April 25, 2006

Black holes are energy-efficient “green” : study

By Deborah Zabarenko

WASHINGTON (Reuters) - Black holes turn out to be "green."

These monstrous matter-sucking drains in space are the most
fuel-efficient engines in the universe, researchers said on

Just how efficient? If a car could use this kind of engine,
it could theoretically go about a billion miles (1.6 billion
km) on a gallon of gas, said Steve Allen of Stanford University
in California.

Unfortunately, no earthly car could do this, as black holes
are fueled by matter lured by the holes' vast gravity. Their
pull is so strong that nothing, not even light, can escape.

Most of the energy released by this matter as it gets close
to the black hole's point of no return -- known as the event
horizon -- shows up in the form of high-energy jets, which spew
forth from magnetized disks of gas.

These jets speed away from the black hole at speeds up to
95 percent of the speed of light, which is 186,000 miles

per second, and create huge bubbles in the hot cosmic gas
of the galaxies.

These bubbles can measure hundreds or even thousands of
light-years across. A light-year is about 6 trillion miles, the
distance light travels in a year.


To come up with their fuel-efficiency estimate, Allen and
his colleagues used NASA's orbiting Chandra X-Ray Observatory
to peer into the inner regions of nine giant elliptical
galaxies. This view gave the scientists an idea of how much
matter -- the black holes' fuel -- was available.

Other Chandra observations were used to figure out how much
power would be required for the jets to produce the big
bubbles, the researchers said in a telephone news briefing. The
energy in the jets is some trillion trillion trillion watts,
Allen said.

The fuel reservoirs for these high-efficiency black hole
engines are so big they could keep things going for hundreds of
billions of years -- many times the current estimated age of
the universe, which is 13.7 billion years.

This fuel-efficient phenomenon may be common to giant
galaxies like the ones the researchers observed, but probably
not to the Milky Way, which contains Earth. These giant
galaxies are some 10 times as massive as ours.

These supermassive black holes are "green" in another way,
said Kim Weaver of NASA's Goddard Space Flight Center: they
appear to limit star formation and galactic sprawl.

The heat coming from the black hole jets is believed to
heat up the gas around the galactic center; without this source
of heat, the gas would cool down and form new stars, Weaver

"This is one way to keep the stars from forming and letting
the galaxies grow bigger," she said.

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