Blue flower, red thorns! Blue flower, red thorns! Blue flower, red thorns! Oh, this would be so much easier if I wasn’t color-blind! -Donkey, from Shrek
Earlier this week, I introduced you to the Red Controversy, the observations recorded around 2000 years ago in Europe asserting that the star, Sirius, appeared red.
Now, taking a look at Sirius today, it is clearly not red:
And, based on what we know about stars, they don’t change color on timescales that quickly. Many of you put forth some very good ideas, and I thank you for the comments. In fact, the most common one was the very first idea considered by scientists who took this seriously: when anything is low on the horizon, it appears red. Case in point: the Moon.
Would this be reasonable? The answer is, unfortunately, no. Why not? We see references to lots of other bright stars that reach similar positions above the horizon in the sky as Sirius does, and their colors are reported accurately. Also, the reddening effect of objects close to the horizon was well-known, and it’s hard to believe that someone as astronomically knowledgeable as Ptolemy would fall for this. Moreover, Ptolemy lived in Egypt, where Sirius gets awfully far above the horizon and doesn’t appear red at all during many times of the year.
Many of you also pointed out that particles in the atmosphere, such as intense volcanic eruptions, can change the apparent colors of astronomical objects. (In fact, we’ve talked about that recently.) But they wouldn’t do it for hundreds of years; Horace, writing in Europe in the 1st Century BC, would not see the same thing as Ptolemy, writing in Africa in the 2nd Century AD. Although, this was a reasonable line of thought, especially considering that there is some evidence from China that they saw Sirius as white during this same time.
(As an aside, the argument that a later source stuck that sentence about Sirius being red in Ptolemy’s work after reading Horace or Aratus (both poets) and deciding that Sirius belonged in that list of red stars as well is possible. However that assertion is impossible to confirm or refute, so it isn’t talked about among scientists.)
So, what could have caused Sirius to appear red in the past, and are any explanations still feasible? There are three major schools of thought.
1.) Bok Globules. These relatively dense clouds of interstellar gas and dust move through our galaxy, blocking out starlight and definitively reddening the brightest stars behind them. This is an intriguing suggestion, because one could not only pass in front of a star, making it appear red, but it could move quickly enough that it could transit in just a few hundred years.
But the argument falls apart on one count: Sirius incredible brightness. Sirius is the brightest star in the sky, and it was reported as the brightest 2000 years ago as well. If you want a Bok Globule to be significant enough to redden it, it would dim Sirius enough so that, at best, it would be around the 15th brightest star in the sky. We also don’t find any Bok Globules in that area of the sky, which hurts that argument significantly. Therefore this idea, although intriguing, is widely rejected.
2.) Sirius’ binary companion. Sirius has a white dwarf companion, known as Sirius B. This star wasn’t always a white dwarf, however! The way most Sun-like stars work is that they burn their fuel until they’re exhausted, and then they expand tremendously and become red giants.
If Sirius B was a red giant 2000 years ago, then the Sirius system would have appeared extremely bright and red! But there’s a tremendous problem: red giants live too long! Once a star is done being a red giant, it blows off its outer layers into a planetary nebula, and the center collapses into a white dwarf, like the Cat’s Eye Nebula, below.
There is no evidence for a planetary nebula, and the timescale for this process to happen is measured in the millions of years, not the hundreds or thousands. Therefore, this idea, although incredibly interesting, cannot be correct.
3.) Maybe Sirius is a trinary star system. This would actually work, and — perhaps most importantly — it’s observationally feasible! How, you ask? Imagine the Red Giant case from case #2. If there were another, denser star close to Sirius B (such as a neutron star or black hole), it could start stealing mass from the Red Giant!
This process could be extraordinarily quick, and could feasibly strip Sirius B bare in just a few thousand years, leaving behind a cold, dead, white-dwarf core.
We don’t yet have the technology to resolve Sirius B to the accuracy necessary to determine whether there is a third star close enough in to account for this, but what an interesting possibility!
It’s very easy to dismiss someone else’s observations, and most scientists do dismiss this ancient set of claims that Sirius was once red. But that doesn’t mean it isn’t worth taking a look at; if we do, in fact, discover a trinary companion to Sirius A and B, we’ll be able to point to these ancient observations and actually verify that they were valid! It’s definitely an outside-the-box claim, but this is one that may turn out to be right!