alleged red colour of Sirius is the ancient Greek astronomer Claudius Ptolemy (A.D. 90 to 168). Around 150 AD, in his famous Almagest, he described Sirius as reddish, along with five other stars, Betelgeuse, Antares, Aldebaran, Arcturus, and Pollux, all of which are clearly of orange or reddish hue. So he was 60 years old and could have had cataracts or been colour blind!  Or, as we only have copies of what Ptolemy wrote there is room for error in copying. A sleepy and overworked scribe might easily have made a slip of the quill (pen) which then got carried into all future copies.  Also apparently there is a version of Ptolemy that does not include Sirius in that list of red stars. Maybe a scribe added it in as a joke; an April Fool’s joke that has lasted almost 2000 years!! But the problem is, according to much of the literature on this subject, Ptolemy (and I will return to him later) was not on his own; many other ancient writers also described Sirius as red, some even writing prior to Ptolemy.

The Greek poet Aratos (3rd century BC) is often listed in support of the ancient red hue of Sirius.  In his poem Phaenomena (326-34), Aratos uses the term poikilos when describing Sirius. This word can be translated as ‘many coloured’.  Where is red coming from? In translating the said poem into Latin Cicero (106-43 BC), the famous Roman orator used the words’ rutilo cum lumine claret fervidus ille Canis’,  which translate to English as ‘with ruddy light fervidly glows that dog’. We must remember that neither Aratos or Cicero were astronomers and true meaning can be lost in translation especially when poetry is the subject matter.

Pliny the Elder (23-79 AD) writing in his monumental work Natural History highlights many astronomical facts. He is often quoted as describing Sirius as reddish. He cites three heavenly bodies as ardens or igneus: the rising Sun, Mars, and Sirius. (Nat. Hist. II 18 47.)  A quick look at a translation (etymology) of these words reveals several meanings including burning, shinning, brilliant and fiery. I do not see that we can rely on this source for the actual colour of Sirius. Pliny could have been referring to the undisputed brightness and brilliance of Sirius.

The narural philosopher Columella (4-70 AD) is invoked as support for the ‘red’ Sirius theory.  Writing about roses he likens their hue to Tyrian purple, the rising Sun, Sirius, and Mars. (De Cultu Hortorum X 286.) He could well be referring to different varities of  roses. And there are all manner of colour of roses. I am not convinced that Columella really intended to tell us anything about the colour of the Dog Star.

Image of Sirius_A_and_B

An artist's impression showing how the binary star system of Sirius A and its diminutive white dwarf companion, Sirius B, might appear to interstellar travellers. (Image credit: NASA, ESA, G. Bacon (STScI) )

 

Returning to Ptolemy, his evidence is the big hurtle to overcome in this debate.He is of course a formidable ally quoted by many in support of the ancient red colour of Sirius.If Ptolemy said it was red then it must have been so.It would be difficult to argue with an intellect as great as Ptolemy’s.I do not argue with Ptolemy but I do take issue with how he is invoked/used to lend credence to the red theory.

In a very interesting article Lynn, W. T. (1887) suggests that Sirius was never red; the whole notion was founded on a mistake. He calls attention to Professior Schjellerup’s detailed translation of Al Sûfi’s account of the heavens in which the famous Arab astronomer used Ptolemy’s Almagest , but doesn’t render Sirius ‘red’.  There is reference here also to the idea that Ptolemy only named five stars as being red, the addition of Sirius to the list being based on a transcription error. I am convinced by the argument and information in this article that the origin of ‘red’ Sirius hypothesis is indeed a red herring. This article is well worth a read; don’t be put off by the French, it is not essential to the basic argument. With Ptolemy discredited as a reliable source for the ‘red’ colour of Sirius I think we should but this debate to bed.

But let’s not be too hasty, there is further support for a white Sirius in Chinese records. Jiang Xiao-yuan of Shanghai Observatory gives an excellent overview and argument on the Chinese perspective.

The Heavenly Wolf as it was known to the Chinese was actually used as a benchmark for the colour white.It is consistently referred to as being white by ancient Chinese writers.No records claim any redness whatsoever. Jiang Xiao-yuan concludes that Ptolemy was simply wrong (… it may be noted that SIMA Qian predated Ptolemy by some two hundred years and since Sima Qian took Sirius as standard for white star, it is quite impossible for the star to have changed into red in the interim. It is therefore obvious that Ptolemy’s statement that Sirius is red cannot be given credence.)  whereas I think that it is more likely that there has been a mistake which has been perpetuated. In other words though I agree with his conclusion, I favour the explaination put forward in W.T. Lynn’s article.

Ancient Chinese sources stating unequivocally that Sirius was all the time white  combined with the toppling of Ptolemy as a proponent of ‘red Sirius’ by Prof Schjellerup really weakens the ‘red’  Sirius theory. It is further undermined by the fact that it is  inconsistent with current  stellar evolutionary theory.

 

image of sirius by HST

This Hubble Space Telescope image shows Sirius A along with its faint, tiny stellar companion, Sirius B. The cross-shaped diffraction spikes and concentric rings around Sirius A, and the small ring around Sirius B, are artifacts produced within the telescope's imaging system. ( Image credit: NASA, ESA,H. Bond (STScI) and M. Barstow (University of Leicester))

 

Let’s consider if Sirius maybe was red in colour 2000 odd years ago. What we see as the brightest star in the night sky is actually a binary star comprising Sirius A, a main sequence star and Sirius B, a white dwarf. The red giant phase of a star, when a star does actually appear red, is a prerequisite to becoming a white dwarf.The possibility that Sirius B could be responsible for ‘red Sirius’ has been rejected by astronomers on the grounds that the timescale of thousands of years is too short and there is no sign of the expected nebulae. A more detailed account of Sirius can be found here . While we always have to keep an open mind I think that the current theory of stellar evolution is safe.

(Aerticle by Mary Bulman, Education Support Officer)