Neutron Star Interior Revealed

By SpaceRef Editor
November 6, 2002
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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

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

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.

For animation, images, and more information, refer to:

SpaceRef staff editor.