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.
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?