Like "black holes", neutron stars are the dead cores of massive stars that exploded as supernovae. Their neutrons are packed together so tightly that a teaspoonful of the material would weigh several billion tons.

For years, science has held that matter might be transformed into exotic states in the dense interiors of neutron stars because of the kind of pressures to which matter is subjected. Some believed that neutrons would break down, freeing the individual sub-atomic particles - known as quarks - of which they are made.

Another theory suggests that such pressure might lead to a form of matter known as a Bose-Einstein condensate (BEC) in which neutrons do not break down into quarks. Their individual identities blur, however, as they start to behave as a single particle.

A new analysis of one of neutron stars now indicates that free quarks - never found by themselves in nature - in fact do not come from the cores of these massive bodies.

The old "exotic matter" theories, as they were known, have been disproved in a study by Feryal Ozel of the University of Arizona. An analysis of the mass and radius of a neutron star known as EXO 0748-676, found that it is probably made of ordinary neutrons.

Ozel calculated neutron star's radius to be 13.8 kilometers (about nine miles). Surprisingly, however, its mass came out to over twice that of the sun, suggesting that the star's neutrons are normal. As the mass of a neutron star increases, it becomes more and more rigid. Otherwise, it would collapse into a black hole under its own gravitational force. Most simulations of quark stars and BEC-containing neutron stars predict they would collapse into a black hole before reaching this great a mass.

"I think the physical measurement procedure is sound," says Frits Paerels of Columbia University in New York, US. "The number that comes out of it is interesting. The mass is surprisingly large."

Most neutron stars whose masses have been measured previously are no more than 1.4 to 1.5 times the mass of the Sun - but their actual sizes in terms of volume have been difficult determine, making the nature of the matter inside unclear.

Ozel says the fact that squishy, exotic states of matter do not seem to occur in a star this massive indicates that "exotic states" simply don't exist in these neutron stars.