Stars are points of light even in telescopes- if they go supernova, their diameter does expand, but given the distance of even nearby stars, they would still be points of light, I suppose?

I am asking because this video suggests that a very large disc would be seen in the sky after a few days if Betelgeuse were to go supernova. How long would it take for us to see a disc instead of a bright spot?

  • $\begingroup$ related, but doesn't answer your question astronomy.stackexchange.com/questions/34711/… $\endgroup$
    – James K
    Sep 6, 2020 at 20:52
  • $\begingroup$ Where does it suggest a "disc"? do you have a time or a quote? $\endgroup$
    – James K
    Sep 6, 2020 at 20:57
  • $\begingroup$ v back of envelope calculation suggests to me a disc might be visible after about a year... $\endgroup$
    – James K
    Sep 6, 2020 at 21:07
  • 1
    $\begingroup$ Supernova expands about 10000 km/s. Distance to Betelgeuse ~642.5 lys. With known telescope resolution, you can simply compute when you can resolve the explosion. $\endgroup$ Sep 7, 2020 at 14:10

1 Answer 1


The human eye has a resolution of roughly 2 arc minutes, or about 0.5 milliradians (15 pixels across a full moon). The distance to Betelgeuse is a bit uncertain, but lets say 600 light years, which is near enough. So to show a disk simply because of the size of the source, it would need to be about 0.3 light years across.

The source can't grow faster than the speed of light, and it reasonably might grow that fast as the radiation from the supernova heats the dust and gas already shed by Betelgeuse. If so it would take about 0.15 years or about 8 weeks to grow large enough to (just) show a visible disk. The actual material ejected in the supernova explosion would move at a fraction of that velocity.

One thing that might happen is that scattering in the Earth's atmosphere might make a very intense point source look larger. Light would arrive at your eye from a number of directions close to the point source, having scattered on its way through the atmosphere. Effects in the eye might also make a very bright point source look larger.

  • $\begingroup$ So long as you can take a photo and demonstrate non-saturated pixels, that might be a good way to verify it's a disk and not a point object smeared by the atmosphere. $\endgroup$ Sep 9, 2020 at 15:44

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