A rare stellar explosion that repeats itself about every 80 years may have been observed and details written down as early as the High Middle Ages. This was discovered by US astronomer Bradley Schaefer, who has been researching T Coronae Borealis for years. This is a so-called recurrent nova, which has so far been reliably observed in the years 1866 and 1946. Schaefer now believes that he has discovered evidence of observations in October 1217 and December 1787 in old records. The former in particular is of particular interest, as it would clearly push our first notice of the object into the past. The star’s next explosion is expected next spring.
Already observed four times
T Coronae Borealis (T CrB) is a binary system consisting of a red giant star and a white dwarf star. To the second, matter continuously flows from the larger of the two stars until a critical mass is reached. This is followed by explosive bursts of brightness, which ensure that the apparent brightness increases from around 10 mag – and therefore fainter than Neptune – to up to 2 mag – about as bright as the Polar Star. This has so far been confirmed with certainty in the years 1866 and 1946, and the next outbreak is, so to speak, imminent. A brightness increase observed in 2015 and later observations suggest another explosion next spring.
Given the expected eruption, Schaefer expects the star will receive attention from the public, the press and the astronomy community in the coming year, he writes now. This is also why he tried to find out whether we know about previous sightings of star explosions. He discovered a fairly clear one in a star catalog by the priest and astronomer Francis Wollaston. He observed the brighter star at least four times in the days before December 28, 1787. Because he couldn’t normally see it with his equipment, the astronomer writes that it must be the double star during a nova.
More compelling pro arguments
Much more exciting and difficult to verify, however, is a possible sighting of the Nova in 1217, more than 500 years earlier. Schaefer found the clues in a chronicle by the medieval historian Burchard von Ursberg. He reports that in the fall of 1217 a “miraculous sign was seen from a certain star in the west.” He himself saw how a faint star in the constellation Northern Crown suddenly became brighter and then became dimmer again. That clearly sounds like T CrB, which had to become brighter at exactly that time, writes Schaefer. At the same time, it could not be a supernova because none of the expected remains can be seen today.
As with his hypothesis about the possible observation in 1787, Schaefer summarizes arguments for and against the fact that the recurrent nova was observed in the monastery near Augsburg. He writes that the term for “star” is clearly used, but there are separate ones for the frequently observed comets. In addition, there is no indication that the object moved in the starry sky. Finally, the US astronomer points out that comets were considered a bad omen at that time. But the bright star is described in the most positive words. Overall, the description sounds exactly as we would expect. Schaefer’s analysis can be viewed online and will appear in the Journal for the History of Astronomy.
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