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1 .This is Scientific American's 60-Second Space. I'm John Matson. Got a minute?
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2 .The year is 774 A.D.
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3 .A blast of energy from outer space hits Earth.
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4 .No one alive then seems to have been aware of it.
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5 .But the event was recorded--in the trees.
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6 .Last year researchers announced that tree-ring data from 774 and 775 show a sudden spike in radioactive carbon 14.
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7 .The uptick points to a burst of charged particles or high-energy radiation that struck Earth.
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8 .But what caused it?
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9 .One suggestion is that a supernova could be the culprit.
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10 .And medieval texts do record the appearance of a "red crucifix" glowing in the night sky, although the timing is uncertain.
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11 .Another astrophysical possibility is a short gamma-ray burst,
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12 .researchers report in the Monthly Notices of the Royal Astronomical Society.
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13 .Short gamma-ray bursts occur when super-dense objects--say, two neutron stars--merge, giving off a quick blast of radiation.
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14 .As long as it was a few thousand light-years away, a short gamma-ray burst in the galaxy might not have caused great harm.
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15 .And they don't glow much in visible light, so it's possible that no one would notice.
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16 .One problem with the theory?
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17 .Short gamma-ray bursts appear to be extremely rare in our galaxy.
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18 .On second thought, that's probably a good thing.
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19 .Thanks for the minute, for Scientific American's 60-Second Space. I'm John Matson.
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