How do black holes affect things near them?

Are we  in danger of being gobbled up by a  black  hole?  Actually, no.  We’re pretty safe.The  gravity  from  a black  hole  is  only dangerous  when you’re  very  close  to it.  Surprisingly,  from a  large  distance, black hole gravity  is no different  than  the gravity  from a star with the same mass. The strength of gravity depends on  the mass of the object and your distance from it. If the Sun were to become a black hole (don’t worry, it’s way too lightweight to ever do that), it would have to shrink so much that its event horizon would be only 6  km  (4 miles) across. From  the Earth’s distance of 150 million km (93 million miles), we’d feel exactly the same gravity as we did when the Sun was a normal star. That’s because the mass didn’t change, and neither did our distance from it. But if we got up close to the black hole, only a few kilometers away, we’d definitely feel the difference! So stellar-mass black holes don’t go around tearing up stars and eating everything  in sight. Stars, gas, planets, and anything else would have  to get up  close and personal  to a black hole  to get trapped. But space  is big. The odds of  that happening are pretty small. Things  are  different  near  a  supermassive  black  hole in the center of a galaxy. Every few hundred thousand years, a star wanders  too close  to  the black hole and gets torn apart. This produces a blast of X-rays that can be visible for decades! Events like this have been seen in other galaxies, and they are a prime target for satellites such as EXIST to reveal otherwise “dormant” black holes.Astronomers have  found another amazing  thing about galaxies:  the stars  in  the  inner parts of a galaxy orbit the  galactic  center  faster  when  the  galaxy’s  central supermassive black hole is more massive. Since those stars’ velocities are due to the mass in the inner part of the galaxy – and even a monster black hole is only a tiny fraction of  that mass – astronomers conclude  that  the total mass of the inner region of a galaxy is proportional to  the  (relatively  very  small) mass of  its  central black hole! It’s as if the formation of that black hole somehow affected  the  formation  of  the  billions  of  normal  stars around  it. EXIST will probe  this suspected  “feedback” between  galaxy  formation  and supermassive  black  holes by  investigating  black holes  in  a  very  large sample of galaxies.