How Was the Mass of the Candidate Black Hole at the Center

Multiple Choice

How was the mass of the candidate black hole at the center of the galaxy M87 estimated?

A) from the amount of mass that is disappearing into it every year
B) from the periodic shift in the wavelengths of spectral lines from a companion object around which the black hole is orbiting
C) from observations of the very high orbital speed of objects close to this center
D) from the intensity of X-rays emitted from it and the frequency of the flickering of the X-ray intensity