A medieval manuscript may be hiding a record of an impending nova

About every 80 years, the faint 10th magnitude star in the Corona Borealis constellation increases dramatically in brightness. The star, T CrB, is known as a repeating nova and last exploded in 1946, peaking at magnitude 2.0, temporarily making it one of the 50 brightest stars in the night sky.

Apart from the 1946 explosion, the only other confirmed explosion of this star was in 1866. arXiv preprint assistant by dr. Bradley Schaefer suggests that a medieval monk may have seen T CrB shine in 1217.

In medieval monasteries, monks kept regular records—lists of significant events throughout the year. In 1217, the abbot of Ursberg Abbey (in southern Germany, west of Augsberg) was Burchard. In that year’s report, he wrote:

“During the fall of [1217], in the evening, a wonderful sign was seen in a certain star in the west. This star was a little to the west of the south, in a place that astrologers call Ariadne’s crown [Corona Borealis]. As we ourselves have noticed, at first it was a faint star which, for a time, shone with great brightness, and then returned to its previous dimness. There was also a bright ray reaching the sky, like a great long ray. This was seen many days in the fall. ”

But was this “wonderful sign” a nova, or one of the many other types of transient events that can grace the night sky?

Schaefer begins to eliminate the possibility that the event could have been a supernova, as any supernova is visible to the naked eye and the latter will leave a remnant that is easily seen. For example, the remnant associated with the supernova in 1054 is the Crab Nebula, easily visible with small telescopes.

Many old supernovae also have remnants associated with them (although sometimes the identification is uncertain, as the historical record was not precise enough in the object’s position in the sky). Since no such remnant is found in this region of the sky, Schaefer concludes that the explosion must not have been very dangerous.

Similarly, Schaefer thinks that a supernova is unlikely, as such an event would appear in several weeks. However, Burchard describes it as only visible for “several days” which is more in line with the average appearance of T CrB around seven days.

But was that the wrong sign of a bright planet? Also impossible, as the Corona Borealis is 45º away from the ecliptic and there is no eye planet – so it wanders far this plane of the solar system.

Maybe a comet? This hypothesis may be appropriate, as comets are much more common than novae. Another report from the monastery of Saint Stephani describes a comet that occurs once a year, but does not give any indication of what season or where in the sky it is.

Even the idea that this historical account described the star is doubtful, as the terminology used is unclear. The author described it as “stella comes” where “comes” is generally used as a subject of counting, although there is another example in the same report where the same phrase is used to describe another event that takes place in 1208, we link to it. bad luck. Instead, comets were generally described as “a star with a tail,” a “torch-like star,” or a “death star.” Therefore, the language used is not clear.

Another argument against the cometary hypothesis is the association of a good omen with the appearance of this star. Historically, comets were considered evil, associated with death and the fall of kingdoms.

Schaefer also discusses the possible sighting of T CrB in 1787. This possible sighting comes from a star catalog published in 1789 by the English astronomer Francis Wollaston. In it, Wollaston lists the star near T CrB coordinates. Although it does not mention the magnitude, the catalog has an estimated magnitude of 7.8 – meaning that, if the star really was T CrB, it should have been seen during the explosion.

Could Wollaston have made a mistake? It’s possible, but not impossible, Schaefer concludes. Wollaston mistakenly identified the star as one from the catalog by William Herschel: V 75. However, Herschel described this star as part of an arc of three stars and 1º from T CrB. This description does not match well with Wollaston’s coordinates and probably describes the star HD 143707. In fact, there are no other stars of the same size within the error range described by Wollaston.

Again, Schaefer considers and discounts other possibilities. He dismisses the comet as impossible, as Wollaston was a trained observer who was familiar with comets. Asteroids far from the ecliptic will never be as bright. The last supernova will remain a bright X-ray source until today. The error in the measurement of another star that gives such a precise alignment of the location of T CrB Schaefer estimates is about 8.5 in 10 million. Faced with another possibility, Schaefer concludes that Wollaston may have caught T CrB at the end of the outburst, recorded its location correctly, and misidentified it as the star V 75 from the Herschel catalog.

As for the next outburst of T CrB, the star has recently decreased in brightness, which was observed in 1945 about eight months before it exploded. If this behavior repeats, Schaefer predicts that the star should shine again in early spring 2024, becoming the brightest nova since CP Puppis erupted in 1942.