WASHINGTON, April 24 - 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 today.
Just how efficient? If a car could use this kind of engine, it could theoretically go about 356 million km on 1 litre of gas 
, said Steve Allen of Stanford University in California.
Unfortunately, no earthly car could do this, as black holes are fuelled 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 magnetised disks of gas.
These jets speed away from the black hole at speeds up to 95 per cent of the speed of light, which is 300,000 km per second, and create huge bubbles in the hot cosmic gas of the galaxies.
:eek: These bubbles can measure hundreds or even thousands of light-years across. A light-year is about 10 trillion km, 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 such as the ones the researchers observed, but probably not to the Milky Way, which contains earth.
These giant galaxies are about 10 times as massive as ours.
These supermassive black holes are ``green’’ in another way, said Kim Weaver of NASA’s Goddard Space Flight Centre: 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 centre; without this source of heat, the gas would cool down and form new stars, Weaver said.
``This is one way to keep the stars from forming and letting the galaxies grow bigger,’’ she said.
More information is available online at http://chandra.harvard.edu.
Maybe I am wrong but I thought that galactic sprawl referred to galaxies expanding and growing larger. First they say it limits it then they got a quote saying it promotes it?