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Millisecond pulsar 1620-26, which is found in the globular cluster M4, is a binary system where an old neutron star spins at high velocity due to accumulation of matter by companion star. Timing measurements reveal the existence of a 0.3-solar-mass companion star, which is a white dwarf with orbital period of 191 days. The pulsar 1620-26 also has a large second derivative parameter in the rotation rate of the pulsar. The presence of this large derivative parameter indicates the presence of a third star in the hierarchical system which has an orbital period of 100 years.

Observations of the compact radio source Sgr A using the Very Long Baseline Array and other radio telescopes bore out previous evidence that Sgr A, located near the Galactic Center, marks the location of a large black hole. Sgr A was imaged at 3.6 and 1.35 centimeter wavelengths and was found to be producing radio emissions that were scattered by electron-density fluctuations along the line of site. These and other findings indicate the presence of a black hole equal to about two times 10 to the sixth power solar masses.