If it happened about 520 years ago, then today we would see it flare up in a matter of hours by about 1 billion times its present brightness. It is a star of the 1st magnitude, and for each 100-fold brightness increase you gain 5 magnitudes, so Betelgeuse would be a star of magnitude -20 or so. The Full Moon is -18 and the sun is -26 so it would be almost as bright as the sun!!

Over a period of months we would see the size of Betelgeuse expand as its outer layers flew away from the star at 10,000 kilometers per second. After a year, it would be surrounded by a bright nebula of x-ray emitting gas about 2,100 times the Earth-Sun distance. At its distance of 520 light years ( 160 parsecs), its diameter ( 160 AU = 1" at this distance) would be about 26 arc seconds in diameter and not resolvable except with a small telescope. But after 150 years, its diameter would be about the diameter of the Full Moon and still growing. Over time, it would become a glowing, multicolored spot of light in the sky fading in brightness, but still the most powerful source of radio and x-ray emission in the entire sky, far brighter than the sun itself.

I do not know what the details of the prompt radiation effects would be but I imagine that starting at detonation, Earth would be bathed with a slightly higher x-ray flux than it gets from the sun. Any neutrino detectors in operation would be suddenly start detecting these fleeting particles by the thousands per hour. This would continue for tens of thousands of years. After about 100,000 years or so, traveling at a speed of 5,000 km/sec, all hell would break loose as the first particles from the expanding gaseous envelope from the explosion begin to overtake the solar system. Then, for the next 10,000 years or longer, humans traveling into space would need some extra protection from enhanced cosmic rays contamination. The magnetosphere of the earth would probably feel the effects of the pressure from this new interstellar wind blowing.

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