How quickly does a star's supernova become apparent to the naked eye?

[Ender55]

If a star in our night sky were to go supernova, how quickly does this become apparent to the naked eye? I know the star collapses very quickly, but what will this look like to the naked eye? Will the star suddenly become brighter within seconds or minutes, or gradually over hours or days?

 

[Ender55]

Within our own galaxy, that is.

 

[Nabeshin]

Roughly speaking, the timescale for brightness growth and decay for a SNe is a days. Exactly how long it takes for the star to reach the naked eye threshold depends more explicitly on how far away it is, but again roughly days as an order of magnitude.

 

[Drakkith]

Well, I took a picture of a supernova in M101 in October, and then again in January, and it was plainly visible in both pictures, though it was probably about 2-3 magnitudes dimmer in January (Best guess, I never measued the magnitude, I'm just going off how bright it looked in the pics). So depending on how close the star was I'd say it could be visible for 3-6 months.

 

[davenn]

If a star in our night sky were to go supernova, how quickly does this become apparent to the naked eye? I know the star collapses very quickly, but what will this look like to the naked eye? Will the star suddenly become brighter within seconds or minutes, or gradually over hours or days?

That really is a little difficult to answer as no one has been observing a star at the time it has gone supernova. They have always been detected after the fact.
The last naked eye SN was SN1987A in the Large Magellanic Cloud at ~168,000 light years away. It was the first one visible for some 300 odd years which was in 1604AD and in our own Milky Way.
That was one of two major astro event hi lights of my life, it was amazing to see a bright visible star in the sky that wasn't the the night before. My mate and I did photos of the supernova and of a comet (possibly Wilson) in the same frame!
(the other major hilite was observing the impact areas of comet Shoemaker-Levy9 into Jupiter)

Wikipedia comments that a small burst of neutrinos were detected some hours before the visible light reached us. They suspect that the neutrino burst was from the core collapse and left the star well before the light of the blast shockwave reached the surface of the star from the core.

That's pretty cool and if the neutrino detector guys were really switched on they would be able to give the optical astronomy fraternity early warning that a supernova was imminent and sky scans could start.

Cheers
Dave