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Sagittarius (Sgr) A* is a unique radio source located at the center of our galaxy. The radiation from Sgr A* may be generated in matter accreting onto a massive black hole. In observations at long wavelengths, the apparent angular size of Sgr A* decreases in the manner expected for emission from a point source scattered by electron density fluctuations along the line of sight. Measurements at a wavelength of 7 millimeters with the nearly completed Very Long Baseline Array indicate a size of 0.7 milliarc seconds, which is consistent with an extrapolation from results at longer wavelengths. The true size of Sgr A* must be less than 0.4 milliarc seconds, or 3.3 astronomical units. The inferred black hole mass is less than 1.5 x [10.sup.6] solar masses according to a recent model for the emission., Sagittarius A* is an enigmatic object. Recent speckle imaging at a wavelength (λ) of 2 μm indicates that the centroid of the stellar sources at the galactic center is much [...]

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.