WASHINGTON — A swarm of ordinary stars buzzing about the dark and unseen center of a distant galaxy has helped two astronomers locate what is believed the most massive black hole ever discovered.

John Kormendy of the University of Hawaii Institute of Astronomy said Thursday that the black hole fills a volume about equal to the solar system, which includes the Earth and sun, but it has a mass equal to about a billion suns.

The black hole is in the galaxy NGC 3115, a stellar grouping that previously was thought to be only a cluster of ordinary stars some 30 million light years from Earth.

But Kormendy said that when he and his co-author, Douglas O. Richstone of the University of Michigan, began plotting the movement of stars about the dark center of NGC 3115, they found evidence of a black hole controlling the star movements and apparently gobbling up stellar material at a furious rate.

Kormendy said the black hole was “weighed” by analyzing the movement of the ordinary stars in NGC 3115 and then calculating the mass of those stars by evaluating their brightness.

The astronomers said the NGC 3115 stars were moving very rapidly and that their speed increased dramatically as they approach the galactic center.

Furthermore, the star density near the center was very high. Kormendy estimated that within the galaxy there is stellar matter equal to about 500 times the mass of the sun in each cubic light year — a cube of about 6 trillion miles per side.

In contrast, the sun is lonely. The nearest single star to the sun is more than 18 trillion miles away.

Kormendy said the mass and the velocity of the stars in NGC 3115 is proof that there has to be a black hole at its center.

“With the stars going at the speed they are going, if you were to take the black hole away the galaxy would just fly apart,” said Kormendy.

Though NGC 3115 appears to be a common galaxy in the constellation Sextans, it at one time could have been one of the brightest galaxies in the heavens, the researchers said.

“The galaxy is several times larger than our own Milky Way, but its stars are mostly old,” Richstone said in a statement. “It contains virtually no gas, and little is going on in the galaxy now other than the stately orbit of its stars.”

The discovery was made using observations from the Canada-France-Hawaii telescope on Mauna Kea in Hawaii.

A black hole is thought to form when stars and other matter in the center of a galaxy clump together, forming a gravitational center that then attracts more and more matter and stars. Eventually, the center becomes so dense and gravitationally powerful that nothing, not even light, can escape. For this reason, black holes cannot actually be seen. Their presence is implied by their location and their gravitational influence on nearby stars.

Kormendy calculated that if the sun, which has a diameter of about 800,000 miles, was compressed to the density of a black hole it would be a sphere about two miles across. A person compressed into the density of a black hole would be only one thousand billionth the size of an atomic nucleus, or “very, very much smaller than an atomic particle,” he said.

A light year is the distance light will travel in a year in a vacuum. It is equal to about six trillion miles.

Kormendy and Richstone are publishing a report on their discovery on Friday in the Astrophysical Journal.