SN 1054
SN 1054, the Crab Supernova, is a supernova that was first observed on, and remained visible until.
The event was recorded in contemporary Chinese astronomy, and references to it are also found in a later Japanese document and in a document from the Islamic world. Furthermore, there are a number of proposed references from European sources recorded in the 15th century, as well as a pictograph associated with the Ancestral Puebloan culture found near the Peñasco Blanco site in New Mexico, United States. The pyramids at Cahokia in the midwestern United States may have been built in response to the supernova's appearance in the sky.
The remnant of SN 1054, which consists of debris ejected during the explosion, is known as the Crab Nebula. It is located in the sky near the star Zeta Tauri. The core of the exploding star formed a pulsar, called the Crab Pulsar. The nebula and the pulsar that it contains are some of the most studied astronomical objects outside the Solar System. It is one of the few Galactic supernovae where the date of the explosion is well known. The two objects are the most luminous in their respective categories. For these reasons, and because of the important role it has repeatedly played in the modern era, SN 1054 is one of the best known supernovae in the history of astronomy.
The Crab Nebula is easily observed by amateur astronomers thanks to its brightness, and was also catalogued early on by professional astronomers, long before its true nature was understood and identified. When the French astronomer Charles Messier watched for the return of Halley's Comet in 1758, he confused the nebula for the comet, as he was unaware of the former's existence. Motivated by this error, he created his catalogue of non-cometary nebulous objects, the Messier Catalogue, to avoid such mistakes in the future. The nebula is catalogued as the first Messier object, or M1.
Identification of the supernova
The Crab Nebula was identified as the supernova remnant of SN 1054 between 1921 and 1942, at first speculatively, with some plausibility by 1939, and beyond reasonable doubt by Jan Oort in 1942.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 nebulae in the sky was completely unknown. Their nature, size and distance were subject to debate. Observing changes in such objects allows astronomers to determine whether their spatial extension is "small" or "large", in the sense that notable fluctuations to an object as vast as our Milky Way cannot be seen over a small time period, such as a few years, whereas such substantial changes are possible if the size of the object does not exceed a diameter of a few light-years. Lampland's comments were confirmed some weeks later by John Charles Duncan, an astronomer at the Mount Wilson Observatory. He benefited from photographic material obtained with equipment and emulsions that had not changed since 1909; as a result the comparison with older snapshots was easy and emphasized a general expansion of the cloud. The points were moving away from the centre, and did so faster as they got further from it.
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, then the generic term for a stellar explosion, in fact covering what is now understood as two distinct 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 Taurus is that of 1054, whose age is estimated to correspond 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 remained 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. 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 are established on the basis of a single document each, and so they cannot be confirmed. Other historical supernovae of which there are written accounts which precede the invention of the telescope are however established with reasonable certitude. Telescope-era supernovae are of course associated with full certitude with their remnant, when one is observed, but none is known within the Milky Way.
Historical records
SN 1054 is one of eight supernovae in the Milky Way that can be identified because written testimony describing the explosion has survived. In the nineteenth century, astronomers began to take an interest in the historic records. They compiled and examined the records as part of their research on recent novae, comets, and later, the supernovae.The first Westerners to attempt a systematic compilation of records from China were the father and son Biot. In 1843, the sinologist Édouard Biot translated for his father, the astronomer and physicist Jean-Baptiste Biot, passages from the astronomical treatise of the 348-volume Chinese encyclopaedia, the Wenxian Tongkao.
Almost 80 years later in 1921, Knut Lundmark undertook a similar effort based on a greater number of sources. In 1942, Jan Oort, convinced that the Crab Nebula was the "guest star" of 1054 described by the Chinese, asked sinologist J.J.L. Duyvendak to help him compile new evidence on the observation of the event.
Chinese astronomy
Star-like objects that appeared temporarily in the sky were generically called "guest stars" by Chinese astronomers. The guest star of 1054 occurred during the reign of the Emperor Renzong of the Song dynasty. The relevant year is recorded in Chinese documents as "the first year of the Zhihe era". Zhihe was an era name used during the reign of Emperor Renzong, and corresponds to the years 1054–1056, so the first year of the Zhihe era corresponds to the year 1054.Some of the Chinese accounts are well preserved and detailed. The oldest and most detailed accounts are from Song Huiyao and Song Shi, historiographical works of which the extant text was redacted perhaps within a few decades of the event. There are also some later records, redacted in the 13th century, which are not necessarily independent of the older ones.
Three accounts are apparently related because they describe the angular distance from the guest star to Zeta Tauri as "perhaps several inches away", but they are in apparent disagreement about the date of appearance of the star. The older two mention the day jichou 己丑, but the third, the Xu Zizhi Tongjian Changbian, the day yichou 乙丑. These terms refer to the Chinese sexagenary cycle, corresponding to numbers 26 and 2 of the cycle, which corresponds, in the context where they are cited, respectively, to 4 July and 10 June.
As the redaction of the third source is of considerably later date and the two characters are similar, this is easily explained as a transcription error, the historical date being jichou 己丑, 4 July.
The description of the guest star's location as "to the south-east of Tianguan, perhaps several inches away" has perplexed modern astronomers, because the Crab Nebula is not situated in the south-east, but to the north-west of Zeta Tauri.
The duration of visibility is explicitly mentioned in chapter 12 of Song Shi, and slightly less accurately, in the Song Huiyao. The last sighting was on 6 April 1056, after a total period of visibility of 642 days.
This duration is supported by the Song Shi. The Song Huiyao by contrast mentions a visibility of the guest star of only 23 days, but this is after mentioning visibility during daylight. This period of 23 days applies in all likelihood solely to visibility during the day, which naturally was much shorter.
Identity of ''Tianguan''
The asterisms of Chinese astronomy were catalogued around the 2nd century BC. The asterisms with the brightest stars in the sky were compiled in a work called Shi Shi, which also includes Tianguan.Identification of Tianguan is comparatively easy, as it is indicated that it is located at the foot of the Five Chariots asterism, the nature of which is left in hardly any doubt by representation on maps of the Chinese sky: it consists of a large pentagon containing the bright stars of the Auriga. As Tianguan is also represented to the north of the Three Stars asterism, the composition of which is well known, corresponding to the bright stars of Orion, its possible localisation is strongly restricted to the immediate proximity of the star ζ Tauri, located between "Five Chariots" and "Three Stars". This star, of medium brightness, is the only star of its level of brightness in this area of the sky, and therefore the only one likely to figure among the asterisms of "Shi Shi". All of these elements, along with some others, allow "Tianguan" to be confirmed beyond reasonable doubt as corresponding to the star ζ Tauri.