From The Albuquerque Tribune, October 13, 2001.

By Dave Finley

It was one of the first stars born in our Milky Way Galaxy, bursting into brilliant life 11 billion years ago, its fresh light spreading across a galaxy still forming in a tumultuous, youthful universe.

A giant more than 30 times more massive than our sun, this star was fated to die in a spectacular explosion fewer than 10 million years after its birth. That's a mere tick of the cosmic clock, compared with the sun's, our star's, expected 10 billion-year lifetime.

When this huge star's powerful death blast briefly outshone the rest of the galaxy, the light rays from its birth had barely reached the nearest neighboring galaxies.

What remained after that explosion was a black hole, nature's densest concentration of matter, wielding a gravitational pull so strong that not even light can escape it. This black hole is the remnant of one of hundreds of thousands of gigantic stars that astronomers say they believe formed at the beginning of our galaxy's history and then quickly burned their way to dramatic destruction.

Until now, however, scientists had little direct evidence of such ancient relics from the Milky Way's infancy.

That changed recently when a team of researchers using the National Science Foundation's Very Long Baseline Array, a continentwide radio telescope headquartered in Socorro, tracked the motion of a black hole, dubbed XTE J1118+480. It is speeding at more than 320,000 mph through the galactic neighborhood 6,000 light-years away from our solar system.

And this black hole's path through our galaxy was the key to revealing its fascinating past.

Our solar system and the majority of stars in the galaxy form a relatively thin disk as they orbit the Milky Way's center.

The oldest stars in the galaxy, however, are found in what astronomers call globular clusters, collections of hundreds of thousands of stars each. The globular clusters orbit the galaxy's center in paths that take them far above and below the Milky Way's main disk.

The researchers, who have reported their findings in the journal Nature, found that XTE J1118+480 is following an orbital path similar to that of the globular clusters.

How did it get into such an orbit?

"There are two possibilities: Either it formed in the galaxy's disk and was somehow kicked out of the disk, or it formed in a globular cluster and was kicked out of the cluster," said Vivek Dhawan, an astronomer at the National Radio Astronomy Observatory in Socorro.

"We think it's much more likely that it was gravitationally ejected from the cluster," said Dhawan, who is part of the team that tracked the black hole backward in time.

If, as the astronomers think, this black hole comes from a globular cluster, that makes it the remnant of a very ancient star. Globular clusters today contain our galaxy's oldest stars, but the stars we now see in those clusters are much less massive than the one from which XTE J1118+480 originated.

The more massive a star, the shorter is its lifetime. The black hole of XTE J1118+480 is seven times more massive than our sun. The star from which it originated had more than four times that much matter before shedding some of its mass during its lifetime and then throwing off even more in the supernova explosion that marked its death. Fewer than 10 million years elapsed between this star's birth and its explosive death.

Astronomers say they believe that, as the Milky Way was starting to form, its first stars were very massive, ones such as XTE J1118+480's precursor.

"The star that preceded this black hole probably formed in a globular cluster even before our galaxy's disk was formed," said Felix Mirabel, another team member and an astrophysicist at the Institute for Astronomy and Space Physics of Argentina and the French Atomic Energy Commission.

Other astronomers had done computer simulations indicating that the black holes left over from these giant early stars began an ever-closer gravitational dance in which the partners finally flung each other completely free of the globular cluster.

That means that there should be hundreds of thousands of lonely black holes wandering the galaxy in eccentric, looping orbits. But so far, XTE J1118+480 is the only one to be found.

"This discovery is the first step toward filling in a missing chapter in the history of our galaxy," Mirabel said. "This also is the first time that a black hole's motion through space has been measured."

The discovery required some luck. The first piece came more than 7 billion years ago, when the black hole's travels began with a partner. Before being flung from its home star cluster, the black hole captured a smaller star that now orbits it. Without the companion, astronomers would not have been able to detect the black hole.

Because light cannot escape a black hole, we cannot see one directly. However, this black hole is sucking material from its captured companion star, and this process makes it visible to a variety of telescopes. XTE J1118+480 is one of several "microquasars" discovered since 1994.

In a microquasar, astronomers say, a black hole or neutron star pulls material from a companion. That material forms a tightly circling disk around the black hole or neutron star before falling onto the denser object. Friction heats this disk to temperatures so great that the in-falling material can emit X-rays. Also, strong magnetic fields in the spiraling disk spit out subatomic particles that emit radio waves.

The first microquasar was discovered by Mirabel and Luis Rodriguez, an astronomer at Mexico's National Autonomous University, using the 27-dish Very Large Array radio telescope west of Socorro. Until now, all known microquasars were part of the Milky Way's disk.

On March 29, 2000, the Rossi X-Ray satellite detected X-rays coming from XTE J1118+480. Astronomers using radio and optical telescopes identified the object as a microquasar.

Mirabel and Dhawan teamed with 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.

Together they decided to use the Very Long Baseline Array to study the newly found microquasar.

The array is a system of 10 radio-telescope antennas spread from Hawaii to St. Croix in the Virgin Islands. Two of its antennas are in New Mexico, at Pie Town and Los Alamos, and the entire system is controlled from an operations center in Socorro.

With more than 5,000 miles separating its farthest antennas, the array provides astronomers with the greatest ability to discern fine detail, called resolving power, of any telescope on Earth or in space. This resolving power is equivalent to the ability to stand in New York and read a newspaper in Los Angeles.

The astronomers pointed the array at XTE J1118+480 in May and July of 2000. Because the array could pinpoint the object's location in the sky with extreme precision, the scientists were able to see that it had moved between the times of their observations.

Even though it is 6,000 light-years away - 1,300 times more distant than the sun's nearest stellar neighbor - the powerful array could track its motion. Again, however, luck played a part. The microquasar had experienced an outburst of activity and was only visible to radio telescopes for about 100 days.

"Because this microquasar happened to be relatively close to the Earth, we were able to track its motion with the VLBA even though it's normally faint," Mirabel explained.

Had the object been farther away in its peculiar galactic orbit or not flared while relatively nearby, the key fact of its galactic motion would have remained hidden.

After discovering the microquasar's motion using the array, the astronomers used an additional tool to learn more abut the object's orbit.

In the 1950s, Palomar Observatory in California made a photographic survey of the sky. The photographic plates from that survey later were scanned and digitized by the Space Telescope Science Institute. Using this digital database, the astronomers were able to trace the motion of the black hole's companion star back 43 years.

Combined with the data from the array, this information allowed them to calculate XTE J1118+480's orbital path backward for millions of years, clearly showing that it moves far above and below the galaxy's disk.

Since its violent exile from the star cluster 7 billion years ago, the black hole has been devouring its companion star. That star now has only about one-third the mass of the sun. The black hole's lengthy meal has stripped the smaller star of its outer layers, exposing its innards.

This whole tale, spanning nearly the entire history of our galaxy, was revealed by creatively using the best resources available to astronomers.

"This is a great example of applying multiple tools of modern astronomy -- telescopes covering different wavelengths and digital databases -- to a single problem," Dhawan said.

Explained Mirabel: "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."

Next will come efforts to learn more about both the gigantic stars that first lighted our galaxy and the gravitational dance that flung them out of their stellar clusters.

With excitement, Mirabel looks to new research: "Now we want to find more of these ancient black holes. There must be hundreds of thousands swirling around in our galaxy."

And, the researchers say, each of them can tell us a new tale about how our galaxy began.

TODAY'S BYLINE

Dave Finley is a science writer and public information officer for the National Radio Astronomy Observatory in Socorro. The observatory operates both the Very Large Array and the Very Long Baseline Array radio telescopes for the National Science Foundation.