Why was the brightest "star" in the night sky regarded as a "disaster star" by ancient people?

Author | Wang Siliang

Review | Dong Chenhui

Editor | Zhao Jingyuan

If we look toward the southern sky in the winter, we will notice an unusually bright star following closely behind the recognizable constellation of Orion. That is the famous Sirius, whose English name is Sirius. Sirius is located in the constellation Canis Major and is the brightest star in the night sky and one of the most familiar stars to people.

Sirius in the winter night sky (Photo credit: Stellarium)

In ancient China, the ancients regarded it as a "disaster star" and believed that it foreshadowed aggression. Sirius is located in the southern sky. To understand what the Northern Song Dynasty writer Su Dongpo said, "Look northwest and shoot Sirius", you need to understand it in conjunction with the arc and arrow star officials in the figure below - unlike the map, the directions in the sky are north at the top, south at the bottom, east at the left, and west at the right. The "wolf" in the figure below is Sirius, which is located in the northwest direction of the arc and arrow star officials. This scene is like someone drawing a bow and shooting in the northwest direction, and his target is Sirius.

Sirius and Arcus (Source: Stellarium)

Sirius has an apparent magnitude of about -1.45, and is so bright that it can be seen with a telescope even at noon. When observing on a dark night, move the telescope towards Sirius, and before Sirius enters the field of view, you can feel a glare gradually spreading from the edge of the field of view until the dazzling light enters it.

Under good observation conditions, Sirius usually appears bright blue-white. When it is low in the sky, the light has to pass through a thicker atmosphere and is seriously affected by atmospheric turbulence, so Sirius will flash in various colors. Many astronomy enthusiasts use this principle to take a novel photo of Sirius: defocus the low-altitude Sirius, then take photos of it and superimpose it, and the resulting photos of Sirius show rainbow colors like the flashing gemstones (such as reference 9).

Interestingly, in some accounts of ancient Western literature, Sirius seems to look red - many ancient writers describe it as "ruddy", "reddish", "hot as fire" (such as Seneca, Cicero, Horace), and even Ptolemy, the master of ancient Greek astronomy, described it this way. There is a theory that the companion star of Sirius may not be a white dwarf 2,000 years ago, but in the red giant stage, and bright enough to affect the overall color perception of the naked eye, but this hypothesis lacks strong evidence to support it, because the time scale required for the evolution from a red giant to a white dwarf is much longer than 2,000 years. Others believe that these documents are wrong, and almost contemporary Chinese ancient books and some Western ancient books all record that Sirius is blue-white (such as Sima Qian's "Records of the Grand Historian·Book of Celestial Officials" said that Venus is "whiter than the wolf", and the "wolf" here refers to Sirius). Maybe Ptolemy and others described the Sirius mentioned above that flickers in the low sky? We don't know.

Sirius and its companion stars (Photographer: Gabriela and Fabio Carvalho)

Sirius is 8.7 light-years away from Earth, but it is the eighth closest known star (the first seven are the Sun, Proxima Centauri, Alpha Centauri A, Alpha Centauri B, Barnard's Star, CN Leo, and HD 95735). Among all the stars visible to the naked eye, it is the closest one except the Sun and the Alpha Centauri system. Because Sirius moves in the same direction as the Ursa Major Moving Group, it was once considered a member of the group, but a 2003 study pointed out that Sirius is relatively young and therefore probably does not belong to the group.

Sirius is an A1-type main sequence star with a luminosity of about 23 times that of the Sun, a diameter of 1.7 times that of the Sun, and a mass of 2 times that of the Sun. The surface temperature is about 10,000 Kelvin, and the core temperature exceeds 20 million Kelvin. Its proper motion is about 1.326 arc seconds per year, and its proper motion angle is 204°. In the past 2,000 years, its position on the celestial sphere has changed by 44 arc minutes, equivalent to 1.5 full moon diameters. The discovery of the proper motion of stars was first announced in 1718 by the British astronomer Edmund Ray, who noticed that the positions of Sirius, Arcturus, and Aldebaran were significantly different from those given by the ancient Greek astronomer Hipparchus.

The companion of Sirius, commonly known as Sirius B, or the "Puppy Star", has been one of the most interesting objects since its discovery and is one of the most massive white dwarfs discovered so far. Its magnitude is about 8.65, and its distance from Sirius varies periodically, ranging from about 3 arc seconds to about 11.5 arc seconds in about 50 years. Although objects as dim as magnitude 10 can be easily identified using small and medium-sized telescopes, it is still an amateur astronomical challenge to identify Sirius B, which is usually drowned in the overwhelming light of Sirius. This usually requires excellent atmospheric conditions, a telescope with excellent optical quality, and correct observation skills. In 2022, the author used the LZOS 105F6.2 with a 3mm eyepiece (217x) to resolve Sirius B in the equatorial tracking mode. After careful focusing to minimize the diffraction ring, he finally succeeded.

The position of Sirius binary star (Source: The Cambridge Double Star Atlas)

If you want to observe Sirius B, Xiaoxing recommends combining the azimuth diagram above for identification or confirmation. Since a certain magnification is required for stable observation, it is recommended that the telescope use electric tracking. If you use a Newtonian reflecting telescope for observation, you should carefully adjust the optical axis in advance and pay attention to the angle to avoid the companion star being disturbed by the starlight caused by the secondary mirror bracket. You can also refer to the practice of foreign enthusiasts and use a sticker to block the light of the main star at the end of the eyepiece near the objective lens, which will make the companion star easier to distinguish. The above figure shows the azimuth and angular distance of the Sirius system under the inverted image, with the bottom being north (PA=0°) and the right being east (PA=90°). In 2025, Sirius B will have the largest angular distance from the main star, when it will be located northeast of the main star. The last closest angular distance occurred in 1975, so the last few years are the best time to observe Sirius B. Interested enthusiasts must not miss it.

References

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