Seeing Red in the Sky

Posted by Ethan on January 25, 2010
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Artists can color the sky red because they know it’s blue. Those of us who aren’t artists must color things the way they really are or people might think we’re stupid.
-Jules Feiffer

Last semester, I was teaching my introduction to astronomy class, and part of the coursework was that each student had to choose a unique research topic and write a research paper based on that topic. Topics varied from cosmology to relativity to the space program to individual planets, but one choice captivated me so much that I bring my version of it to you now.

Astronomy was, arguably, the very first science that was approached scientifically. Since ancient times, astronomical observations were made first, and then models were made to explain those observations. Perhaps the most famous ancient astronomer, Ptolemy, wrote in his Almagest a list of six very bright red stars in the sky.

These six stars were as follows, with images above their descriptions:

1.) Betelgeuse, which is a red supergiant and the ninth brightest star in the sky.

2.) Antares, which is also a red supergiant and is the 16th brightest star in the sky.

3.) Aldebaran, an orange giant that’s the thirteenth brightest star in the night sky,

4.) Arcturus, an orange giant, variable star that is the third brightest star in the sky.

5.) Pollux, another orange giant (which looks a little yellower than the others) and the 17th brightest star in the sky. And finally…

6.) Sirius, the single brightest star in the night sky. Only, take a look at Sirius up there. Does something strike you as different about Sirius from the other five? Yes, they’re all very bright, but Sirius, unlike the others, looks blue!

Don’t believe your eyes? Take a look at Hubble’s view of Sirius:

Yep, that’s anything but red. In fact, Sirius was used — as far back as ancient China — as the standard for “white”.

Is it possible that Ptolemy made a mistake? If so, he’s not the only one. Horace, Seneca, and Aratus — all writing in Europe around 2000 years ago — each describe Sirius as being red in color. (And do so up to 200 years before Ptolemy.) Cicero and Germanicus also translated Aratus and wrote about his work, and felt no need to mention Sirius’ redness as being bizarre at all.

But is there any way to make sense out of this? Is it possible that Sirius once really was red, and has changed color to blue-white over only the past 2000 years? Or are all of our ancient sources here — including Ptolemy — simply unreliable? Moreover, what to make of the other ancient sources (mostly from China) that contradict this?

The explanation will be forthcoming on Wednesday, but in the meanwhile, I’d like to know your take based on the evidence I presented here. Ptolemy is pretty reliable as far as sources go, and the fact that all six stars are bright and that five of them are definitively red are pretty strong circumstantial evidence that Sirius once was, too. On the other hand, we have some ancient evidence to the contrary as well as a plethora of modern evidence that… well… it isn’t red! Pretend you’re an astronomer and someone presents this to you.

What would you do? How would you go about deciding whether this was confirmable, plausible, or whether you could bust this myth?

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  1. #1 Tex
    January 25, 2010

    IANAA, but my guess is that Sirius is red sometimes and white sometimes for the same reason the sun is. Low on the horizon, or at times when there is lots of haze in the atmosphere, all of the blue is scattered and lost in the atmosphere, leaving only longer red wavelengths to reach our eyes.

    You might not notice this with other, fainter stars, since scattering the blue part of their light might reduce them below the limits of visibility. Or if they really are mostly red to start with, they would have little of the shorter wavelengths to lose in the first place.

    This could be easily tested by setting off a really big volcano and looking to see if it turns Sirius red again.

  2. #2 Kevin C.
    January 25, 2010

    To add to Tex’s comment, I would note that variations in latitude make a difference as to the elevation of a star in the night sky, and thus the amount of blue light scattered out by the atmosphere; observers in Europe, being farther north, will always see Sirius closer to the horizon (and thus through more atmosphere), than more southern observers.

  3. #3 Daniel J. Andrews
    January 25, 2010

    My first guess is the same as Tex’s–it was low on the horizon from his observation area. But still it is fairly apparent that a star may appear red when low and I can’t imagine Ptolemy being fooled this way. So I’d have to see what other writers would say on the matter, and importantly, see when they talked about it–if they listed it as white, then over a hundred year span talked about it as red, then white again a hundred years later, we could assume it actually did change colour in that time span, as opposed to people calling it red and white in the same century).

