How long does the supernova stage of a star last? How long does the peak luminosity of a supernova last? Hours, days, weeks? If a supernova were close and bright enough to be seen during the day on earth, how long before it would be invisible in the day time?
The explosion of a supernova occurs in a star in a very short timespan of about 100 seconds. When a star undergoes a supernova explosion, it dies leaving behind a remnant: either a neutron star or a black hole.
November 2002 Update by Karen: Below is figure which shows the typical light curves for the two main types of supernova. A light curve is a plot of how bright the supernova is over time. As you can see from the diagram, the answer to your question depends on the type of supernova you are asking about. Type I supernovae, (which are believed to result from matter falling onto a white dwarf in a binary system), are typically brighter, but fall off more quickly, with the peak brightness lasting only a few hours to days. Type II supernovae (which are thought to be the result of core collapse of a massive star) generally have a plateau in brightness before dimming more slowly. Their peak brightness can last several months.
(Image take from Mr. Galaxy's Intro to Supernovae)
The vertical axis on the figure shows the absolute magnitude of the Supernova. Magnitude are a brightness unit commonly used by astronomers. You can read an explanation here if you don't already know about them. The absolute magitude just says how bright the supernova would be at a distance of 10 parsecs (a parsec is another astronomers unit, 1 parsec = 3.26 light years). To see an object in during the day it must have an apparant magnitude of less than (smaller magnitude means brighter object!) about -4 (you can see Venus during the day at it's brightest which is about -4.4), so how long the supernova would be visible during the day would depend on how bright it was at peak, which depends on it's absolute magnitude and how far away it is. The type II supernova which created the Crab nebula is estimated to have had a peak brightness of -6 magnitudes and was visible during the day for 23 days (in 1054)! It is about 6500 light years (or 3000 parsecs away) so had an absolute magnitude of about -17 (remember the light curves in the figure are 'typical', but that not all supernova are exactly the same).
Hope that gives you an idea of the answer to your questions.
This page was last updated June 27, 2015.