Black hole engine is the new green

An artist's impression of a black hole engine. Picture / Reuters

WASHINGTON - Black holes turn out to be "green". These monstrous matter-sucking drains in space are the most fuel-efficient engines in the universe, researchers say.

Just how efficient? If a car could use this kind of engine, it could theoretically go about 1.6 billion kilometres on 4.5 litres of petrol, 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 up to 95 per cent of the speed of light, which is 300,000km 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 10 trillion kilometres, the distance that 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. 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 10 times as massive as ours.

These 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 from the black hole jets is believed to heat the gas around the galactic centre; without this heat source, the gas would cool and form new stars, Weaver said.

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

- REUTERS


INTO THE BLACK

* Black holes are not holes in the usual sense but rather part of space from which nothing can return.

* They are concentrations of mass great enough that the force of gravity prevents anything past its point of no return or "event horizon" from escaping.

* The gravitational field is so strong that the escape velocity past its event horizon exceeds the speed of light. This implies that nothing, not even light, inside the event horizon can escape its grip.