Testing the General Relativistic "No-Hair" Theorems Using the Galactic Center Black Hole Sagittarius A*

If a class of stars orbits the central black hole in our galaxy in short period (~ 0. 1 year), high eccentricity (~ 0. 9) orbits, they will experience precessions of their orbital planes induced by both relativistic frame-dragging and the quadrupolar gravity of the hole, at levels that could be as large as 10 microarcseconds per year, if the black hole is rotating faster than 1/2 of its maximum rotation rate. Astrometric observations of the orbits of at least two such stars can in principle lead to a determination of the angular momentum vector J of the black hole and its quadrupole moment Q₂. This could lead to a test of the general relativistic no-hair theorems, which demand that Q₂ = - J²/M. Future high-precision adaptive infrared optics instruments make make such a fundamental test of the black-hole paradigm possible.