posted June 24, 2006 04:29 PM              

Jun. 22, 2006. 08:21 AM
PETER CALAMAI
SCIENCE WRITER

An artist's representation shows the binary star system observed by NASA. This binary consists of a black hole and a normal star shown in blue. Gas is being pulled away from the star and falling onto a red disk spinning around the black hole. Some of this gas spirals in towards the black hole, generating copious amounts of light along the way.

Scientists have cracked a huge cosmic paradox — how black holes can be the darkest objects known but also responsible for a quarter of all light and other radiation produced in the universe since the Big Bang.

The same cosmological breakthrough is also being heralded as showing the way to understand better how galaxies and stars form, considered the current Grand Challenge in astronomy.

And, as people once said about magic tricks, it’s all done with magnets — or at least, magnetic fields.

A team of American astronomers announced yesterday the best evidence yet that invisible magnetic fields explain how black stars consume their surroundings and also trigger intense stellar beacons called quasars, the brightest objects in the universe.

The findings, published in the journal Nature, brought praise from Jeremy Heyl, who holds a Canada Research Chair in black holes and neutron stars at the University of British Columbia.

“Up until now most of the evidence we had for magnetic fields was circumstantial. This is like finding someone with a gun that’s still smoking. Even if you didn’t see them pull the trigger, you’re pretty sure who’s responsible,” he said.

Led by astrophysicist Jon Miller from the University of Michigan, the U.S. researchers used observations by a specialized NASA X-ray satellite to eliminate two other theories of what allows matter to fall into black holes despite a natural tendency to stay in orbit.

The team looked at a system called J1655 made up of a black hole and a companion star about 11,000 light years from Earth, next door in astronomical terms. The black hole sucks material from the star into an “accretion disk” that surrounds the black hole, somewhat like Saturn’s rings.

In April 2005, Miller and colleagues recorded the cosmic duo for 18 hours as the black hole was “gorging” itself on the star, giving off extra-strong “burps” or signals recorded by the satellite Chandra, NASA’s X-ray observatory.

“It’s very likely magnetic fields that are making this process possible and making it luminous as well,” Miller told a webcast news conference.

The scientists say the magnetic fields of J1655 — 10,000 stronger than the Earth’s — channel jets of super-hot matter that shoot out from the accretion disk.

These jets drain away enough energy that the rotating material is sucked down by the intense gravity of the black hole.

Energy lost as the material is sucked towards the black hole is emitted as radiation across the entire spectrum, including X-rays and the intense light beams of quasars.

Without this magnetic interference or some other drag, the matter would stay in orbit forever just like planets around the sun.

“It’s like a missing link. We could see this light from the vicinity of black holes but we didn’t know how it was produced,” said Meg Urry, a Yale university astronomy professor familiar with the research.

Urry and others said the magnetic field discovery is likely to trigger further waves of research into other disks found around young Sun-like stars while planets are being formed and also around ultra-dense objects called neutron stars.

“Every galaxy has a black hole at its centre and this process explains why galaxies don’t get any bigger,” said Urry.

Knowing the role of magnetic fields will also help astronomers get a better fix on the power and duration of these interstellar jets.

Heyl said jets formed by super massive black holes are thought to shoot over vast distances, as far as the Earth is from the centre of the Milky Way galaxy.

“And that gets us into the question of how galaxies are formed and how stars are formed. It’s all connected in a way,” he said.

Astronomers estimate that radiation from around black holes accounts for about a quarter of all radiation since the universe began in the Big Bang. A relative few black holes produce so much radiation because they are a hundred times better than stars at converting matter into energy, Heyl said.

http://www.thestar.com/NASApp/cs/Cont entServer?pagename=thestar/Layout/Article_Type1&call_pageid=971358637177&c=Article&cid=1150970532767