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Monday, November 15, 1999 Published at 11:00 GMT


Sci/Tech

Scientists 'create' a neutron star

The Z machine is the most powerful X-ray generator on Earth

By BBC News Online Science Editor Dr David Whitehouse

Laboratory physicists have simulated some of the effects found around one of the strangest and most dangerous objects in the known Universe.

In an inconspicuous building in the desert of New Mexico, a machine has been creating temperatures that rival that found at the centre of the Sun. It is helping physicists discover what happens to matter when it falls into the grip of black holes and neutron stars.

The experiments are taking place at the Sandia National Laboratories using the so-called Z machine - the most powerful X-ray generator on Earth.

Exploding stars

Some space-borne observatories are sensitive to X-rays, high energy radiation from the hot and violent parts of the cosmos. X-rays also come from the surfaces of active stars, the remnants of exploded stars and from matter being super-heated as it is being sucked into a black hole or onto the surface of a super-dense object called a neutron star.


[ image: Engineers prepare Z]
Engineers prepare Z
When these titanic events occur astronomers using observatories like the Chandra X-ray observatory look for the presence of iron because its X-ray signature tells them what conditions are like in these violent events.

Creating a neutron star or black hole on Earth for scientists to study would be something of a problem. Even it were technically possible, it would destroy our planet.

But Mark Foord, one of the project leaders, says because "neutron stars and black holes radiate X-rays similar in effect to those emanated by Z, we realised we have a chance to test astrophysical theoretical models that have never been tested experimentally."

The results will further human understanding of black holes and neutron stars.

Super-hot plasma

Colleague Jim Bailey adds, "We're looking at the atomic physics of ionised iron so that they can be compared with theoretical calculations.

"We have a collaboration with four or five groups around the world whose main job is to analyse data from Chandra," says Dr Foord. "They have made some predictions that we're going to compare to our data."

The experiments involve placing thin slivers of iron at the heart of the Z-machine where it is exposed to temperatures of more than one million degrees for a few billionths of a second. The electrons orbiting the atomic nuclei are stripped away and the iron becomes a super-hot plasma radiating X-rays.


[ image: The iron is vapourised in this chamber]
The iron is vapourised in this chamber
"Iron has thousands of spectral lines," says Dr Foord. "We know their signatures but they differ in intensity depending on which electrons, or how many, have been stripped away in the ionising heat of a neutron star. Calculating these relative intensities is uncertain.

"This makes it difficult to predict how highly ionised iron is, or should be, in other star systems. If we can measure in the lab what the actual figures are, we learn how to interpret our data from the stars.

"Our first two shots were in late October and we are now analysing the data," says Dr Foord. "It appears that we were successful at producing highly ionised iron and were able to obtain an accurate measurement of the radiation produced from the Z."



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