Sirius is the brightest star in the night sky. It can be found in all its splendor in the winter skies of the northern hemisphere , more than twice as bright as the next brightest star in the sky: Canopus, which can be seen mainly from the southern hemisphere.
When Sirius rises above the horizon, it dominates over the rest of the stars that accompany it on its journey through the celestial vault. Only the Sun, the Moon, Venus, Jupiter and on rare occasions Mars shine brighter than it . For this reason, it has aroused the interest of scholars and the curious ever since we human beings began to direct our gaze to the night sky . If you go outside somewhere away from the light pollution of cities shortly after sunset and look to the southwest you might still be able to see it. In autumn and winter you could enjoy it practically all night.
If I asked you what color it is, you would probably answer white or, perhaps, bluish . And yet we have records from ancient astronomers who would disagree. Ptolemy described it in his Megale Syntaxis as reddish , along with other nearby stars that do have this color, such as Betelgeuse or Aldebaran . The Greek writer Aratus , born four centuries before Ptolemy, and even Seneca , also described her in this way.
In a dimmer star, flickering resulting from atmospheric distortion could lead to some uncertainty as to what color that star shines. But the brightest star in the night sky should not present this problem. If we compare Sirius with Betelgeuse (they are quite close in the sky), there is no doubt that the former has a bluish or whitish color while the latter shines with reddish and orange tones.
So it would seem that Sirius has changed its appearance during the last two thousand years . To try to understand why, let’s first understand what determines the color of a star. The main difference between Sirius, Rigel or Regulus and Betelgeuse, Arcturus or Antares is in their surface temperature. While the second group hardly reaches 4,000 ºC of temperature in its photosphere, the first group does not go below 9,500 ºC . These stars, and the rest of the stars in the universe, emit radiation as would what is known in physics as a “black body [link to “Stars and the relationship between their color and temperature”]”. This black body would be any object that only emits light due to its temperature , absorbing any light that reaches it and reflecting absolutely nothing.
The blackbody equations, which were derived in the late 19th century, tell us that the higher the temperature of the object, the more energetic the emitted light will be . Hence relatively cool stars emit light mainly in red and orange tones while hotter stars emit light mainly blue.
When they are in the main sequence , that is, in the phase of nuclear fusion of hydrogen and helium, the smallest stars tend to shine with a reddish hue and the most massive ones with a bluish hue . Intermediate stars, like our sun, shine yellowish or whitish. However, when the most massive stars leave this main sequence they can expand considerably . When this happens, even though their core reaches temperatures of millions of degrees, their surface cools, turning them into red giant stars .
Well, if we take the testimonies of these ancient thinkers as true, it would seem that Sirius has undergone some transformation during the last 20 centuries. Sirius is actually a binary system , located just over 8 light-years away (unbelievably close in astronomical terms). The largest star, the one responsible for the color we observe today, is Sirius A , a star about twice the mass of the Sun and slightly larger than the Sun, around which orbits a white dwarf , Sirius B , with a mass similar to that of our star but a diameter more similar to that of the Earth.
Therefore, one option for the change of color of Sirius would be the simple evolution of this pair of stars . It is known that, after their red giant phase, some stars completely shed their outer layers and become white dwarfs, objects in which nuclear fusion no longer occurs and that continue to shine due to the residual heat of their previous phase. This is in fact what we think will happen to the Sun in about 5 billion years . However, two thousand years is too short a period for the Sirius system to have undergone such a radical change in appearance, and such an event should have left a trail in the form of a nebula around the stellar pair .
Another option would be some kind of interaction with a third star , smaller and fainter than the two known ones, which would have temporarily given the set that color. But again, there is no proof that such a companion exists . And if it did exist, it would be too small a body (at most a few times the mass of Jupiter) and incapable of causing such significant changes in Sirius.
All this makes the mystery around the changing color of Sirius remain, a mystery that has been with us for several centuries and that could be due to laws and processes that we do not know, as well as to the error or poeticization of ancient thinkers.
REFERENCES:
C. Ceragioli, 1995, The Debate Concerning ‘Red’ Sirius, Journal for the History of Astronomy. 26 (3), doi:10.1177/002182869502600301
CB Whittet, 1999), A physical interpretation of the ‘red Sirius’ anomaly, Monthly Notices of the Royal Astronomical Society. 310 (2), doi:10.1046/j.1365-8711.1999.02975.x
