Well, after 900 years the crab nebula is about 10 light years in diameter.
Given those kinds of numbers, the exploding Supernova would start out as a point source when it exploded, and gradually growing to be maybe the size of the moon in the sky over a couple of thousand years.
What would determine its brightness? I guess nearby neighbors could make it a reflection nebula. Is the nebula itself going to be bright enough to be visible from Earth? I dunno.
“Is the nebula itself going to be bright enough to be visible from Earth?”
Well, the Crab nebula has a magnitude of 8 to 8.4. Using the same brightness ratio as before gives a magnitude of 2-2.4 for a betelgeuese nebula; and that’s after it’s cooled for nearly a thousand years. By way of comparison, M31, the galaxy in Andromeda which is visible to the unaided eye, has a magnitude of only 3.4.
Actually, while the supernova is at its brightest, most telescopes would be massive overkill. You don’t need a 10 meter telescope to observe a -18.5 magnitude object! In fact, you’d have to have a ridiculously short exposure time to do anything–even take spectra.
Similarly, neither world-class telescopes nor Hubble are routinely used to observe the Moon because it’s so damn bright. IIRC, there have been only a few observations of the Moon with HST–one I know was a spectrum of earthshine off dark side of the Moon.