in the constellation Aquila
14 times the mass of the Sun
Diameter roughly 50 miles (85 km) equal to the size of a large city
Measuring the motions of a companion star
Because a black hole is both massive and compact, it exerts a strong gravitational pull on the material around it. Astronomers can deduce the presence of a stellar-mass black hole (one that is a few times as massive as the Sun) by measuring the velocity of a companion star in a binary system. In a system that contains a black hole and another type of star (one that produces visible light or other forms of energy), the orbital speeds of the two component stars is much greater than in a system with two "normal" stars (stars that are similar to the Sun). Measuring the orbital speeds of the two components in a binary system, along with the distance between the stars, reveals the system's total mass. Using other techniques, astronomers can determine the mass of the luminous companion. By subtracting that from the system's total mass, they can determine the mass of the dark companion, which reveals whether it is a black hole or a less-dense object like a neutron star. This technique is like the one that astronomers use to deduce the masses of planets in solar systems other than our own.
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This document was last modified: June 28, 2011.