At what point would we discover a black hole headed directly toward us?

CCWilson

I can understand that if you happened to notice a deflection from expected of a particular star or stars that you might be able to figure out what's going on. But as a practical matter, with so incredibly many stars out there, how would that be picked up? Are there computer programs that compare the night sky with the expected night sky? I guess the general question is, how likely is it that a minor deviation from expected position of a random star would be flagged for further investigation?

mfb

With GAIA, I am quite sure that those deviations would get noticed if they affect several stars or give odd apparent motions, as GAIA highly relies on its own stellar data for calibration.