Black holes have a very intense gravitational field.
We do see some stars like Cygnus X1, which seem to have companion is accreting mass that has been ejected by the visible star. The mass falling into this compact object is heating up to very high temperatures, something which can happen in intense gravitational fields. Mass of this invisible object is determined by observing the orbit of the visible star around it. Temperature of the falling mass is measured by observing the X ray emission from the heated material. Ph. D., Division Director.