SN 1054 - Modern Identification of The Supernova

Modern Identification of The Supernova

In modern times, the star of 1054 was identified as a supernova between the 1920s and the 1940s.

In 1921, Carl Otto Lampland was the first to announce that he had seen changes in the structure of the Crab Nebula. This announcement occurred at a time when the nature of the nebulas in the sky was completely unknown. Their nature, size and distance was subject to debate. Observing changes in such objects allows astronomers to determine if their spatial extension is “small” or “large”, in the sense that notable changes in an object as vast as our Milky Way cannot be seen over a small time period, such a few years, while such changes are possible if the size of the object does not exceed a few light-years. Lampland's comments were confirmed some weeks later by John Charles Duncan, an astronomer at the Mount Wilson Observatory. He benefitted from photographic material which has not changed since 1909 and in fact made the comparison with older snapshots easy, highlighting a general movement of expansion of the cloud. The points were moving away from the centre, and did so faster as they got further from the centre.

Also in 1921, Knut Lundmark compiled the data for the “guest stars” mentioned in the Chinese chronicles known in the West. He based this on older works, having analysed various sources such as the Wenxian Tongkao, studied for the first time from an astronomical perspective by Jean-Baptiste Biot in the middle of the 19th century. Lundmark gives a list of 60 suspected novae, the generic term for a stellar explosion, in fact covering two defined phenomena, novae and supernovae. The nova of 1054, already mentioned by the Biots in 1843,' is part of the list. It stipulates the location of this guest star in a note at the bottom of the page as being “close to NGC 1952”, one of the names for the Crab Nebula, but it does not seem to create an explicit link between them.

In 1928, Edwin Hubble was the first to note that the changing aspect of the Crab nebula, which was growing bigger in size, suggests that it is the remains of a stellar explosion. He realised that the apparent speed of change in its size signifies that the explosion which it comes from occurred only nine centuries ago, which puts the date of the explosion in the period covered by Lundmark's compilation. He also noted that the only possible nova in the region of the Taurus constellation (where the cloud is located) is that of 1054, whose age is estimated to correspond precisely to an explosion dating from the start of the second millennium. Hubble therefore deduced, correctly, that this cloud was the remains of the explosion which was observed by Chinese astronomers.

Hubble's comment remains relatively unknown as the physical phenomenon of the explosion was not known at the time. Eleven years later, when the fact that supernovae are very bright phenomena was highlighted by Walter Baade and Fritz Zwicky and when their nature was suggested by Zwicky Nicholas Mayall proposed that the star of 1054 was actually a supernova, based on the speed of expansion of the cloud, measured by spectroscopy, which allows astronomers to determine its physical size and distance, which he estimated at 5000 light-years. This was under the assumption that the velocities of expansion along the line of sight and perpendicularly to it were identical. Based on the reference to the brightness of the star which featured in the first documents discovered in 1934, he deduced that it was a supernova rather than a nova.

This deduction was subsequently refined, which pushed Mayall and Jan Oort in 1942 to analyse historic accounts relating to the guest star more closely (see section Compilation of Historic Accounts below). These new accounts, globally and mutually concordant, confirm the initial conclusions by Mayall and Oort in 1939 and the identification of the guest star of 1054 is established beyond all reasonable doubt. Most other historical supernovas are not confirmed so conclusively: supernovas of the first millennium (SN 185, SN 386 and SN 393) are established on the basis of a single document each, and so they cannot be confirmed; in relation to the supposed historical supernova which followed the one in 1054, SN 1181, there are legitimate doubts concerning the proposed remnant (3C58) and an object of less than 1000 years of age. Other historical supernovae of which there are written accounts which precede the invention of the Telescope (SN 1006, SN 1572 and SN 1604) are however established with certitude.

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