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Rainer Schödel, Reinhard Genzel and collaborators combined NTT imaging with VLT adaptive-optics infrared observations. Silicon-monoxide masers registered stellar images to the radio position of Sagittarius A*. Their 2002 observations included S2's closest approach; a fitted Kepler orbit constrained a concentrated central mass. Rejecting a particular extended cluster also required its density and survival assumptions.^[ref]

Andrea Ghez and collaborators combined Keck speckle imaging and laser-guide-star adaptive optics with infrared spectra. Image fitting supplied angular positions; spectral lines supplied velocities. The 2008 analysis treated optical distortion, coordinate alignment and blended sources: a shifting photocentre can imitate motion. Joint angular and spectral fitting constrained both distance and mass, with assumptions about central radial motion affecting the result. Apparatus calibration and confusion tests are therefore part of the dynamical inference.^[ref]

One half went to Roger Penrose for showing that black hole formation is a robust prediction of general relativity. One quarter each went to Reinhard Genzel and Andrea Ghez for discovering a supermassive compact object at the centre of the Milky Way.^[ref]

Reinhard Genzel investigates the central mass of the Milky Way using the motion of nearby stars. He coauthored the 2002 report led by Rainer Schödel that followed a close stellar orbit around Sagittarius A*.

An orbital path tests more than a high stellar speed: its curvature, timing and closest approach constrain how tightly the attracting mass is concentrated. The measured short-period orbit required several million solar masses in a compact central region, providing strong dynamical evidence for a supermassive black hole.^[ref]