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Astronomers say they may have solved a cosmic mystery: why gravitational monsters known as black holes are inept at swallowing their prey.

A black hole can form in space when a large star dies and its matter gets crunched into a much smaller volume. The resulting gravitational pull is so great that even light can't escape. Given this power, one theory was that black holes indiscriminately consumed everything that passed within their reach.

However, scientists recently observed that this scenario isn't always the case—and they now believe they understand why.

"We found that when matter flows toward a black hole, more than 99% of it is ejected" away from it, said Q. Daniel Wang, astrophysicist at the University of Massachusetts Amherst, who led a team that cracked the mystery.

Scientists had plenty of theories about how black holes worked, but few facts based on observations.

A black hole's gravitational force can be measured by X-ray emissions, which show the amount of heat generated. When researchers had previously looked at black holes' X-ray signals, they were surprisingly faint, suggesting that black holes weren't generating large amounts of energy in the form of radiation.

The latest research provides an answer based on actual data from a space-borne X-ray telescope, known as Chandra, operated by the National Aeronautics and Space Administration. Dr. Wang's data, published Thursday in the journal Science, were collected for nearly five weeks, with Chandra aimed at the massive black hole at the center of the Milky Way.

One observation was that the distribution of hot gas streaming toward the Milky Way's black hole matched the distribution patterns of hundreds of young, hot stars orbiting the black hole. This is significant because it indicates that the hot gas had been blown off from the stars in the direction of the black hole.

"No one has observed this connection before," said Dr. Wang.

Astronomers knew that gas falling toward a black hole got hotter and should glow. So they had been puzzled to see the area around the Milky Way black hole glowing less than expected.

Dr. Wang's team has now shown that more than 99% of the gas streaming toward the Milky Way black hole is being repelled, which explains why radiation from the black hole was faint.

"Either all the gas that's pouring toward the black hole is not getting in, or it's getting sucked in without shining," says Jeremy Schnittman, a NASA astrophysicist not involved in the study. The latter theory, he adds, "now seems to be ruled out."

The explanation: the amount of gas and other matter getting pulled toward a black hole depends not just on the gravity it feels, but also on the speed and direction of movement of the particles.

"The particles are moving very fast, competing with gravity," notes Dr. Wang. Typically, the particles win, which is why most of the gas is rejected from the black hole.

Other astrophysicists have lately been monitoring a different event—a gas cloud, known as G2, being stretched by the gravitational force of the Milky Way black hole.

Thursday's findings in Science have implications for that event as well.

The G2 cloud is orbiting the inner region surrounding the black hole, and it could brighten when it interacts with the flow of the ambient gas from the stars. The extent of brightening depends on the density of the ambient gas; Dr. Wang says HIS FINDINGS imply that the G2 brightening should be substantially lower than what scientists had expected.

A version of this article appeared August 29, 2013, on page A4 in the U.S. edition of The Wall Street Journal, with the headline: Scientists Shed New Light on Black Holes.