The scientists believe the black hole is the remnant of a massive star that lived out its brief life billions of years ago and later was gravitationally kicked from its home star cluster to wander the Galaxy with its companion. 

The discovery was made with observations from the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope and the Rossi X-ray satellite. 

Important supporting evidence came from studying optical images made for the Palomar Observatory Sky Survey (POSS) taken 43 years apart. The POSS images were digitized by the Space Telescope Science Institute to support the Hubble Space Telescope observing programs and also as a service to the astronomical community.
This huge database, called the Digital Sky Survey, allows astronomers to quickly and easily measure stellar motion across the sky. The DSS scans confirmed the motion of the black hole and companion star. 

The DSS scans, combined with data from both the radio and optical images, allowed the astronomers to calculate the object's orbital path around the galactic center.

"This also is the first time that a black hole's motion through space has been measured," Mirabel added. A black hole is a dense concentration of mass with a gravitational pull so strong that not even light can escape it. The research is reported in the September 13, 2001 issue of the scientific journal Nature. 

The object is called XTE J1118+480 and was discovered by the Rossi X- Ray satellite on March 29, 2000. Later observations with optical and radio telescopes showed that it is about 6,000 light-years from Earth, and it is a "microquasar" in which material sucked by the black hole from its companion star forms a hot, spinning disk that spits out "jets" of subatomic particles that emit radio waves. 
Most of the stars in our Milky Way galaxy are within a thin disk, called the plane of the Galaxy.

However, there also are globular clusters, each containing hundreds of thousands of the oldest stars in the Galaxy, which orbit the Galaxy's center in paths that take them far from the Galaxy's plane. XTE J1118+480 orbits the Galaxy's center in a path similar to those of the globular clusters, moving at 300,000 miles per hour (145 kilometers per second) relative to the Earth.

A massive star ends its life by exploding as a supernova, leaving either a neutron star or a black hole as a remnant. Some neutron stars show rapid motion, thought to result from a sideways "kick" during the supernova explosion. "This black hole has much more mass -- about seven times the mass of our Sun -- than any neutron star," said Dhawan. "To accelerate it to its present speed would require a kick from the supernova that we consider improbable," Dhawan added. 

"We think it's more likely that it was gravitationally ejected from the globular cluster," Dhawan said. Simulations of the gravitational interactions in globular clusters have shown that the black holes resulting from the collapse of the most massive stars should eventually be ejected from the cluster. 

"The star that preceded this black hole probably formed in a globular cluster even before our Galaxy's disk was formed," Mirabel said. "What we're doing here is the astronomical equivalent of archaeology, seeing traces of the intense burst of star formation that took place during an early stage of our Galaxy's development." 

The black hole has consumed so much of its companion star that the inner layers of the smaller star -- only about one-third the mass of the Sun -- now are exposed. The scientists believe the black hole captured the companion before being ejected from the globular cluster, as if it were grabbing a snack for the road.

The astronomers used the VLBA to observe XTE J1118+480 in May and July of 2000, using the VLBA's great resolving power, or ability to see fine detail, to precisely measure the object's movement against the backdrop of more-distant celestial bodies. 

"With the VLBA, we could start observing soon after this object was discovered and get extremely precise information on its position. Then, we were able to use the digitized data from the Palomar surveys to extend backward the time span of our information. This is a great example of applying multiple tools of modern astronomy -- telescopes covering different wavelengths and digital databases -- to a single problem," said Dhawan. 

In addition to Mirabel and Dhawan, the research was performed by Roberto Mignani of the European Southern Observatory; Irapuan Rodrigues, who is a fellow of the Brazilian National Research Council at the French Atomic Energy Commission; and Fabrizia Guglielmetti of the Space Telescope Science Institute in Baltimore, MD.