THE SOUTHWORTH PLANETARIUM
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
2021-2022: CXLVI
"Anybody who considers protocol unimportant has never dealt with a cat." -Robert Heinlein
THE DAILY ASTRONOMER
Thursday, July 7, 2022
The Red Sirius Mystery
Dead people tell no tales; which makes them horrid conversationalists. Extracting information from those folks who've advanced to the next plane is exceedingly difficult. Consequently, those still milling about on sphere three must sometimes try to fathom the phrases and ideas of those who've moved on. Such interpretations are not easy and, more often than not, remain frustratingly elusive. We have to rely on our own ideas and theories that, though based on sound scientific principles, might well be misguided themselves.
This introduction brings us directly to the "Red Sirius Mystery," a particularly problematic sequence of Sirius observations that continue to confound astronomers. Sirius is well known as it is the night sky's brightest star. It represents either the nose or collar of Canis Major, the greater of Orion's two hunting dogs. One cannot currently observe as it remains too close to the Sun. Wait, however, until mid August, when it will reappear low in the southeastern pre-dawn sky. Sirius spotting is a sublime pleasure, for it glows white hot throughout the winter night. Therein lies the mystery! Sirius appears white. And, in fact, as it is an "A"* star, Sirius exhibits a blue-white color.
So, one wonders, why have some observers in the distant past referred to Sirius as "reddish?" Perhaps most notable amongst these observers is famed Greek astronomer Claudius Ptolemy (AD 90 – c. AD 168)**, who assigned a 'red' color to Sirius, just as he did to Betelgeuse, Antares, and a couple other stars which are genuinely red. A recurrence of this observation was found in De cursu stellarum ratio by Saint Gregory (538-594). Designed to assist in the timing of prayers, specifically as related to stellar risings and settings, this work also referred to Sirius' color as 'red.' However, to thicken the plot, Chinese astronomers of the same time period record it consistently as white.
Why would a white star, such as Sirius, appear red to anyone?
The short answer is, 'we don't know.'
The longer answer, which winds up in the same place as the shorter one, requires an unhealthy dose of astronomy. First, we know that Sirius has a companion called Sirius B, a white dwarf star. Well, the better term would be 'white dwarf' stellar remnant,' as a white dwarf forms after an active star concludes its core thermonuclear fusion process and casts away its outer layer as a planetary nebula: a gaseous shell rapidly expanding away from the white dwarf core. Our Sun, and all stars of comparable mass (up to 8 times as massive, generally) will transform into white dwarfs when their life cycles end. Before this transformation, however, the star will expand into the red giant stage.*** Once the fusion reactions end in the core, the outer layers are cast off as a planetary nebula. The remaining core becomes the white dwarf.
One might think that therein lies a solution: Sirius B might have been a red giant a couple thousand years ago and, as it was larger, it would have been more luminous and therefore more readily visible. Unfortunately, this 'solution' is problematic. Astronomers have already estimated the white dwarf's age as being about 124 million years.*** Besides, if Sirius B were a red giant 2,000 years ago, we should still see the resultant planetary nebula: the expanding gaseous layers becoming increasingly more rarefied through space. The Ring Nebula (Lyra the Harp) is one of the best known planetary nebula. It formed approximately 7000 years ago and as it is approximately 2300 light years away, we're seeing it just before its 5000th birthday. The nebula is still distinct and expanding. We would certainly see the nebula still, had it formed 2000 years ago.
Another possible solution relates to the atmosphere and how it distorts colors. One may well have noticed the ember shading of the rising moon and Sun. Such atmospheric 'coloring' can occur with stars, but generally only when the star is low on the horizon. The amount of atmospheric gases between an observer and outer space decreases with increasing altitude. i.e., it is greatest along the horizon. Dust particles are principally responsible for the reddening and the dust density along a sight line will also increase as one observes closer to the horizon due to the increased air amounts.) Here, we encounter another problem because Sirius would have been higher in Ptolemy's sky than for us.**** At its highest altitude, Sirius would have been almost halfway between the horizon and the zenith (see fourth footnote.) One cannot imagine that the discoloration would have been significant enough at this latitude to make Sirius appear red. It might have appeared somewhat discolored when it was low on either horizon, but Ptolemy's description seemed to indicate that it was intrinsically red: a description he would most likely have assigned to it only have repeated observations at various latitudes.
