Astronomers have found the most massive neutron star yet measured ? one nearly twice the mass of our sun. The discovery indicates that, as their name suggests, these stellar remnants really are made mostly of neutrons, as opposed to more exotic particles.
Neutron stars are fast-spinning remnants left behind in the aftermath of supernovas: huge star explosions where protons are crushed together with electrons to form neutrons. They are typically small, with diameters of about 12 miles (19.3 km) or so, but yet so massive they weigh as much as the sun.
But the new, precise neutron star measurements have revealed an object more massive than any neutron star yet observed. At nearly twice the mass of our sun, the star is about 20 percent more massive than the last neutron star record-holder of 1.67 solar masses. [Top 10 Star Mysteries]
"We didn't really know for sure that neutron stars could get quite this massive until we made this measurement ? it was very surprising and exciting," researcher Paul Demorest, an astronomer at the National Radio Astronomy Observatory, told SPACE.com. "The typical thinking was that most neutron stars clustered pretty tightly around 1.4 solar masses."
While stars come in all sizes and can be dozens or hundreds of times the mass of the sun, neutron stars ? because of their properties ? are unique in that astronomers have long-thought they were limited to masses around 1.4 times solar masses.
The record-breaking neutron star is called PSR J1614-2230 and is roughly 3,000 light-years from Earth.
What's it really made of?
Neutron stars are made of ultra-dense matter. A chunk of a neutron star the size of a sugar cube can weigh about 100 million tons. This extraordinary density makes neutron stars ideal ways to study the densest and exotic states of matter known to physics that require far too much energy to replicate in stable form here on Earth.
While astronomers have long thought that neutron stars are composed solely of neutrons, some scientists have recently proposed they might also contain more exotic subatomic particles as well, such as hyperons and kaon condensates, which possess so-called "strange quarks."
Although quarks ? the building blocks of protons and neutrons ? are generally thought to always be confined atomic nuclei in nature, some researchers had also suggested neutron stars might contain unbound "free quarks."
Most massive neutron star
To learn more about neutron stars, investigators focused on PSR J1614-2230, which is a millisecond pulsar, a neutron star that emits radio pulses and spins completely around roughly every three thousandths of a second. Millisecond pulsars spin very reliably, serving as very stable timekeepers ? changes of even a few millionths of a second can be detected.
This pulsar is a binary, in mutual orbit with a companion star, a white dwarf.
To determine the neutron star's mass, researchers measured a delay in the travel time of its radio pulses resulting from them getting distorted by the companion star's gravitational field. This effect, called the Shapiro delay, varies systematically as the paired stars orbit each other, and precise analysis of it allowed scientists to determine the white dwarf's mass.
Since the investigators know the orbital characteristics of the binary system as a whole, knowing the companion star's mass enabled them to calculate the pulsar's mass as well.
"We got very lucky with this system," said researcher Scott Ransom, an astronomer at the National Radio Astronomy Observatory in Charlottesville, Va.
The paired stars are in an orbit almost perfectly edge-on from Earth, making the variation in the distortions of the radio pulses more pronounced, researchers said. Also, the white dwarf is unusually massive for a star of its type, meaning its gravitational field had an especially profound effect on the pulses.
"This unique combination made the Shapiro delay much stronger and thus easier to measure," Ransom added.
The scientists narrowed the pulsar's mass to 1.97 times the mass of the sun, give or take 0.04 solar masses.
This high mass rules out nearly all currently proposed models for neutron star matter that involve exotic particles such as hyperons and kaon condensates, Demorest explained. Those exotic particles are more essentially squishier than neutrons, and if a neutron star that massive did possess those particles, it could squeeze together so much that it would collapse into a black hole.
Although the matter in the neutron stars could be made of quark matter, it could only support a star this massive if they strongly interact with each other as they do in normal atomic nuclei and not if they were free, added researcher Feryal Ozel of the University of Arizona.
These new findings could also shed light on the origins of gamma ray bursts, the most powerful explosions in the universe. A leading explanation for the cause of one type of gamma-ray burst ? the "short-duration" bursts ? is that they are caused by colliding neutron stars. The fact that neutron stars can be as massive as PSR J1614-2230 hints these collisions would be powerful enough to generate these bursts.
"Pulsars in general give us a great opportunity to study exotic physics, and this system is a fantastic laboratory sitting out there, giving us valuable information with wide-ranging implications," Ransom said. "It is amazing to me that one simple number ? the mass of this neutron star ? can tell us so much about so many different aspects of physics and astronomy."
The research is detailed in the Oct. 28 issue of the journal Nature. They will also detail their calculations regarding neutron stars and free quarks in Astrophysical Journal Letters.