[5] SPACE: IT REALLY IS A NEUTRON STAR (pp51-54; N&V) Astronomers have analyzed X-ray light coming from a super-heavy object called a neutron star well enough to more accurately depict the stars dimensions and confirm that theories about the objects are on track.
Neutron stars created when old stars explode and some of the core matter typically weighing more than our entire Sun -- collapses into a region just 6 or 7 miles (roughly 10 kilometers) across. A teaspoonful would weigh a billion tons. Neutron stars are thought to be composed almost entirely of neutrons, subatomic particles able to huddle close together on the verge of normal physics.
While they are considered cosmic corpses, neutron stars can remain active.
The object of the new study has an orbiting companion, another relatively normal star. The neutron star siphons hydrogen and helium from the companion, and when enough builds up on its surface, a thermonuclear reaction generates an
outburst of energy. The new study examined 28 these outbursts with the European Space Agencys XMM-Newton X-ray observatory.
"We have been able to measure the ratio of the mass to the radius of the star, a quantity that depends on the properties of the material that makes up the interior of the star," said Frederik Paerels, a Columbia University researcher who worked on the new study. The work was led by Jean Cottam of NASAs Goddard Space Flight Center and will be detailed in the Nov. 7 issue of the journal Nature.
The technique is expected to eventually allow researchers to determine whether more exotic stars exist in even denser configurations. One example is the theoretical quark star.
Theorists have speculated that at certain high densities, neutrons dissolve into their constituent quarks, and the matter makes a transition to free quark matter, Paerels explained. "This is truly exotic, because quarks, under ordinary conditions, cannot exist as free, single particles. This is what is meant by an 'exotic' star or a quark star."
In April, another research group using the Chandra X-Ray Observatory announced evidence that one suspected neutron star, named
RXJ-1856, might be a quark star or some other wild object more dense than a neutron star. That finding has not been confirmed.The classification of the newly studied star, named EXO07482-676, is not certain either.
"Our current measurement tells us that the object could be either a neutron star or an exotic star," Paerels told SPACE.com. "But this is the first time that this technique has been applied to a compact star, which means that we may now finally be able to experimentally determine whether exotic stars indeed exist. For this, we'll need more data, on different stars, and we will have to put together an accurate theoretical model of what the outside of a compact star really looks like."
The work does not provide any clues to the possible existence of quark stars, Paerels emphasized. It is important, nonetheless, because it puts a constraint on the ratio of mass-to-radius in a neutron star, and that constraint turns out to be consistent with theoretical predictions.
"That is a success," Paerels said. "Current theory is close, evidently, and passes the test set by our data."
While firm proof for the existence of a quark star would be "tremendously exciting," Paerels thinks such a finding "is still fairly far away."
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