Image caption Artist's concept: Pulsars are so-called because of the way their radio emission is detected at Earth

Astronomers have discovered what they say is the mightiest neutron star yet.

The super-dense object, which lies some 3,000 light-years from Earth, is about twice as massive as our Sun.

That is 20% greater than the previous record holder, the US-Dutch team behind the observation tells the journal Nature.

Like all neutron stars, the object's matter is packed into an incredibly small space probably no bigger than the centre of a big city like London.

"The typical size of a neutron star is something like 10km in radius," said Dr Paul Demorest from the National Radio Astronomy Observatory (NRAO), Charlottesville, US.

The size is easy to understand but the density is much more extreme than anything we know here on Earth Dr Paul Demorest, National Radio Astronomy Observatory

"It's approximately the size of a city, which for an astronomical object is interesting because people can conceive of it pretty easily; and yet in that space it has the mass in this case about two times our Sun. So the size is easy to understand but the density is much more extreme than anything we know here on Earth," the study's lead author told BBC News.

The finding is important, says Dr Demorest's team, because it puts constraints on the type of exotic material that can form a neutron star.

Such objects are thought to be the remnant cores of once giant stars that blew themselves apart at the ends of their lives.

Theory holds that all atomic material not dispersed in this supernova blast collapses to form a body made up almost entirely of neutrons - the tiny particles that appear in the nuclei of many atoms.

As well being fantastically compact, the cores also spin incredibly fast.

This particular object, classified as PSR J1614-2230, revolves 317 times a second.

It is what is termed a pulsar - so-called because it sends out lighthouse-like beams of radio waves that are seen as radio "pulses" every time they sweep over the Earth.

Image caption The observations were made using the Green Bank Telescope in West Virginia

The pulses are akin to the ticks of a clock, and the properties of stable neutron stars make for ultra-precise time-pieces.

This was how the team, observing with the Green Bank Telescope in West Virginia, was able to measure the object's mass.

Because PSR J1614-2230 also circles a companion star, its pulses - as received at Earth - are disturbed by the neighbour's gravity.

"The way it works is that as the pulses travel from the neutron star past the companion, they slow down a little bit.

"And how we see that on Earth is that the pulses arrive a little later than we would otherwise expect when the neutron star is lined up behind the companion," Dr Demorest said.

The team could use this effect to calculate the masses of both bodies.

The group reports a pulsar mass 1.97 times that of our Sun - significantly greater than the previous precise record of 1.67 solar masses.

The result is said to put limits on the type of dense matter that can make up the cores of these bizarre objects.

Some scientists had suggested exotic particles such as hyperons, kaon condensates or free quarks could exist deep inside neutron stars. But Dr Demorest and colleagues believe their observations preclude this possibility.

"It's simply that if those particles were formed, the star would get too dense and collapse into a black hole prior to this point," the NRAO researcher said.

Jonathan.Amos-INTERNET@bbc.co.uk