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Cataclysms such as novas, kilonovas, supernova, magnetars, quasars, gamma ray bursts are rarely visible from Earth. Under what conditions (such as distance, day vs. night, naked eye vs. level of optics) would such phenomena be visible from the Earth's surface, and what would they look like in terms of shape, initial duration, brightness, and so forth?

For instance, I have seen questions like how long does a supernova last and I have read that they can be visible for some time afterwards as a nebula forms, but this doesn't tell me what the initial burst would look like. For instance, would it be possible to see a supernova in a neighboring galaxy such as Andromeda? If so, how long would it be visible with the naked eye? Would it look like a flash, last a few seconds, several? I assume with a supernova not in our own galaxy there would be no subsequent light from the expanding shell since it would be too far away, whereas in our own galaxy if it were close enough we would subsequently see it, but in that case would there be a short burst of light, following by darkness and then the re-emergence of light, or would it be a continuously visible phenomena?

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Any of these phenomena would be visible to the naked eye if they occured in the Milky Way, or a neighbouring galaxy (such as the Magellanic clouds) and was not obscured by dust. At the distance of stars, all of these would appear star-like.

Supernovae have been seen, most recently in 1987, when there was a supernova in the Large Magellanic cloud. It had a peak magnitude of 2.9, (about as bright as gamma ursa minor, the star at the bottom of the Little bear constellation, furthest from the polestar). It was visible, but not spectacularly bright. Supernovae that have occurred in the Milky way have been very bright, outshining all the other stars. A supernova in 1006 may have been brighter than Venus. If it is close enough a supernova could be seen during the day.

All of these are highly energetic, and if you were close enough to see any detail (such as a nebula) with the naked eye, you would be toast. The exposive events (Supernovae, Kilonovae, Gamma ray bursts) would fade over time. Magnetars and Quasars are stable and would continue to shine. Remnant nebulae from Supernovae are visible in telescopes, the most well known is the Crab Nebula, M1.

A supernova in Andromeda would likely be naked eye visible, but not impressively bright, though it depends on the acutal power of the explosion, and other factors, like dust in Andromeda.

Supernovae are visible for months. Kilonovae fade faster. Gamma ray bursts may only last for a few seconds (though may be associated with a supernova that lasts longer). The visible element of a GRB might be visible, there was one that might have reached naked eye visibility, though there is no evidence that anyone actually saw it (it would only have been marginally visible in ideal conditions). Quasars are stable objects that don't fade over time, they are active supermassive black holes.

There are lots of other, more local things that can produce a "star" that brightens then fades: Planes have headlights, and as they turn the appear like a star that brightens then fades over a few seconds. If they are distant enough, the other lights might not be visible.

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    $\begingroup$ Possibly worth mentioning the Crab supernova? $\endgroup$ – Rob Jeffries Oct 21 '17 at 7:00
  • $\begingroup$ Thanks, very informative! So what I saw definitely wasn't a supernova since it only lasted maybe 5 seconds, maybe a flare? $\endgroup$ – Michael Oct 21 '17 at 7:59
  • $\begingroup$ Specifically, just before 1320UTC (Oct 20) while on a walk I saw a light I would estimate between mag -2 and -5 mostly white but with a bit of blue at the bottom. I thought it was a plane but I stopped and it was stationary and seemed to elongate slightly horizontally. I realized it was in the wrong direction to be Venus as it was easterly but then as I watched it suddenly winked out, although I thought I saw a dimmer star (maybe doubletake). Pulled up a star map on my phone and estimated it somewhere between Scheat and Lacerta although may be off; lost exact track in opening the app.. $\endgroup$ – Michael Oct 21 '17 at 8:11
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    $\begingroup$ That's the headlamps of a distant plane that is turning. As it turns the lights first point towards you, then point away, giving the appearance of a star that appears then fades. $\endgroup$ – James K Oct 21 '17 at 10:47
  • $\begingroup$ @JamesK That's what I thought at first too and why I did the doubletake, because it is close to a flight path and I could see 3 or 4 other planes in the normal approach pattern. But the other plane lights had a yellowish hue and were larger and less point-like, and watching for a few minutes after in the same area I saw nothing. Given the normal air traffic pattern I am used to seeing (and I see a lot) it didn't match. It's also possible something weird is going on with my eyes... On a previous night a few weeks back I saw a mag 6 satellite going in a specific vector nearly across... $\endgroup$ – Michael Oct 21 '17 at 16:13

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