Question: What should the critical mass of a black hole be in order for the star to be torn apart? How does one find this critical mass? — Safar
Answer: The general answer to your question depends upon a number of factors related to the star/black hole system, but we can look at one example to get an idea as to what size of black hole we need to tear a star apart. Such an event has in fact been observed earlier this year (see the Scientific American article describing this event). Spectroscopic measurements of a flare from a star/black hole system as the star was being devoured was used to determine what chemical species exist in the material being eaten by the Black Hole. This indicated that the material being eaten was from the core of a Red Giant star. Also, by measuring this system while the Black Hole dined on the Red Giant star over more than a year scientists were able to determine the mass of the Black Hole, which they found needed to be about three million times the mass of our Sun.