An artist's concept of a neutron star. Photo: Robert Mallozzi, U. Alabama/Huntsville & NASA

As if daily nuclear explosions on neutron stars releasing more energy in 10 seconds than the Sun does in a week weren't fantastic enough, a NASA astronomer observed a far more powerful blast lasting 1,000 times longer.

Dr. Tod Strohmayer of NASA Goddard Space Flight Center in Greenbelt, Md., observed a three-hour burst on a neutron star 20,000 light years from Earth. The burst was likely caused by over a year's worth of stored carbon -- the nuclear ash from daily helium-fueled explosions -- packed so tightly below the neutron star surface that it finally fused and exploded.

Dr. Strohmayer's results, which may mark the first observation of a carbon- fueled thermonuclear explosion on a neutron star, were presented this week at the meeting of the High Energy Astrophysics Division of the American Astronomical Society in Honolulu, Hawaii.

"This was quite a find," said Dr. Strohmayer. "We had suspected that such an explosion could exist; but because they are so rare, we didn't know if we could actually observe one. Such a long burst -- with a rich assortment of X-ray data -- provides new insights into the physics of neutron stars and thermonuclear explosions, particularly about what is happening underneath the surface."

A neutron star is the skeletal remains of a star once several times more massive than the Sun that exhausted its nuclear fuel and subsequently exploded its outer shell. The remaining core, still possessing about a Sun's worth of mass, collapses to a sphere no larger than Honolulu, about 7 miles in diameter.

Typical neutron star bursts last about 10 seconds, a thousand times shorter and 500-1,000 times less energetic than this burst in binary star system 4U 1820-30, observed with the Rossi X-ray Timing Explorer. The three-hour burst from the tiny neutron star released 20 times more energy than the Sun does in a year. Put another way, this three-hour burst from a 7-mile-wide neutron star released more than a trillion times the amount of energy used by the entire United States in 1999. Also, the typical 10-second bursts on neutron stars release about a billion times more energy than the 1999 U.S. consumption.

The neutron star in 4U 1820-30 is in tight orbit with a low-mass dwarf star composed of mostly helium. Their orbital period of 11 minutes is the shortest known of any binary system. In fact, the stars are so close that the orbit would easily fit inside the Sun. The neutron star's strong gravitational field attracts gas from the companion star. This gas eventually rains down upon the surface of a neutron star, a journey visible in many forms of light, particularly X rays.

When enough gas builds up on the neutron star surface -- in the case, helium gas -- the increased pressure raises the temperature and initiates helium fusion, a nuclear reaction that manifests itself as an X-ray burst. X-ray bursts often erupt on neutron stars in binary systems several times a day.

Dr. Strohmayer said an "ordinary" burst may have triggered the much longer one. The data reveal that there was a burst that lasted about 10 seconds before fading. Then, a few seconds later, the longer burst ignited ... and kept on going.

The three-hour nuclear inferno was likely fueled by carbon, the ashes of helium fusion, Dr. Strohmayer said.

"Over the course of a year or two, more and more helium rains down upon the neutron star," said Dr. Strohmayer. "This helium ignites and produces carbon. The carbon ash builds up under layers of new helium and other gaseous metals. When enough carbon builds up -- and the pressure raises the temperature to many times that of our Sun's core -- carbon will begin to fuse."

Dr. Strohmayer estimated that it would take about a billion trillion pounds of carbon and a temperature of a billion degrees to create the three-hour explosion on this particular neutron star. At the rate at which material is crashing down on the star's surface, Dr. Strohmayer estimated that it would take about 1 to 2 years for that much carbon to build up. The amount of carbon consumed in the explosion was about mass of Pluto or 1/10th the total mass of the Moon.

The explosion itself is interesting to scientists because (1) it lasted so long, providing plenty of X-ray photons to analyze, and (2) it showed such unique fluctuations or patterns during those three hours, providing the meat for in-depth analysis of the physics of neutron stars, accretion, and thermonuclear reactions.

Little is known about the interior of neutron stars, as opposed to hydrogen- burning stars, such as the Sun. Their densities are far greater than anything that can be reproduced in a laboratory. Dr. Strohmayer said that scientists could use data from carbon-fueled explosions, like the one observed by the Rossi Explorer, to probe beneath the neutron star surface. Two key interests are determining the neutron star's equation of state, or how compressible the material is, and also understanding the physics of matter at the Universe's most extreme density and gravity.

"With the Rossi Explorer, we are now not only seeing the brilliant activity on the surface of a neutron star, we are also seeing what goes on underneath," said Dr. Jean Swank of NASA Goddard, Project Scientist for the Rossi Explorer. "This recent observation provides information from much deeper under the surface than we have been getting with smaller bursts, where the accreting gas is being converted into neutron star material."

The Rossi X-ray Timing Explorer is the result of a NASA collaboration with Massachusetts Institute of Technology and University of California, San Diego, assembled and operated by NASA Goddard. Launched in 1995, The Rossi Explorer is a unique type of X-ray observatory that measures the rapid fluctuation of X-ray activity in pulsars and neutron stars, magnetars, black holes and active galactic nuclei. The Rossi Explorer looks for changes in X-ray patterns over timescales as short as a millisecond, which can reveal the physics of how matter is behaving under the force of extreme gravity. As such, the Rossi Explorer can test the Theory of General Relativity and laws of physics in ways not possible in Earth-bound laboratories.