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Follow this question. If a star at the same distance from us as, say, Proxima Centauri had exploded as a Supernova 4 years ago (I know, it can't explode as supernova, let's say it can)...

How bright will it be?

It will impact us (life on earth and/or technology)?

What about Sirius?

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Proxima Centauri can't explode as supernova as it is about 2 orders of magnitude too small and light.

But if a supernova were to happen from that distance? How bright would it be?

You can use the formula: $$\text{Apparent magnitude} = \text{Absolute magnitude}+5log_{10}(distance)-5$$ where the distance is in parsecs.

Proxima centauri is 1.3 parsecs distant.

A supernova typically has an absolute magnitude between -15 and -19 (although they can go brighter or dimmer in the case of hypernovas etc). So Let's use -15 as the low end and -19 as the high end.

Plugging into the equation, we get an apparent magnitude of between -18 and -23.

For comparison, the sun as seen from earth has an apparent magnitude of about -26, so the supernova would be between 1500 and 15 times dimmer than the sun from earth.

Perusing the list in the Wikipedia article on apparent magnitude, we find the brightness of the supernova would be between the brightness of the Sun viewed from Jupiter and the Sun viewed from Pluto. In any case, that is plenty of light to read by, and plenty of light to see colour by.

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    $\begingroup$ Would the ejected material or the high energy radiation & particles effect us? $\endgroup$ – SpaceBread Oct 31 '19 at 8:58
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    $\begingroup$ @SpaceBread Nice question - radiation dose collected by us in Sieverts, I think it would worth a new question. $\endgroup$ – peterh - Reinstate Monica Mar 6 at 9:01

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