Sometimes in astronomy an object has to disappear before you can take its full measure. Just ask Michael Nowak, who has examined the changing output of a wild and crazy star as it dimmed, brightened and blinked out completely every few hours.

The object is a dense neutron star. It's about as big as Boston yet weighs more than our Sun.

Nowak and his colleagues used the orbiting Chandra X-ray Observatory to examine changes in output from the neutron star caused by a fortuitous eclipse by its companion. Only a small percentage of binary systems are arranged such that one star passes in front of the other as seen from our Earth.

The researchers also found a second and more subtle eclipse event that they think is caused by a blob of hot gas also orbiting the dense central object.

Further, the new observations support an old idea that a third star may be involved in the system.

The neutron star, called 4U 2129+47, is thought to be no more than 6.2 miles in diameter (10 kilometers). Like all such objects, it is the decrepit remnant of an exploded star, now said to be composed entirely of neutrons, which can pack together so tightly that a teaspoonful weighs more than an elephant.

The companion star, roughly one-third as massive as our Sun, orbits the neutron star and feeds material to it. This stuff, mostly hydrogen gas, develops into a fairly flat disk -- common to such systems -- as it swirls inward toward the neutron star.

Nowak explained what happens next:

"As new material comes in ... it first freefalls in a spiral, and then rams into the edge of a disk," he said. "This locally heats up the disk and raises a bulge on the edge of the disk."

Because the bulge came from the orbiting companion star, it orbits the neutron star at the same pace. When the bulge passes in front of the neutron star, it creates the subtle eclipse -- a drop in X-rays observed by Chandra.

X-rays are among the most intense forms of electromagnetic radiation along of spectrum that includes radio waves and visible light.

Nowak describes the subtle X-ray eclipse as something like the change in visible light caused by a cloud passing in front of the Sun. The more pronounced eclipse, created when the binary star completely blocks the X-rays coming from the neutron star, can be thought of as similar to a total solar eclipse observed from Earth when the Moon gets in the way.

If 4U 2129+47 were nearby, and if you had X-ray vision, the eclipse of the binary companion star would cause the neutron star to wink out every 5.25 hours, for about 26-minutes, and then turn back on just as suddenly.

About 20 years ago, this neutron star had an outburst of energy at least 3,000 times greater than what is produces today. Nowak said the X-rays then would have emanated from an expanded disk loaded with extra material. The new observations allow calculations showing how the disk has shrunk, probably to about half its previous size.

He said the difference from active to quiet states, as observed in X-rays, can be compared in visible light to a star that would range from being the brightest in the sky to the dimmest possibly visible to the unaided eye.

The new observations also strengthen a long-held speculation that 4U 2129+47 may have a second stellar companion, bringing the system's total of gravitationally bound stars to three.

X-ray observations so far have not allowed an accurate determination of the distance to 4U 2129+47. But in roughly the same location of the sky, another star -- an object slightly more massive than our Sun called an F star -- has been observed in visible light and determined to be about 20,000 light-years away.

"Because Chandra has fabulous resolution in the X-rays, we can tell that to very high accuracy that the X-ray image is sitting right on top of the optical image of the F star," Nowak said.

The observations also allowed the researchers to estimate how much interstellar material exists between Earth and the neutron star, and it's about the same amount that has been calculated to exist between Earth and the F star.