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 recurrent nova and last flared in 1946, peaking at magnitude 2.0, temporarily making it one of the 50 brightest stars in the night sky.
Apart from the 1946 flare, the only other confirmed sighting of this star’s outburst was in 1866. But new research from Dr. Bradley Schaefer suggests that a medieval monk may have spied T CrB clears up in 1217.
In medieval monasteries, monks regularly made chronicles, a list of notable events that happened throughout the year. In 1217, the abbot of Ursberg Abbey (in southern Germany, west of Augsberg) was Burchard. In the chronicle of that year, he wrote:
In the fall season of [1217], early in the evening, a wonderful sign was seen in a certain western star. This star was a little west of the south, in what astrologers call Ariadne’s Crown [Corona Borealis]. As we ourselves have observed, it was originally a faint star which, for a time, shone with great light, and then returned to its original faintness. There was also a very bright ray that reached the sky, like a great high beam. This was seen for many days that fall
But was this “wonderful sign” a nova, or one of many other types of transient events that could grace the night sky?
Schaefer first rules out the possibility that the event could have been a supernova, since any recent supernova visible to the naked eye would leave an easily detectable remnant. For example, the remnant associated with a supernova in 1054 is the Crab Nebula, easily visible even with small telescopes. Several older supernovae also have associated remnants (although the identification is sometimes uncertain, as the historical record was not precise enough about the object’s location in the sky). Since no such remnants are found in this region of the sky, Schaefer concludes that the eruption must not have been particularly destructive.
Similarly, Schaefer considers a supernova unlikely, as such an event would have been visible for several weeks. However, Burchard describes it as only being visible for “many days”, which is more in line with T CrB’s average visibility of about 7 days.
But was the sighting a misidentification of a bright planet? Also impossible, since the Corona Borealis is 45º away from the ecliptic and no planet to the naked eye is so far from this plane in the solar system.
Maybe a comet? This hypothesis has some merit, since comets are more common than these novae. Another chronicle of the monastery of Sant Esteve describes a possible comet in the same year, but gives no indication as to what season or where in the sky.
Even the idea that this other chronicle described a comet is in doubt, as the terminology used is vague. The author described him as a “ve star” where “comes” is generally used as a title for an earl, although there is another instance in the same chronicle where the same phrase is used to describe another transitory event in 1208, associating it with a bad omen. Rather, comets are generally described as a “tail star”, “torch-like star” or a “death star”. Therefore, the language used is ambiguous at best.
Another argument against the comet hypothesis is the association of a positive omen with the appearance of this star. Historically, comets were taken as negative omens, associated with death and the fall of kingdoms.
Schaefer also mentions a 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 a star close to the coordinates of T CrB. Although it does not specify a magnitude, the catalog has a magnitude limit of 7.8, meaning that if the star was indeed T CrB, it should have been observed during an outburst.
Could Wollaston have made a mistake? Possible, but unlikely, concludes Schaefer. Wollaston did incorrectly identify the star as one from a catalog by William Herschel: V 75. However, Herschel described this star as part of an arc of three stars and 1º of ? CrB. This description does not match Wollaston’s coordinates well and most likely describes the star HD 143707. In fact, there are no other stars of a similar magnitude within the error range described by Wollston.
Again, Schaefer considers and discounts other possibilities. He dismisses a comet as unlikely, as Wollaston was a trained observer who was familiar with comets. Asteroids this far from the ecliptic could never be that bright. A recent supernova would remain a bright X-ray source to this day. An error in the measurement of another star that gives such precise coordinates for the location of T CrB Schaefer estimates about 8.5 in 10 million. Faced with no viable alternative, Schaefer concludes that Wollaston probably caught T CrB at the end of an outburst, recorded its position accurately, and misidentified it as the star V 75 in Herschel’s catalog.
As for the next eruption of T CrB, the star recently began to decrease in brightness, which was observed to happen in 1945 about 8 months before its eruption. If this behavior occurs again, Schaefer predicts the star should shine again in the early spring of 2024, becoming the brightest nova since then CP puppies broke out in 1942.
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