maybe both ?
Sign in sky maybe was a large comet that later hit a large sunspot causing a bigger than usual CME ? To affect any event on sun a comet would have to be big and very fast, perhaps from outside solar system.
Sometime between 1,237 and 1,238 years ago, the earth was inundated by a massive blast of high-energy radiation greater than any known to have occurred either before or since – but no one knows its source. This startling fact was uncovered by tree-ring analysis – the same technique that has proved so useful to climate …
"... Here we report 14C measurements in annual rings of Japanese cedar trees from ad 750 to ad 820 ..."
"... When averaged over 10 years, the data are consistent with the decadal IntCal 14C data from North American and European trees ..."
Radiocarbon dates are always a little odd. As we got better information on the half-life, and better instruments, the dates have shifted. There is always some doubt about the relationship between radiocarbon years and calendar years. And we know that the balance between C13 and C14 shifts: all those fossil fuels are effectively C14-free.
With the half-life of C14, everything since this event is more radioactive. And the chances are we already know that. A sample which is 7000 years old in radiocarbon years has a caibrated age a thousand years different. What really makes this event remarkable is that the scientists have found a year for it, and no apparent explanation from other sources.
It isn't going to wreck archaeology
The Anglo-saxon chronicle had something:
"A.D. 774. This year the Northumbrians banished their king,
Alred, from York at Easter-tide; and chose Ethelred, the son of
Mull, for their lord, who reigned four winters. This year also
appeared in the heavens a red crucifix, after sunset; the
Mercians and the men of Kent fought at Otford; and wonderful
serpents were seen in the land of the South-Saxons."
I wonder what they saw that they interpreted as a "red crucifix". I don't have the original text handy, only a translation.
"A planetary nebula could easily be mistaken for a crucifix so I guess a nearby one would easily cover all the requirements. We just need to find the culprit."
Not the planetary nebulae I know. Besides, they are not large enough to be resolved by the naked eye. Planetary nebulae are not supernova remnants, but form when a star roughly the size of the sun blasts off its outer layers. However, let us not forget that the 1054 supernova was seen in the east, but not by western observers (too busy bashing each other's brains in ?). I do not know of a nearby SN-remnant which could be a candidate. A gamma-ray burst may be the culprit, as others have noted.
As this event is not so much impossible, but very, very improbable, I suggest the Heart of Gold is to blame
NGC 6537 looks very like a cross - and many other planetary nebula exhibit the hourglass appearance that could be interpreted as a cross. The helix nebula is the size of the full moon and 2.5 light years across and may be a candidate - not sure how fast its expanding!
A nearby one may have dispersed by now, or possibly have blown itself away with a greater explosion later on - maybe the crab nebula was planetary before it went supernova?
"NGC 6537 looks very like a cross - and many other planetary nebula exhibit the hourglass appearance that could be interpreted as a cross. The helix nebula is the size of the full moon and 2.5 light years across and may be a candidate - not sure how fast its expanding!
A nearby one may have dispersed by now, or possibly have blown itself away with a greater explosion later on - maybe the crab nebula was planetary before it went supernova?"
The Red Spider (NGC 6537) is pretty unique in shape, and at 1.5 arcminutes is not very big (and would have been smaller in the past). The hourglass-type side-on nebulae like the Dumbbell are not that cruciform, and though the Helix is the size of the full moon, it is very difficult to spot, even through my 8" scope, as its surface brightness is very low. This is a key problem: high surface brightness planetaries are small, one big enough to resolve by eye have very low surface brightness.
If the white dwarf at the centre of a planetary is part of a binary (spotted one such system last year), it could go supernova (Type Ia), otherwise this is unlikely (not enough mass). There is no indication that the Crab pulsar is part of a binary, I think. There may be a supernova remnant as yet undiscovered, of course. Supernovae embedded in a star forming region could generate strange light echoes on the surrounding dust and gas.
Finally, the "red crucifix" in the sky may have been atmospheric, rather than deep sky. A curious illumination of clouds after sunset, noctilucent clouds in a strange formation, auroras, or a bright meteor which exploded (and form cruciform patterns). And finally, if your king or local lord had stated he saw a red cross in the sky (after imbibing some bad mead, maybe) stating you could not see it might be a terminal career move ;-)
The crab nebular is already recorded with the date it went "up" in Chinese records AFAIK. There are quite a few cross shape nebular or novas that have been photographed. I can't remember if it's the angle or due to rotation/double star systems that does it, but it's quite common.
A "red crucifix" and "wonderful serpents" are both very good descriptions of highly active Aurora Borealis, which form all sorts of marvellous shapes. If they're strong enough you'll begin to see coherent plasma formations taking on the form of pillars, crosses, humanoid figures, chalices and all sorts of fancy things. Serpents, which were traditionally sinuous, flying creatures and not necessarily snakes, would be fairly run of the mill.
