NASA: Exotic Innards of a Neutron Star Revealed in a Series of Explosions (Photos)
Goddard Space Flight Center Media November 6, 2002 - (date of web publication) [Note: Modern scientific discoveries, especially astrophysical observations, have tremendously
opened up man's visual field. Some substances such as neutron stars and black holes are far beyond
people's daily life experiences and their existence was only a matter of speculation a few decades
ago. Now they have been verified through scientific observations and experiments. Once again,
science has proved the existence of something that people have once denied or even opposed because
it was not visible or not recognized by the current science and technology.] Amid the fury of 28 thermonuclear blasts on a neutron star's surface, scientists using the
European Space Agency's (ESA) XMM-Newton X-ray satellite have obtained a key measurement revealing
the nature of matter inside these enigmatic objects. The result, capturing for the first time the ratio between such an ultra-dense star's mass and
radius in an extreme gravity environment, is featured in the November 7 issue of Nature. Dr.
Jean Cottam of NASA's Goddard Space Flight Center in Greenbelt, Md., leads this international
effort. The neutron star -- the core remains of a star once bigger than the Sun yet now small enough to
fit within the Washington Beltway -- contains densely packed matter under forces that perhaps
existed at the moment of the Big Bang but which cannot be duplicated on Earth. The contents offer a
crucial test for theories describing the fundamental nature of matter and energy. Cottam and her team probed the neutron star's interior by measuring for the first time how light
passing through the star's half-inch atmosphere is warped by extreme gravity, a phenomenon called
the gravitational redshift. The extent of the gravitational redshift, as predicted by Einstein,
depends directly on the neutron star's mass and radius. The mass-to-radius ratio, in turn,
determines the density and nature of the star's internal matter, called the equation of state. "It is only during these bursts that the region is suddenly flooded with light and we were
able to detect within that light the imprint, or signature, of material under extreme gravitational
forces," said Cottam. The neutron star is part of a binary star system named EXO 0748-676, located in the constellation
Volans, or Flying Fish, about 30,000 light-years away in the Milky Way galaxy, visible in southern
skies with a large backyard telescope. Scientists estimate that neutron stars contain the mass of about 1.4 Suns compacted into about a
10-mile-wide sphere (16 kilometers). At such density, all the space is squeezed out of the atoms
inside the neutron star, and protons and electrons squeeze into neutrons, leaving a neutron
superfluid, a liquid that flows without friction. By understanding the precise ratio of mass to radius, and thus pressure to density, scientists
can determine the nature of this superfluid and speculate on the presence of exotic matter and
forces within -- the type of phenomena that particle physicists search for in earthbound particle
accelerators. Today's announcement states that EXO 0748-676's mass-to-radius ratio is 0.152 solar masses per
kilometer, based on a gravitational redshift measurement of 0.35. This provides the first
observational evidence that neutron stars are indeed made of tightly packed neutrons, as predicted
by theory estimating mass-radius, density-pressure ratios. "Unlike the Sun, with an interior well understood, neutron stars have been like a black
box," said co-author Dr. Frits Paerels of Columbia University in New York. "We have bored
our first small hole into a neutron star. Now theorists will have a go at the little sample we have
offered them," he said. More important, said co-author Dr. Mariano Mendez of SRON, the National Institute for Space
Research in the Netherlands, "We have now established a means to probe the bizarre interior of
a 10-mile-wide chunk of neutrons thousands of light-years away -- based on gravitational redshift.
With the fantastic light-collecting potential of XMM-Newton, we can measure the mass-to-radius
ratios of other neutron stars, perhaps uncovering a quark star." In a quark star, which is denser than a neutron star and has a different mass-to-radius ratio,
neutrons are squeezed so tightly they liberate the subatomic quark particles and gluons that are the
building blocks of atomic matter. To obtain its measurement, the team needed the fantastic radiance provided by thermonuclear
bursts, which illuminate matter very close to the neutron star surface where gravity is strongest.
The team spotted the 28 bursts during a series of XMM-Newton observations of the neutron star
totaling 93 hours. There are dozens of known binary systems with neutron stars, like EXO 0748-676,
where such bursting is seen several times a day, the result of gas pouring onto the neutron star
from its companion star. ESA's XMM-Newton was launched in December 1999. NASA helped fund mission development and supports
guest observatory time. Goddard Space Flight Center hosts the U.S. guest visitor-support center.
Jean Cottam joins Goddard through a grant from the National Research Council. http://www.gsfc.nasa.gov/topstory/20021003nsexplosion.html
Chinese version available at
http://minghui.ca/mh/articles/2002/11/27/40096.html
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