Sunday, 10 July 2022

Does the Nebra Disc represent a Bronze Age supernova?

Discovered with a hoard of Bronze Aged weapons in 1999 by two metal-detectorists in a prehistoric enclosure in the Ziegelroda Forest, 60 km west of Leipzig in the German State of Saxony-Anhalt, and subsequently traded several times on the black market before coming to the notice of archaeologists, the Nebra Disc is now considered to be one of the oldest known depictions of celestial objects. 

The disc depicts a cluster of stars, which can be confidently identified as the Pleiades flanked by a disc interpreted as the Sun and a crescent interpreted as a waning Moon, against a starry background with a blue-green patina. Additional arcs are present on two edges of the disc, although these are now thought to have been added later. Organic material found with the disc has been carbon dated to 1600-2000 BC, while material adhered to one of the swords found with it has been dated to between 1600 and 1560 BC. This gives an estimated burial time for the object, although the disc is likely to be older. Its style of manufacture has been linked to the Unetice Culture, which was found in Central Europe between about 2300 and 1600 BC. 

X-ray fluorescence trace element analysis of the metals used in the disc has suggested that the copper used in its manufacture originated from  Bischofshofen in Austria, the gold of the outer crescents (thought to have been added later) probably came from the Carpathian Mountains, while the gold of the original stars and discs probably came from Cornwall. 

The disc was clearly a high value object, with considerable effort and resources going into its manufacture, and therefore is likely to have had specific meaning for its makers, but that meaning remains obscure to us today. The objects at its centre have clearly been chosen carefully, yet their conjunction is highly unlikely. A waning Moon would not occur close to a Sun-disc in nature; the Moon does occasionally pass close to, or even in front of, the Sun, but is always in the New Moon (invisible) phase, as all the phases of the Moon are driven by reflected sunlight. This has led to the conclusion that the selection of objects is purely symbolic in nature, not directly related to a specific observation.

In a paper published in the Journal of Anthropological and Archaeological Sciences on 24 March 2022, Rosario Gianluca Pizzone and Roberta Spartá of the Laboratori Nazionali del Sud at the Istituto nazionale di fisica nucleare, make the proposal that the large disc seen on the Nabra Disc does not in fact represent the Sun, but is instead a depiction of a supernova explosion.

The Nebra disc as it appears now (on display in Halle Museum of Prehistory). The Pleiades cluster shows up in the top part, amid the golden disc and the waning Moon. The solar arcs on the right and bottom of the picture were added in a post-construction phase (see text for details). Pizzone & Spartá (2022).

Pizzone and Spartá note that the large gold disc is depicted in a part of the sky (the Auriga-Taurus astersim) known to be very active in terms of stellar formation and supernova explosions (the two are connected, with the largest stars, which are the ones that undergo such explosions, having very short lives). This area contains a number of supernova remnants, and is home to the only historically recorded supernova explosion, which led to the formation of the Crab Nebula in 1054 and was recorded by Chinese scholars.

Armed with this knowledge, Pizzone and Spartá conducted a survey of the Auriga-Taurus asterism, looking for young supernova remnants which might be associated with the event depicted on the Nebra Disc. This survey uncovered five such young supernovae. The first of these, SH2 224, is estimated to be between 13 000 and 24 000 years old, and is located 4500 parsecs (1470 light years) from our Solar System. The second, SH2 221 (HB9) is estimated to be 4000-6600 years old and is roughly 800 parsecs (2600 light years) away. The third, Semeis 147, is thought to be 30 000-40 000 years old and is 1200-1500 parsecs (3900-4500 light years away). The fourth, the Crab Nebula, can be precisely dated at 966 years old, and is 2000 parsecs (6500 light years) away. The final object, IC443, is between 3000 and 30 000 years old, and located 1500 parsecs (4500 light years) from us.

Pizzone and Spartá observe that one of these supernovae, SH2 221 (HB9) has a lower age range which coincides roughly with the possible creation of the Nebra Disc. They estimate, from the size of this object and its estimated distance, that at its peak this explosion would have been as bright as the Full Moon, a striking event which would likely have been seen as being of great importance to any Bronze Age European observers. 

They further note that several stone carvings, of roughly the same age, in the Burzahom archaeological site in Srinagar, Jammu and Kashmir, India, depict stick figures observing two light-emitting objects in the sky. The precise meaning of these has also been debated, with the objects possibly being the Sun and Moon or two stars, but could also conceivably represent a supernova explosion close in the sky to the Moon.

Photograph of a stone carving from Burzahom, and drawing of the same. Joglekar et al. (2007).

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