Unprecedented observations of light from a neutron star have illuminated some of the mysteries surrounding these strange stellar leftovers thought to conceal new states of matter, scientists said Wednesday.
Probing into the super-small, hot and dense collapsed star's core with cutting-edge space-based instruments, astrophysicists have uncovered more clues to the fundamental nature of the universe, they said.
A neutron star's features offer scientists the opportunity to study an exotic realm they cannot explore on their home planet, researchers told United Press International. It is composed of the core remains of a star once bigger than our 800,000-mile-diameter sun, yet now small enough to fit within Manhattan Island. It brims with tightly packed matter under forces some scientists speculate existed at the moment of the Big Bang -- forces so extreme they cannot be duplicated on Earth.
Although described as neutron stars since the 1930s, their actual composition remained unproven. A team of astrophysicists set out to learn what a neutron star is made of by teasing out cues from the burned-out star's interior, brought to light by the brilliant flashes of thermonuclear explosions. From the data collected by a state-of-the-art X-ray satellite and a key measurement calculated from their observations, the space sleuths deduced the neutron star is indeed made of neutrons and not some more extreme form of matter.
"We have now bored our first small hole into a neutron star," said study co-author astrophysicist Frits Paerels of Columbia University in New York City. "Unlike the sun, with an interior well understood, neutron stars have been like a black box."
These remnants of exploded stars are so dense a paper clip made from their material would outweigh Mount Everest. Their gravitational collapse crushes atoms, leaving compressed wads of neutrons, the charge-less elementary particles at the center of atomic nuclei. Getting to the core of these oddball bodies might reveal a fantastic world of strange quarks and other unimaginable matter that transcends terrestrial limits, physicists speculate.
Using the Earth-orbiting XMM-Newton X-ray observatory, launched by the European Space Agency in 1999, the researchers analyzed X-rays streaming from the neutron star dubbed EXO0748-676. It is located in the constellation Volans, or Flying Fish, some 30,000 light-years away. A light-year is the distance that light, traveling at about 186,000 miles per second, covers in one year -- nearly 6 trillion miles.
The scientists gleaned the star's interior by measuring how light passing through its quarter-inch-thick atmosphere is warped by extreme gravity in a process called the gravitational redshift. Although gravity cannot slow the speed of light, it stretches out light's wavelengths in predictable and measurable ways. The extent of the stretch, or shift, as predicted by Albert Einstein, depends directly on the star's mass and radius. The mass-to-radius ratio, in turn, determines the density and nature of the star's internal matter, in a formula called the equation of state.
EXO0748-676's mass is known from its gravitational pull on another star it is orbiting, making it a simple matter to calculate the neutron star's diameter and density, said study co-author Jean Cottam of the National Aeronautics and Space Administration's Goddard Space Flight Center in Greenbelt, Md.
The measurement is telling. If the star had been made of more exotic forms of matter, it would have been even denser and had a greater mass-to-radius ratio.
"We finally have data with enough light in it -- light that we are sure is coming from the surface of the neutron star -- that we can measure the fundamental properties of neutron stars, just as we do with ordinary stars," Paerels said.
This first such measurement of a neutron star provides quantitative evidence of the nature of the matter at the star's core, Cottam told UPI.
"The mass-to-radius ratio suggests that this is made of 'ordinary' neutrons, protons, and electrons, albeit at tremendous density," said Cole Miller of the University of Maryland in College Park, who analyzed the findings. "This tends to rule out the most compact models involving strange matter for all neutron stars, not just this one."
In normal matter, protons and neutrons are made of two types of quarks -- elementary particles that cannot be further subdivided -- called up quarks and down quarks. Strange matter is composed of up, down and strange quarks. By some speculative accounts, strange matter might have formed in the early universe and some of its remnants might still exist.
The new data and measurement set the testing ground for current theories describing the fundamental nature of matter and energy, the investigators said. The breakthrough -- made possible by special X-ray detectors designed by astrophysicists in the United States, the Netherlands, England and Switzerland -- is detailed in the Nov. 7 issue of the British journal Nature.
The detectors ride aboard the X-ray Multi-Mirror satellite, the most powerful X-ray telescope ever sent into orbit.
"No other instrument would have had a chance," Miller told UPI.
Celestial objects such as neutron stars generate X-rays -- reflectors of the distant past when stars were born and died and projectors of the future. These emanations are blocked by Earth's atmosphere, however, and only from space can such sources be spotted, located and analyzed in detail.
XMM-Newton, the largest European-built science satellite, carries three advanced X-ray telescopes. Each contains 58 high-precision concentric mirrors that offer the largest collecting area of any such instrument to catch X-rays. The mirror modules allow XMM-Newton to detect millions of sources, far more than any previous X-ray mission.
"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," said study co-author Mariano Mendez of SRON, the National Institute for Space Research in the Netherlands:
To obtain the measurement, the team needed the fantastic radiance provided by thermonuclear bursts, which illuminate matter close to the neutron star surface where its gravity is strongest.
"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," Cottam said.
The team spotted 28 explosions during 93 hours of observations.
"This particular object undergoes bursts of thermonuclear fusion," Miller told UPI. "This is completely expected if EXO0748-676 is a neutron star, but not if it is a star composed of pure strange matter. Therefore, one would have to alter the strange star model further to accommodate the bursts. Other exotic models (e.g., ones in which just the core is made of unusual matter) are not as easy to rule out."
"With the fantastic light-collecting potential of XMM-Newton, we can now measure the mass-to-radius ratios of other neutron stars, perhaps uncovering a quark star," Mendez said.
Quark stars are even denser than neutron stars. However, they are not compact enough to become black holes, objects with such strong gravitational pull even light cannot escape them. The neutrons in these weird objects are squeezed so tightly they free the subatomic quark particles and gluons -- the building blocks of atomic matter.
The ultra-tiny particles that make up much of the universe are impossible to find on their own on Earth. Rather, quarks huddle in groups of three, making up the protons and neutrons inside ordinary atoms. Their existence has come to light in experiments in giant accelerator machines. They appear fleetingly in the debris from atoms smashed together at very high speeds.
"If strange stars exist, or other exotic matter is in the cores of neutron stars, this tells us something fundamental we didn't know before about nuclear interactions and high densities," Miller said.
"(The findings) point to exciting ways of learning about matter in an exotic environment," he added. "I also find it cool that observations, with telescopes, of objects thousands of light-years away can tell us new things about matter at subnuclear scales! In a sense, that brings together the unity of the universe, at large scales and small."
