If Betelgeuse Explodes, Just How Bright Will It Get?

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By RHETT ALLAIN, Wired. 

Excerpt: Betelgeuse, ...red supergiant has dimmed repeatedly in the past few years, which could mean that it's ready to go full supernova quite soon—and by “soon” we mean within the next 10,000 years. ...If Betelgeuse does blow, it will be the brightest supernova ever witnessed by humans. ...A supernova that was observed in 2015 (ASASSN-15h) had a peak luminosity of around 2 x 1038 watts. That's more power output than 500 billion suns. It's crazy. Oh, you didn't see that one? Yeah, because it was in a different galaxy. Betelgeuse is in our back yard, astronomically speaking. ...to a Betelgeuse supernova... start with a luminosity of 2 x 1038 watts, like that supernova in 2015. For the distance, I'm going to use 500 light-years. ...With that, the intensity of the light received by Earth would be 0.711 watts per square meter. ...Crunching the numbers gives me a brightness magnitude of –18.5—which ...will be by far the brightest object in the night sky. For comparison, a full moon has a magnitude of –12.6, so this supernova would also be easily visible even during the day. With the naked eye, the supernova would still look like just a single point of light—because, hey, it's pretty close for a star, but it's still far away. You wouldn't see a disk as with our own sun or moon, but it would be by far the brightest dot you've ever seen in the night sky, and it would probably last for weeks. But is it dangerous? Well, this is still a lot lower than the brightness of the sun, which has a magnitude of –26.8.... 

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