What exactly does a Supernova look like?

[Cooly568]

This was a question that came to me over today.

We've seen remnants of Supernova's, but what does one look like in the visible spectrum look like, or would like, to us?

Or is this something we don't have enough data to predict yet?

[Elthy]

It would just be bright, very, very bright. To cite XKCD (What if? 73):

Here's a question to give you a sense of scale:

Which of the following would be brighter, in terms of the amount of energy delivered to your retina:

A supernova, seen from as far away as the Sun is from the Earth, or

The detonation of a hydrogen bomb pressed against your eyeball?

Applying the physicist rule of thumb ("However big you think supernovae are, they're bigger than that.") suggests that the supernova is brighter. And indeed, it is ... by nine orders of magnitude.

[maltesh]

This was a question that came to me over today.

We've seen remnants of Supernova's, but what does one look like in the visible spectrum look like, or would like, to us?

Or is this something we don't have enough data to predict yet?

Humanity's seen supernovas. There just hasn't been one in this galaxy visible from Earth since the invention of the astronomical telescope. That last one would have been Kepler's Supernova, in 1604.

[Vanamonde]

You know what Venus looks like at its brightest? Historical descriptions make it sound something like that. Possibly even visible in daylight, but still just a bright dot without color or feature.

[cantab]

From a distance, they look like a new star that appears, brightens over the course of a week or two (though less if it's not spotted right away), then fades over a few months. This light is produced by hot ejected material, heated by radioactive decay of heavy nuclei and/or kinetic energy from the blast depending on the type of supernova. At shorter wavelengths the peak is much narrower and corresponding more closely to the actual supernova explosion.

As long as you're far enough to be outside the expanding hot ejecta, I expect you'll see more or less the same thing. I'm not sure how big said ejecta gets, but I'd guess several AU. If you're so close that you're inside the ejecta, you'll basically be inside a searing fireball akin to the outer layers of the Sun.

[lajoswinkler]

From far away, when angular diameter of the star is insignificant, it looks like an extremely bright star. It does not suddenly turn brighter. It's a gradual increase of luminosity.

From closer up, it does not look like what you've seen in documentaries. Giant stars are so huge that even if the ejecting matter was going at the speed of light, it would look (ignoring the c limit for observer, and appropriatelly dimmed) slow. Ejected highly radioactive plasma certainly doesn't go near light speed, so it's even slower. Days of primary explosion, then years of dimming and material cooling, forming a nice picture for everyone in the neighborhood.

The explosion itself is like a slowly growing ball or bubble of extreme luminosity.

[bartekkru99]

Just think about, how would it look like if it was 100 lightyears, from us, if the star 20.000 light years was visible during the night...