How to take arbitrarily long exposures

[keeton]

Are there any digital cameras being produced that have the option to keep the lense open for up to an hour [ or any length of time] to capture a subject in very low light

[McQ]

Nearly all digital SLR cameras have a "bulb mode", which does just that. You can take an exposure all the way until the camera battery runs out, if such a long exposure is needed. Even then, you could use a battery pack to extend the potential exposure time to twice as long.

The only disadvantage is that longer exposures have an increased tendency for fixed pattern noise.