The brightest apparent magnitude for a nova in recent times
(since the 17th century) was -1.1 recorded for Nova Aquilae in
1918. The next brightest was Nova Persei of 1901 at zero
magnitude. The rate of detection over the 20th century suggests
that naked eye novae occur on average about once every three
years.
When a nova outburst takes place, the binary star concerned
increases in brightness by about 10 magnitudes, so the brightness
of a star before it becomes a nova is the chief factor determining
how bright it will become. The explosion throws off a shell of
gas which expands outwards around the star and may be
observed decades later.
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