Jul 042015
 

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On this date in 1054 a supernova (SN 1054) was first noted by Chinese, Japanese, Arab, and possibly Puebloan observers near the star Zeta Tauri. For several months it remained bright enough to be seen during the day. Its remnants form the Crab Nebula according to modern astronomy, although some debate lingers on this point because the Chinese accounts (relatively accurate, but not entirely interpretable now because of the difficulty of understanding the contemporary Chinese measurement system), place the supernova in a position that is not fully compatible with that of the Crab Nebula.

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The core of the exploding star formed a pulsar, called the Crab Pulsar (or PSR B0531+21). The nebula and the pulsar it contains are 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 the best known supernova in the history of astronomy.

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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. Due to this error, he created his catalog of non-cometary nebulous objects, the Messier Catalogue, to avoid such mistakes in the future. The nebula is cataloged as the first Messier object, or M1.

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The Crab Nebula was identified as the supernova remnant of SN 1054 between 1921 and 1942, at first speculatively (1920s), with some plausibility by 1939, and with virtual certainly 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 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 the Milky Way cannot be seen over a small time period, such as 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 benefited from photographic material obtained with equipment and emulsions which had not changed since 1909, which made the comparison with older snapshots easy, highlighting a general expansion of the cloud. The points were moving away from the center, and did so faster as they got further from the center.

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 analyzed 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. 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 realized 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 to an explosion dating from the start of the second millennium. Hubble therefore deduced that this cloud was the remains of the explosion which was observed by Chinese astronomers.

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In Chinese annals stars which appeared temporarily in the sky were generically called “guest stars” (kè xīng 客星) by Chinese astronomers. The guest star of 1054 occurred during the reign of the Emperor Renzong of the Song dynasty (960–1279). 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 C.E., so the first year of the Zhihe era corresponds to the year 1054.

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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, which corresponds, in the context where they are cited, to 4 July.

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The location of the guest star 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. According to the Song Huiyao the visibility of the guest star was for 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.

The Song Huiyao recounts the observation of the guest star, by focusing on the astronomical aspect, but gives more important information relating to the visibility of the star, by day and by night.

Zhihe era, first year, seventh lunar month, 22nd day. […] Yang Weide declared: “I humbly observe that a guest star has appeared; above the star there is a feeble yellow glimmer. If one examines the divination regarding the Emperor, the interpretation [of the presence of this guest star] is the following: The fact that the star has not overrun Bi and that its brightness must represent a person of great value. I demand that the Office of Historiography is informed of this.” All officials congratulated the Emperor, who ordered his congratulations be [back] forwarded to the Office of Historiography. First year of the era of Jiayou, third lunar month, the director of the Astronomical Office said “The guest star has disappeared, which means the departure of the host [that it represents].” Previously, during the first year of the Zhihe era, during the fifth lunar month, it had appeared at dawn, in the direction of the east, under the watch of Tiānguān (天關, Zeta Tauri). It had been seen in daylight, like Venus. It had rays stemming in all directions, and its color was reddish white. Altogether visible for 23 days.

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I gave my recipe for crab sandwiches here https://www.bookofdaystales.com/tony-hancock/ which you could make today. But since 4th July is a big day for outdoor cooking in the U.S., I recommend a traditional crab boil for celebrating the Crab Nebula. It is simplicity itself. You will need a galvanized bucket or tub (well scrubbed) and a good campfire of hot coals – I did mine in my fire pit in my garden in New York. The essentials are that you bring salted water in the tub to the boil over the fire and then add live crabs, corn on the cob, small new potatoes (unpeeled), and crab boil seasoning of your choice (such as Old Bay or Zatarain’s). In Louisiana they sometimes also add andouilli sausage, onions and/or crayfish – basically your choice. Add what you want.

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Bring the water back to the boil and cook for 10 minutes, or until the crabs are bright red. Strain off the water and dump the boil mix on to newspaper on your picnic table, and dig in. You can use hammers or nutcrackers for the tricky bits of the crabs. Eat everything with your fingers using nut picks, or similar, to dig out the crab meat from inaccessible places.. The boil seasoning gets all over your hands and, therefore seasons the meat and vegetables as you eat. I’m getting hungry (and missing Louisiana) !!

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