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Popular science articles suggest that when Betelgeuse goes supernova, for some days it would appear as bright as the full moon. But I have no intuition about a celestial point source that bright.

  • Would staring at it for some time risk eye damage?
  • With the sun behind clouds and the supernova unobstructed, could it cast obvious shadows in daytime?
  • Would it affect weather forecasts' UV index (risk of sunburn)?
  • Might other analogies hint at what optical phenomena it might produce?
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A typical Type II supernova has a peak absolute magnitude of -17. Betegeuse is ~170 pc away, so its supernova would be 289 times dimmer, corresponding to a magnitude increase of 6.15, so its apparent magnitude would be ~-10.85, which is slightly dimmer than the full moon, but would appear brighter as it would appear as a point source.

Venus, which peaks at magnitude -4.6, can cast shadows, albeit very faint ones; as it is similar to a point source, we can extrapolate to Betelgeuse to conclude that it definitely could, but not at day as the Sun would outshine it by a factor of over 1000000. Even if the Sun were behind clouds, it would not be able to cast any shadows, just like how the brighter Moon can't do so either.

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