I've been looking a little at this myself lately. The original source, unfortunately, is not verifiable. However, it's basically not inconsistent with existing observations of Betelgeuse, which indicate that after a long period of fluctuation in size with a period on the order of several years, it has been shrinking continuously (and at an increasing rate) since 1993. If this does reflect a permanent change in the internal state of the star and Betelgeuse has entered its final collapse, we would expect it to become progressively more non-spherical as it collapses. I imagine I don't need to point out that the brightness estimates in the initial article are WAY off, though.



The original post uses SN1054 as a reference. Let's do that. And let's also look at SN1006, although SN1006 was probably a Type 1a supernova. SN1054, at a distance estimated at 6300LY, is estimated based on accounts written at the time to have reached apparent magnitude -6. SN1006, the brightest extra-solar stellar event ever observed and recorded by humans, reached absolute magnitude -7.5 from a distance estimated at 7200LY. Measurements of Betelgeuse's distance vary from 495LY to about 620LY, with the "best compromise" estimates considered to lie between 530LY and 570LY.



To start with, if we assume Betelgeuse would be like a SN1054 event, we can predict that simply by virtue of being between 11 and 12 times closer, a Type II supernova of Betelgeuse would appear somewhere in the region of 130 to 140 times brighter than SN1054 merely by virtue of being closer. This would mean an apparent magnitude somewhere around -12, making it a little less than half as bright as the full moon (magnitude -12.92).



This, however, assumes that Betelgeuse is just like the progenitor star of SN1054, which it is not. We believe that SN1054's progenitor star was between 9 and 11 solar masses. This is about half the mass of Betelgeuse, estimated at 20 solar masses. We can therefore expect Betelgeuse's supernova to be a considerably more energetic event than SN1054, but we don't know by how much. So let's also compare to SN1006, which was a full 1.5 magnitudes brighter than SN1054 despite being almost 1000LY further away. We can readily calculate that SN1006 had to have an absolute luminosity on the order of 5.2 times that of SN1054. This is a little under two magnitudes.



So, if we assume that a Type II supernova of Betelgeuse would be at least as luminous as SN1054 (a not unreasonable assumption), but no more luminous than SN1006, we can ballpark its probable apparent magnitude at somewhere in the range of -12 to -14, or, in other words, from roughly half to roughly twice the maximum brightness of the full moon. (Remember, though, to visual observation it will be a point source. It will be piercingly brilliant.) But there is no way it's going to be 12 to 13 magnitudes (absolute) brighter than SN1006, which is what it would take to rival the Sun.



So, as bright as the full moon? Very likely. As bright as the sun? Not a chance, unless something far more catastrophic happens than our understanding of stellar evolution would currently lead us to expect.