About every 80 years, a faint 10th-magnitude star in the constellation Corona Borealis dramatically increases in brightness. This star, T CrB, is known as a recurring nova and its last eruption occurred in 1946, peaking at 2.0, making it temporarily one of the 50 brightest stars in the night sky. Aside from the 1946 explosion, the only confirmed time the star exploded was in 1866. But new research published on the arXiv preprint server by Dr. Bradley Schaefer suggests that a medieval monk may have observed the enlightenment of T CrB in 1217. In medieval monasteries, monks regularly kept a chronicle, a list of notable events that occurred during the year. In 1217, the abbot of Ursberg Monastery (in southern Germany, west of Augsberg) was Burchard. In the chronicle of that year, he wrote: “In the autumn of [1217], in the early evening, a wonderful sign was observed in a certain star in the west. This star was located a little west of the south, where the Astrologers call the crown of Ariadne [Corona Borealis]. As We have observed, it is initially a faint star, for a time shines brightly, then returns to its original weakness. In addition There was also a very bright ray of light pointing up into the sky, like a tall beam of light. This was observed for several days that fall. »

But is this “wonderful sign” a nova or one of the many other types of transient events that can adorn the night sky? Schaefer first ruled out the possibility that the event could be a supernova, because any recent supernova visible to the naked eye would have left behind easily detectable remnants. For example, the remnant associated with the supernova of 1054 is the Crab Nebula, which is easily visible even with a small telescope. Some older supernovae have also been associated with remnants (although identification is sometimes uncertain because historical records are not precise enough about the object’s location in the sky). Since no remains were found in this region of the sky, Schaefer concluded that the eruption must not have been particularly destructive. Likewise, Schaefer thinks a supernova is unlikely because such an event would be visible for several weeks. However, Burchard described it as being visible for only “a few days”, which is more consistent with T CrB’s average visibility of about seven days. The 2° star field is centered on T CrB, revealing the inherent uncertainty in a given star’s coordinates as well as its star sign and distance from ? CrB identified HD 143707 as a likely candidate for the Herschel star and T CrB as the most likely candidate for the Wollaston star. Taken from Schaefer’s article. Provider: arXiv (2023). DOI: 10.48550/arxiv.2308.13668 But is this observation a misidentification of a bright planet? It’s also impossible, because Corona Borealis is 45 degrees from the ecliptic plane and no naked-eye planet can extend that far from this plane of the solar system.

Perhaps a comet? This hypothesis has some merit, as comets are more frequent than such novae. Another chronicle from Saint Stephani monastery describes a possible comet in the same year, but does not give any indication as to what season or where in the sky. Even the notion that this other chronicle did describe a comet is in doubt, as the terminology used is vague. The author described it as a “stella comes” where “comes” is generally used as a title for a Count, although there is another instance in the same chronicle where the same phrase is used to describe another transient event in 1208, associating it with an ill omen. Instead, comets are often described as “tail stars”, “torch stars” or “death stars”. So the language used is vague to say the least. Another argument against the comet hypothesis is the connection between a positive omen and the appearance of this star. Historically, comets were considered a bad omen, associated with death and the fall of kingdoms. Schaefer also mentioned a possible observation of T CrB in 1787. This potential observation comes from a star catalog published in 1789 by British astronomer Francis Wollaston. In it, Wollaston listed a star near coordinates T CrB. Although it does not specify a magnitude, the catalog has a limiting magnitude of 7.8, meaning that if the star is indeed T CrB it should have been observed during the flare. Could Wollaston have made a mistake? It’s possible, but unlikely, Schaefer concluded. Wollaston incorrectly identified this star as the star in William Herschel’s catalog: V 75. However, Herschel described this star as part of an arc of three stars and 1° of T CrB. This description does not closely match Wollaston’s coordinates and most likely describes the star HD 143707. Indeed, no other star with a similar luminosity falls within the error range described by Wollston.

Again, Schaefer considered and eliminated other possibilities. He dismissed the possibility of a comet because Wollaston was a skilled observer familiar with comets. Asteroids far away from the ecliptic can never be that bright. A recent supernova remains a bright source of X-rays today. The error in the measurement of another star that gave such precise coordinates for the position of T CrB Schaefer is estimated to be about 8.5 10 million. Faced with no viable alternative, Schaefer concluded that Wollaston could have captured T CrB at the end of an eruption, recorded its location accurately, and mistakenly identified it as the star. star V 75 in Herschel’s catalog. As for the upcoming flare of T CrB, the star has recently begun to lose brightness, which was observed in 1945, about 8 months before the flare. If this phenomenon repeats, Schaefer predicts the star will shine again in early spring 2024, becoming the brightest nova since CP Puppis erupted in 1942.

Source: 1217 AD, arXiv (2023). DOI: 10.48550/arxiv.2308.13668