    I’d also check the Almagest myself to see if indeed he did list Sirius, and if he did, if it is the same star we know as Sirius today. Always check your sources.

    Perhaps Sirius blew off a layer of itself which dimmed the star (implausible because I’m sure someone would have already looked for this expanding layer, and this wouldn’t be a mystery).

    Maybe an orbiting companion eclipsed it? (look for wobbles in Sirius?).

    Passing dust cloud? (any interstellar dust in the region? I didn’t think so but would need to check).

    Or maybe there is a word connection between Sirius and the dog days of summer where “heat”, or “hot” was later translated as “fiery”, which became “fiery red”, and then “red” (ok, a stretch, but the etymology might be interesting).

    I have a vague recollection of reading something about this (or similar “red” puzzle about how ancient writers referred to a wine-coloured sea) either in Burnham’s 3 volume celestial handbook or a Sky and Telescope or Astronomy Today magazine article in the mid-80s. I think it came down to meanings of Greek words and how they’d changed over time (but I read this a long time ago so can’t remember…I just remember it was fascinating).

  4. #4 Nick
    January 26, 2010

    I like the atmosphere hypothesis.

    My next Hypothesis is that someone considered authoritative (like Aristotle)mistakenly described Sirius as red and such was the weight of his authority that no Western writer of the time would dispute him, even though every illiterate shephard clearly knew that Sirius was white, even if they didn’t know its name.

  5. #5 Andrew
    January 26, 2010

    [url=http://serendip.brynmawr.edu/exchange/node/61]This[/url] source seems to confirm that the Greeks would use color adjectives to describe not only the color but also the characteristics of the thing. Hence hair being described as “azure” when it’s loose a flowing (like the sea).

    Mental perception can sharpen or reduce color perception, and the limits of the language you speak can affect color perception as well (as with native Russian speakers having an increased ability to differentiate between shades of blue, because all blues must be described as being a “light” or “dark” blue, with no simple word for “blue”).

    In other words, I like Daniel’s hypothesis.

  6. #6 Wayne Robinson
    January 26, 2010

    Could Ptolemy have had the rare form of blue colour blindness? So when he looked at Sirius, it looked just slightly redder, and then when it was close to the horizon, the thicker atmosphere made it look even redder? If it was just the proximity to the horizon and the scattering of blue light, I would have thought that the effect would have been greater in China (much further north than Egypt (Ptolemy was either Greek or Egyptian and died in Alexandria) and the dust storms from Mongolia (which made China such a good place for observing the Sun) should have caused greater reddening of Sirius there.

  7. #7 Jérôme ^
    January 26, 2010

    The answer lies in the limited colour vocabulary the Greeks had. For instance, Homer used to call the Egean sea purple. (In Latin, there was not even a word for « blue », which is why the French word « bleu » now comes from German, and the Spanish and Italian ones « azul » and « azzure » from Arabic; in Greek, there was « cyanos » but it might not have been precisely blue). The Greeks were distinguishing colours by luminance/saturation instead of by hue (they must have had special eyes, maybe akin to those of the mantis shrimp).

  8. #8 Jeffrey
    January 26, 2010

    If I remember correctly, to the ancient Greeks and certain Romans influenced by them colors were less, well, colors than qualities. The clear noon sky was called “bronze” because it was clear and bright like a polished shield and blood was “green” because it was fresh and alive, like human sap.

  9. #9 slw
    January 26, 2010

    Unless my memory deceives me, the ancient greeks did not actually have a word for the color blue.

    Ah, yes, a quick google search comes up with this:
    http://serendip.brynmawr.edu/exchange/node/61

  10. #10 vagueofgodalming
    January 26, 2010

    Whatever the explanation, it must also account for why other bright white stars in a similar latitude to Sirius (Rigel, Spica, Fomalhaut) did not make the list despite being just as bright as Antares (which is also well south of the celestial equator). I think this kills the “Greeks didn’t have a word for ‘blue’” hypothesis, but not anything based on synaesthesia relating to Sirius specifically.

