What's inside a neutron star? Don't assume it's just neutrons.

Yes, neutron stars are mostly made of neutrons. But deep inside they might harbor exotic particles called free quarks, the building blocks of protons and neutrons. Scientists study neutron stars to understand the nature of matter and energy.

A neutron star is not a place you would want to visit. These are the dense remnants of collapsed stars. Stars several times more massive than our Sun will ultimately shed most of their material in an explosion called a supernova. The core, about the mass of Earth, remains; and it collapses into a sphere only about 15 kilometers wide. That's a tight fit. And that's why strange things happen in, on, and around a neutron star.

The neutron star magnetic field is billions of times stronger than Earth's, enough to shuffle your body's molecules long before you even land. The featureless surface is no fun either. Crushing gravity ensures that the star is a near-perfect sphere, compressing all matter so that a sand-grain-sized scoop of neutron star material would weigh as much as a cruise ship on Earth.

Beneath the neutron star crust, a kilometer-thick plate of crystalline matter, lies the great unknown. The popular theory is that the neutron star interior is made up of a neutron superfluid -- a fluid without friction.With the help of X-ray satellites, scientists are journeying (virtually) to the center of neutron star. Matter might be so compressed there that it breaks down into quarks and gluons. Gluons, like "glue," are the strong force that hold protons, neutrons and thus atom together.

To dig inside a neutron star, no simple drill bit will do. Scientists gain insight into the interior from afar through events called glitches, a sudden change in the neutron star's precise spin rate.

Glitches speed up neutron star spins. The neutron star's magnetic field slows the crust's spin, yet the superfluid interior spins down at a slower rate. As the difference of the two speeds widens, the superfluid reaches a critical threshold and, like a flywheel, suddenly transfers its built-up angular momentum to the crust, causing the "exterior" of the neutron star to spin faster. The glitch's size -- that is, how much it quickens the spin --is a reflection of the mass, density, and other characteristics of the superfluid.

Given enough pressure, the neutron superfluid will dissolve into quarks, perhaps in the core. Quarks normally exist in pairs or triplets and are bound to each other by gluons. Theory has it that quarks cannot exist in a free state, but that theory may fall by the wayside in a neutron star's inner core. No one knows for sure. Glitches can only provide information about the amount of superfluid, not quarks.

Some neutron stars might pull down enough gas from a companion star to add enough pressure (sort of like weight) on its surface to squeeze the neutron superfluid interior into free quarks. Or, as the neutron star's spin slows down over millions of years, the centrifugal force from within could ease enough to allow the crust to collapse, liberating quarks. In either case, the diameter of the neutron star would shrink, quickening the spin. The neutron star would become a theorized quark star, full of quarks instead of neutrons. If it collapsed any further, it would become a black hole. A quark star is a stepping stone to a black hole.

To understand the nature of the neutron star's interior, astronomers need to determine the ratio between the star's mass and radius. Masses are well known, particularly for neutron stars in binary systems in which the orbital period is dictated by mass and distance. No one has accurately measured a neutron star radius, however, because these objects are so tiny.

A team led by Jean Cottam of NASA Goddard Space Flight Center have determined the mass-to-radius ratio of one neutron star. With the XMM-Newton satellite, the team observed how light passing through the star's centimeter-high atmosphere is warped by extreme gravity, called the gravitational redshift. The extent of the gravitational redshift, as predicted by Albert 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 -- that is, whether the star is compact enough to house free quarks.

It turns out that this neutron star likely doesn't harbor free quarks. But others certainly might. The search continues.

Chris Wanjek is a science writer supporting the Beyond Einstein initiative, a roadmap to understand the forces of nature beyond General Relativity and Quantum Mechanics through the study of the Universe from the Big Bang to black holes.