Nikon 5000 - Manual (Page 133)

Taking Photographs—Advanced Photography
119
Setting the Time Limit for Long-Time Exposures: Maximum Bulb Duration
In manual exposure mode, a shutter speed setting
of BULB allows the shutter to remain open as long
as the shutter-release button is pressed (
95).
When Maximum Bulb Duration is set to 1 min
(the default setting), the maximum for such long
time-exposures is sixty seconds. You can extend
this limit to five minutes by choosing 5 min. Note
that longer exposure times increase the amount of
noise visible in the final photograph.