  11. #11 Frans
    January 26, 2010

    At what age did Ptolemy made his observation(s) of Sirius? Could it be that he had cataract? This would result in a yellowing of the lens, which in turn would lead the lens to selectively absorb more short-wavelength light (blues and greens). The image that would impinge on the retina would appear more reddish. It would not (or much less) affect the colour of stars that are tainted red already.

    This hypothesis does have an issue with the remark made in post #10, though………

  12. #12 ENT-TT
    January 26, 2010

    A guy named Robert Temple suggested a short-term fusion hypothesis… Some hydrogen on the outer atmosphere of Sirius-B (white dwarf) may have drifted into its core. Since the core contains carbon and oxygen, they could each serve as a nuclear catalyst, causing the hydrogen to fuse into helium. The heat bubble from the fusion reaction might cause the thin hydrogen layer to balloon in size by several orders of magnitude. When it cools down again, it would burn bright red, and likely stay that way for a couple hundred years. Once the bubble collapses, it would turn blue again. Since stars don’t usually change color over night, it would be accepted that someone had simply been mistaken, and you wouldn’t expect the likes of Ptolemy to be wrong about a repeatable, verifiable observation.

  13. #13 Russell
    January 26, 2010

    Homer also called the sea wine dark. And it will look that at times. And, at other times, copper.

  14. #14 Eric Lund
    January 26, 2010

    Actually, the horizon theory may have something to it, because unlike other stars there was specific reason to look for Sirius when it was near the horizon. The ancient Egyptians associated the appearance of Sirius at the dawn horizon with the annual Nile floods, and they reset their calendar accordingly. I don’t know if the association still held in Ptolemy’s day, since precession of the equinox would have shifted the timing by a noticeable amount over a period of thousands of years.

    Chinese agriculture had different seasonal dependencies, so they had no particular reason to look for Sirius near the horizon. Thus they saw it as white.

  15. #15 Chris
    January 26, 2010

    Perhaps, the time of day when the star was observed has something to do with it. If Sirius were observed at daytime with the sun “just high enough over the horizon”, it (really they) would appear to be not as bluw as the sky if you were benchmarking color.

  16. #16 cehegarty
    January 26, 2010

    I don’t know for sure, but I think it has something to do with particles in the atmosphere making Sirius appear red. That’s just my guess. Now I’m going to have to keep myself from looking it up somewhere until tomorrow.

  17. #17 Omega Centauri
    January 26, 2010

    IANAA, but I started grad school with the intent. I am hard pressed to come up with an astronomical way to get it red. Sirius is main sequence, so its evolution is going to be pretty slow. Sirius B is a white dwarf, although I don’t know its age (since it formed at the center of a planatery nebula), I would wager that that was many millions of years ago, so any leftover gas/dust seems unlikely. And the star is really close to earth so the possibility of a dust cloud passing between us an Sirius seems really low.

    I do note that looked at through a telescope (i’ts ridiculously bright for that), but atmospheric distortion makes it shimmer in all the colors of the rainbow, as the focusing/defocusing effects which cause twinkling sometimes make one color stand out. So someone staring closely at it should percieve red as well as green as well as blue colors. So if his list was bright stars, for which he could (sometimes) see as red, that might be an explanation. Sirius is seriously (pun inteneded) brighter than the others, so this effect might have been missed with some of the other bright ones (Vega, and Capella come to mind).

  18. #18 Sol
    January 26, 2010

    I would have to make sure the location of the Sirius he talks about and where Sirius actually is match, but the first explanation I thought of was Venus. I have no idea if at that time they were at the point of distinguishing planets from stars, but Venus is a large and very bright red point of light in the night sky.

  19. #19 Jeffrey
    January 26, 2010

    I don’t see how atmospheric conditions would account for it. Greece is at about the same latitude as China, so they wouldn’t be seeing it at very different angles. I suppose the amount of dust in the sky might be different because of geology/proximity to deserts/prevailing winds, but I don’t see that making that big of a difference. And the Greeks certainly knew about planets, including Venus (hence the name planet from planetes).

    I still think that it’s a matter of what colors meant to the Greeks. IANAC (I am not a classicist) but maybe “red” referred to another quality entirely–brightness or where in the sky they are.