The notion that a hitherto unseen third body may have contributed to the coloration seems similarly implausible, as the Sirius system is quite close (8.4 light years) and any such body, or its remains, would have been discovered by now.
Some have suggested that the 'red' description is more metaphorical and observational. The 'red' being indicative of some sinister quality associated with the star, as opposed to an intrinsic aspect. Colors do have associations, of course, and perhaps the ancient astronomers were not above the occasional poetic phrasing. This answer might be more a reaction to exasperation than thoughtful supposition, as we haven't yet fathomed why anyone would have considered Sirius to be red. The notion that it was a Marxist sympathizer, an idea advanced by someone who thought the entire inquiry a pointless exercise that required an equally frivolous explanation, appeals richly to us. After all the astronomy we've attempted to do today, it is heartening for us to end the song on a stupid note.
*We won't spend an inordinate amount of time on this matter (perhaps, in some people's opinions, we already have). Astronomers have a nifty little letter sequence that indicates a star's spectral type or "color index." The sequence is O B A F G K M (Just think of the litigation-precipitating mnemonic "Oh, be a fine girl, kiss me.") The O stars are the hottest (effective temperatures higher than 59,000 F ==> 33,000 K) and exhibit a blue color; The M's are the red stars, and have the lowest effective temperature of the active stars, (down to 3800 F --> 2400 K). A stars, like Sirius, are among the hottest and have an apparent blue-white color. Although, it should be noted that the "white" color of most stars result from our own eye limitations. We cannot perceive color at low light levels, so most stars merely appear white. (Notable exceptions: Betelgeuse, in Orion, and Antares, in Scorpius, are extraordinarily large red supergiants and they both appear red in the sky.)
**We remember this fellow as the one who developed the Ptolemaic solar system model which placed Earth in the dead center and described all the planets, Sun and Moon and revolving around it. Though derided as fraudulent -for he apparently fabricated data to support his system- Ptolemy showed a high level of mathematical aptitude by creating a model that explained a planet's retrograde motion while retaining the two rigid Greek paradigms of a central Earth and perfectly circular planetary orbits.
***Determining stellar ages is hardly a trivial matter. Deducing the "cooling" age of a white dwarf isn't much easier. Essentially, they can know the white dwarf's mass because it is a component in a binary star system. (One can derive the mass of a binary star system-and its two members- if one knows the period and the separation distances.) Once this mass is known, astronomers can calculate the mass of the star that produced it, called the "progenitor." Through modeling, we can understand how hot a white dwarf star will become once created, as this relates directly to its mass. (White dwarf stars have different masses, up to about 1.4 times the mass of the Sun.) By comparing the assumed initial temperature with the current measured temperature, one can estimate the white dwarf''s age as the cooling rate is already known. (Cooling rates are a complex mess, too, but never mind.)
****Sirius' declination is -17 degrees, meaning that it is 17 degrees below the Celestial Equator, the projection of Earth's equator onto the sky. With this one piece of information, we can determine a star's maximum altitude in our sky, as that will occur whenever the star culminates (or crosses the meridian, which marks due south.) We're at latitude 43 degrees north (almost), so the Celestial Equator's highest altitude equals 90 - 43 = 47 degrees. Since Sirius is 17 degrees south of that line, its culmination angle will be 30 degrees. Ptolemy's home was Alexandria, Egypt (approx latitude 31 degrees N.) In Alexandria, the Celestial Equator passes 90 - 31 = 59 degrees above due south. Sirius would therefore have a culmination angle of 59 - 17 = 42 degrees.
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