Could a Black Hole eat An Entire galaxy?

Suppose a fair sized galaxy (like Andromeda) were to encounter a wandering black hole. The black hole (if it had the right velocity) would settle into a spiral orbit around the galaxie’s centor of mass. Presumably, it would then encounter stars on the outer edge of the galaxy, and consume them. As its mass increased, it would move closer and closer in, devouring more and more stars. Could it wind up by absorbing the entire mass of the galaxy? Is there a limit to how much mass a BH can absorb?
Man, that would be a REAL “deathstar”! :wally

There’s not theoretical upper bound to the mass of a black hole, so in theory one could consume an entire galaxy. However, in practice, the dynamics of such a situation would probably not permit this to happen. Galaxies are huge and black holes, however massive, do not have infinite “sucking power”; their gravitational fields obey the inverse square law, just like that of any other object. Chances are that a galaxy would survive mostly intact; in fact, it is now theorized that many, if not most, galaxies have supermassive black holes in their centers.

A black hole’s gravitational field is approximately the same as the exterior gravitaional field of a star of the same mass. Compared to the larger stars the black holes (excluding spermassive black holes) have very little mass. So a black hole orbitin the centre of a galaxy will generally behave like a smallish star. Stellar collisons are very, very rare in galaxies due to the enormous distance between stars so the effect of a single black hole (again excluding supermassive black holes) on a galaxy is fairly minimal, infact it is suspected that the Milky Way contains quite a number of balck holes.

We really need The Bad Astronomer but supermassive back holes at the galactic core is pretty much accepted as fact.

http://antwrp.gsfc.nasa.gov/apod/ap991017.html
http://news.bbc.co.uk/hi/english/ sci/tech/newsid_1526000/1526724.stm
http://www.cnn.com/2002/TECH/space/10/16/black.hole/

So essentially the whole galaxy is a massive accretion disk.

Hmmm… accidentally hit Submit there. Since the whole Milky Way Galaxy is essentially an accretion disk, yes, eventually it will eat pretty much the whole galaxy - more in fact as we’re due to collide / merge with the Andromeda Galaxy at some point in the far future. Some matter will be ejected from the North and South poles of the accretion disk very near the hole, forming cosmic jets.

The final point to be made with this question is that, because there is no upper limit to the mass of a black hole, you can continue theoretical calculations on what its size and density would be at masses equivalent to a globular cluster, a dwarf galaxy, a giant galaxy, a cluster of galaxies, etc.

And for the estimated observable mass of the Universe, the diameter of a black hole that size would be three billion light years. Add in the mass equivalent of the dark matter of the “missing mass” and the diameter of the resulting theoretical black hole becomes that of the observed Universe.