New black hole could be clue to stars snuffed out billions of years ago

A black hole discovered in the sun's galactic neighborhood may help astronomers fill key gaps in the Milky Way's history.

In today's edition of the journal Nature, astronomers report the discovery of a small black hole orbiting the galaxy in a region largely inhabited by stars older than the Milky Way itself.

The team suggests that the object, perhaps 5 billion to 7 billion years old, could be a remnant of one of these ancient stars.

"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 formation," says Felix Mirabel, an astrophysicist at the Institute for Astronomy and Space Physics of Argentina who led the team.

Black holes are so dense that nothing can travel fast enough to escape their gravity - not even light. Astronomers detect them by their effect on their surroundings. These effects show up as "last gasp" radiation, given off when matter orbiting nearby accelerates, heats, and plunges into the black hole.

Last week, astronomers using the Chandra X-Ray telescope reported that they had detected radiation from matter falling into a supermassive black hole - a voracious behemoth 3 million times as massive as the sun - at the center of the Milky Way. The work represents "smoking gun" evidence that a black hole lies at the Milky Way's heart.

The Mirabel team's black hole is more modest. Currently, about 6,000 light-years from Earth, the object is six times as large as the sun. Its companion, a star with one-third the sun's mass, orbits the black hole at a distance of 172 million miles.

Astronomer Vivek Dhawan, a member of the team who works at the National Radio Astronomy Observatory's Very Large Array radio telescope in Socorro, N.M., notes that the group was fortunate that the object, dubbed XTE J1118+480, gave off tell-tale X-ray and radio emissions for 100 days, not just short bursts astronomers typically see. This allowed the team to get a snapshot at the beginning and at the end of the period to see how the black hole's position shifted. This information, along with historical observations of the companion star, allowed the team to calculate the black hole's orbit in the galaxy's halo.

Astronomers have spotted roughly a dozen black holes in the halo, the team notes. But this work represents the first time astronomers have established a black hole's orbit around the galaxy.

"There are 100,000 or more dead stars in the Milky Way's halo, and perhaps more than 10,000 black holes. We really don't know," Dr. Dhawan says.

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