Monday 21 November 2016

It happened on the 4th of July 1054

No, not Independence Day! The Vikings had only just arrived in Newfoundland, let alone Columbus or Mayflower settlers. On this particular day chinese astronomers recorded the appearance of an unusually bright star in the sky. It was five times brighter than Venus and remained clearly visible even during daylight for 23 days. It then continued to remain visible to the naked eye at night for another two years. The carefully compiled chinese records are confirmed by later Japanese and Arab documents and it was also portrayed by the Pueblos and Mayas and accounts of it are still being told in Aboriginal legends. 

Strangely enough, no trace of this mysterious new star can be found in contemporary European literature, apart from some unreliable sources dated many centuries later. However, we have to consider that Europe was going through a period of disaster. Edward the Confessor's reign over England was weakened by internal struggles and local barons incessantly conspired to seize more power. Holy Roman emperor Henry III was constantly on the war path against the Hungarians, Flemings and Poles, neglecting his German homeland that lay in ruin. The Normans conducted violent raids against Byzantium. Only months earlier Pope Leo IX had excommunicated the Patriarch of Constantinople, bringing about the great schism between the Catholic and Orthodox churces, and had died soon afterwards, leaving also Rome in turmoil. 

The most likely reason why it wasn't mentioned is that many people in Europe must've believed that the star announced the end of the world. In the early Middle Ages it was generally thought that the universe had been created to last for 6.000 years (1.000 for every day of creation), 5.000 of which had already passed by the time Jesus was born. So anyone living in Europe around the year 1.000 would not have felt very much at ease and the sudden appearance of that new star must've struck many people with terrible fear.

But two years later the star had gone, seemingly without leaving a trace, and business in Europe and the rest of the world was still as usual. It was not until 1921 that scientists lay the connection between a strange nebula, discovered in 1731, and the events described in the chinese chronicles. This nebula, M1 or popularly nicknamed the Crab Nebula, happens to be the home of the strongest X-ray source that we observe in our sky: the Crab Nebula's pulsating radio star, or pulsar. It is what remains of a star so violently compressed by a supernova explosion that it's become a tiny but heavy ball of neutrons that rotates at a very high speed. Pulsars have an extremely powerful magnetic field and emit radiation in a beam not necessarily aligned with their rotation axis. This means that we observe regular radiation "pulses" that come from those stars according to their rotation speed. To put it simple, they're oversized lighthouses. In the case of our Crab Pulsar, the lighthouse shines in our direction 30,2 times per second. Now that's fast, isn't it? The nebula which hides the pulsar to visual observers is all the rest of our exploded giant star and over time scientists have been able to measure a significant increase in size. Not surprising since the supernova blast expelled the star's atmosphere at an incredible 20.000km per second (!) and even today it keeps expanding at 1.400km/s. Given time, the still fairly young and compact Crab Nebula will be ripped apart and dissipate into space, just like the Veil Nebula which is a remnant of a supernova that happened some 5.000 years earlier and of which only delicate filaments of gas can still be observed. Considering its respectable distance of more than 6.000 lightyears, the Crab Nebula is still one of the brightest planetary nebulae in the sky. Imagine what an explosion it must have been!

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