Tonight I could see the sky very clearly and was excited, but my camera would not take a pic because it said that the subject was too dark and that I had to use a flash. I was using bulb and the maximum ISO, which is not
that high in the Nikon D5100. I tried to increase/compensate the exposure, but it still would not take a pic.
Anyway, any advice?

The other day it took a pic OK, even if there was noise because I didn't use a tripod. And there are stars in the photo, so it is capable of it... But today it would not do it...
Thanks!