[Bern]

Is there a time limit to shooting in B (bulb), how long can I shoot in B for star trails? can the sensor be damaged from a long time exposure? Pentax K5 is the camera.

[Adam]

No, several minutes, and no (but you can get a lot of noise if the sensor gets hot and you disable slow shutter speed noise reduction).

Keep in mind that you need a remote control for the shutter to stay open without the shutter button being depressed.

A nice technique is to shoot a series of 30-second shots and stacking them rather than having one long exposure. You can then also take a reference photo at the end of the set and use it for noise reduction (dark frame subtraction).