A large CME impacting earth would generate very strong, very southerly aurora.
Additional evidence: the magnetic north pole was much closer to Europe in the 700s and 800s AD (it's moved quite a bit since then) which means that aurora would be more southerly in Europe to begin with, which would explain the serpents - bright glowing, snaking shapes in the sky would be seen as serpents and the flying, dragon-like "worm" and might even have been related back to the norse mythology of Jormungand.
So the question becomes, was that CME powerful enough to generate more coherent shapes? If it were at the level of the Carrington Event (look it up) then it could well have been responsible for both the sky sightings and the C14 increase.
Graham, you are probably right; the aurora borealis would cover the facts and explain the descriptions. That would make high levels of solar activity the likely (though not definite) cause.
Then again, I seem to recall that the years in the ASC cannot always be taken literally, as events and years were made to fit a pattern. The order is probably valid, but the exact years might not be reliable in all cases. I read that years ago, though, so maybe they have been determined to be reliable.
They can be a trifle sketchy. I seem to recall that they started as annotations to Easter tables which were set up to calculate when Easter would fall in a given year. There were a couple of ways that these could be out; again, I'll have to try to remember to look at this when I get home rather than at work. At least this post-dates the Synod of Whitby :)
If it was something naked-eye visible and of duration >24 hours, it would have been seen globally and recorded by quite a few civilisations more advanced than the Europeans at that date. China, for example.
Unless, possibly, it was in the Southern hemisphere near the (celestial) South pole. Could that have escaped notice?
The event will probably only have lasted for a short period of time during the year (days or perhaps a week or two). This is not long enough to have any noticeable impact on climate.
If you are thinking of Svenmark's Cloud Theory then a couple of cloudier days during a year would be lost in the noise and natural variability of weather.
Also there aren't any proxies with a high enough resolution and/or accuracy to pick out an individual years temperature.
A gamma ray burst from the far side of the Milky Way could have done the job. As a GRB only lasts seconds, if it hit the day side of the earth it could have been completely missed by human observers.
(A nearby GRB would have been lethal but from the far side of the galaxy and with the dust and gas in the way the biological effects would have been minor.)
Would have had to have been a very small one. we don't know enough about GRBs to know if a small one is possible. A regular-sized one, of the sort we observe from cosmologically distant galaxies, would sterlilize the entire galaxy within which it occurred. The biosphere-killing mechanism is atmospheric ionisation, leading to the creation of huge amounts of Nitrogen Oxides, followed by deadly acid rain and ocean acidification.
A GRB in Andromeda?
The statement that if it were of solar origin, then aurora would've been commented on, or it would've done serious damage to the ozone layer assumes a CME type event lasting at most a few days. However, the growth took place over a year, so perhaps a slower, longer lived event could've done it without having to have killed everything, or make people think that they were at a nightclub?
It seems to me that we don't fully understand the processes in the sun (hence the questions about the recent solar minimum, and the glut of solar science instuments being launched like SOLO - yay)
What about an event on Earth rather than in space?
During the brief era of nuclear bomb testing, we raised the C-14 level massively (see http://www.radiocarbon.com/carbon-dating-bomb-carbon.htm ). I'm not saying someone detonated a nuke in 774 - though a mushroom cloud might well have been interpreted as a "red crucifix" at that time:) - but a terrestrial event would not need to be of cosmic scale energy to affect our atmosphere.
If it were an Earth event, the effect would vary a lot more over area & time than if it bathed the earth uniformly from space, and that would show up as differing spikes in the C-14 in tree rings - one way to test the idea.
Another point though: if most ring surveys are averaged over 5-10 years, might there not be other single year spikes we aren't aware of? Maybe they are common, in which case this data wouldn't indicate an anomaly at all.
If so, one would expect many other radio-isotopes to be anomalous. I'd rather expect someone to have noticed the evidence of atmospheric H-bomb testing during the dark ages in (say) skeletons, but I guess it might be masked by the evidence of atmospheric H-bomb testing during the cold war.
Chain reactions can't happen in natural uranium on Earth as the level of fissile U235 is too low to sustain the reaction. Well it is now - if you go back in time, the proportion of U235 in uranium rises to a point that self-sustaining reactions could occur. So far one site is known where this happened, it was discovered at Oklo, Gabon in 1972 by French geologists mapping a uranium deposit. They discovered the level of U235 at Oklo was even lower than normal. About 1.7 billion years ago, the ore would have been about 3% U235 (compared to about 0.7% today), water circulating through a uranium ore acted as a natural moderator allowing a self-sustaining chain reaction to run at very low power for hundreds of thousands of years.
Personally I go with the idea that this C14 spike was caused by Camelot's nuclear testing programme.
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