  20. #20 Andrew
    January 26, 2010

    Actually, come to think of it, wouldn’t a volcanic eruption in the Greek Islands have thrown dust into the local upper atmosphere for several decades if it were powerful enough?

    Ptolemy was A.D. 90 to 168, and Santorini did erupt in A.D. 40, at a “VEI” of 3. Perhaps more interestingly, Vesuvius erupted with a VEI of 5 in A.D. 79, wiping out Pompeii. And Italy isn’t that far from the Greek Islands…

  21. #21 Treppenwitz
    January 27, 2010

    A dust cloud passing between Earth and Sirius was my guess. It wouldn’t explain conflicting reports from the same time period, though.

  22. #22 MadScientist
    January 27, 2010

    Perhaps a location which has a lot of dust in the air all the time and with Sirius always near the horizon? Then again China spans a huge range of latitudes (definitely higher latitudes than Greece) and dust in the air would have been fairly common even 2000 years ago.

    On another topic: what do people think the fate of the universe would be? With expansion and gradual conversion of hydrogen to heavier elements will we eventually have a cold dead universe, or are there reasons to believe there will always be stars (and therefore planets and possibly other life forms).

  23. #23 Joe
    January 27, 2010

    Andrew,

    I did think of Pompeii (although I forgot about Thera), but Horace died in 8 BC or so. So we would need something providing air dust even earlier. And I would imagine that the dust would have mostly settled out 30-40 years later.

    I still think that an atmospheric phenomenon is the most likely case. Latitude can’t be too much of an issue because most Chinese population centers at the time were between Alexandria and Rome. Changes in the star can’t be right because of the Chinese observations. I know that educated Romans were obsessed with Greek culture, and I’m no expert in Latin writing style, but random adoption of the Greek way of discerning color in Latin poetry by several major writers seems farfetched. I would want to see the context, at least.

  24. #24 Alan B.
    January 27, 2010

    We have only copies of copies of copies of what Ptolemy wrote. I remember reading once of a version of Ptolemy that did not have Sirius listed, suggesting that it was added by a scribe.

  25. #25 Jon
    January 28, 2010

    When Sirius is low in the horizon it turns red, just like the Sun.

  26. #26 judith weingarten
    February 1, 2010

    It’s not so simple. See Caniculares Dies (The Dog Days) with much meditation on the colour of Sirius, as the ancients may have seen it.

  27. #27 Wo
    August 14, 2010

    About the color, it has nothing to do with the earths position. I can be anything, from space dust, to our suns flares getting in the way, atmosphere pollution or even a dust storm in the earth. Plus if you do some digging you will see that its not always the same days , but totaly random days !

  28. #28 wo
    August 14, 2010

    Of course when it is on its lower point in the horizon its has to do with the position. Just remember of and old simple but really revealing experiment. When ‘white light” goes through an colorless object ( water, glass, etc) the “white light” becomes many different lights like a rainbow, only that we can see a color between red and yellow!

  29. #29 Wo
    August 14, 2010

    Oh and do not forget that, a star color shows how powerful, what kind of chemicals it burns and thats its Temperature. Our Sun for example burns Helium, even though it sound extreme but i have heard that Greeks called the Sun (Ηλιος) Helios because of that! Something thats has no base on anything, its just a saying !

  30. #30 Nicolasin
    February 7, 2012

    Hey ho ;o)
    Stars can change colour instantly as they are electrical in nature and connected to the whole as well as being conscious beings and having a singularity at the center….they can change colour simply by changing frequency…no problem there….our sun may well have changed colour multipul times and may well in the not so distance future…have a fun day all ;o)

  31. #31 cat
    April 6, 2012

    We all need to think about this, could he have been color blind? It was 2000 years ago!!, also look at all the toxins that is in the air now, could that change its appearance?

  32. #32 Dean Williams
    May 20, 2012

    Dust in the atmosphere? Color blindness? No! Atmospheric distortions? Does no one see what’s obvious? When a star is moving away from our position, its light shifts to the red. When a star is moving toward earth, the doppler shift is blue. ALL stars have orbital paths. It is clear that Sirius has “rounded the corner” in its orbit relative to our position and is now approaching the earth in its great cycle. 2000 years ago it was indeed receding; it is now coming closer to us and therefore appears blue.