Astronomers discover one of the most massive neutron stars ever measured, packing 2.17 times the mass of the sun

The neutron star, called J0740+6620, was detected approximately 4,600 light-years from Earth and is only around 15 miles across


                            Astronomers discover one of the most massive neutron stars ever measured, packing 2.17 times the mass of the sun

Scientists have discovered one of the most massive neutron stars to date, which packs 2.17 times the mass of the sun into a sphere only 20-30 kilometers or about 15 miles across. To put into perspective how massive the star is, the sun is 333,000 times the mass of the Earth. The breakthrough was achieved through the Green Bank Telescope in Pocahontas County, West Virginia.

The neutron star - spotted by members of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) Physics Frontiers Center - is a rapidly rotating millisecond pulsar, called J0740+6620. The Center is funded by the National Science Foundation. 

"This measurement approaches the limits of how massive and compact a single object can become without crushing itself down into a black hole. Recent work involving gravitational waves observed from colliding neutron stars by LIGO (Laser Interferometer Gravitational-Wave Observatory) suggests that 2.17 solar masses might be very near that limit," say the researchers in their findings, published in Nature Astronomy.

According to Eberly distinguished professor of physics and astronomy Maura McLaughlin from West Virginia University, the discovery is one of many 'serendipitous' results that have emerged during routine observations taken as part of a search for gravitational waves. The star was detected approximately 4,600 light-years from Earth. One light-year is about six trillion miles.

"At Green Bank, we are trying to detect gravitational waves from pulsars. To do that, we need to observe lots of millisecond pulsars, which are rapidly rotating neutron stars. This (the discovery) is not a gravitational wave detection paper but one of many important results which have arisen from our observations," says McLaughlin.

What are neutron stars?

Neutron stars are the collapsed cores of some giant stars, which pack roughly the mass of the sun into a region the size of a city. They are created when giant stars die in supernovas (the biggest explosion that humans have ever seen) and their cores collapse, with the protons and electrons melting into each other to form neutrons.

An illustration of a neutron star. (B. Saxton (NRAO/AUI/NSF))

Astronomers and physicists have studied and marveled at these objects for decades, but many mysteries remain about the nature of their interiors: Do crushed neutrons become "superfluid" and flow freely? Do they breakdown into a soup of subatomic quarks or other exotic particles? What is the tipping point when gravity wins out over matter and forms a black hole? The latest observation has brought scientists closer to finding the answers.

"Neutron stars are as mysterious as they are fascinating. These city-sized objects are essentially ginormous atomic nuclei. They are so massive that their interiors take on weird properties. Finding the maximum mass that physics and nature will allow can teach us a great deal about this otherwise inaccessible realm in astrophysics," says Thankful Cromartie, a graduate student at the University of Virginia and Grote Reber pre-doctoral fellow at the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia.

According to the research team, to understand and visualize the mass of the neutron star detected, just a single sugar-cube worth of neutron-star material would weigh 100 million tons on Earth, or about the same as the entire human population.

"These stars are very exotic. We don't know what they are made of and one critical question is, 'How massive can you make one of these stars?' It has implications for very exotic material that we simply can't create in a laboratory on Earth," says McLaughlin.

In 2018, researchers from the Universitat Politècnica de Catalunya (UPC) and the Canary Islands Institute of Astrophysics (IAC) used an innovative method to measure the mass of one of the heaviest neutron stars known to date. Discovered in 2011 and called PSR J2215+5135, the researchers found the star to be of about 2.3 solar masses.

The discovery

Researchers Maura McLaughlin and Duncan Lorimer are standing on top of the Green Bank Telescope, which they used to help detect the most massive neutron star ever. (Scott Lituchy/West Virginia University)

Pulsars get their name because of the twin beams of radio waves they emit from their magnetic poles. These beams sweep across space in a lighthouse-like fashion. Some rotate hundreds of times each second. "Since pulsars spin with such phenomenal speed and regularity, astronomers can use them as the cosmic equivalent of "atomic clocks." "Such precise timekeeping helps study the nature of spacetime, measure the masses of stellar objects, and improve the understanding of general relativity," say the researchers.

In the case of this binary system, the "cosmic precision provided a pathway" for astronomers to calculate the mass of the two stars. 

"As the ticking pulsar passes behind its white dwarf companion, there is a subtle (on the order of 10 millionths of a second) delay in the arrival time of the signals. This phenomenon is known as "Shapiro Delay." In essence, gravity from the white dwarf star slightly warps the space surrounding it, in accordance with Einstein's general theory of relativity. This warping means the pulses from the rotating neutron star have to travel just a little bit farther as they wend their way around the distortions of spacetime caused by the white dwarf," the findings state.

Astronomers can use the amount of that delay to calculate the mass of the white dwarf, which, in turn, provides a mass measurement of the neutron star. "Once the mass of one of the co-orbiting bodies is known, it is a relatively straightforward process to accurately determine the mass of the other," says the study.

According to Scott Ransom, an astronomer at NRAO and co-author on the paper, the orientation of the binary star system created a fantastic cosmic laboratory. "Neutron stars have this tipping point where their interior densities get so extreme that the force of gravity overwhelms even the ability of neutrons to resist further collapse. Each "most massive" neutron star we find brings us closer to identifying that tipping point and helping us to understand the physics of matter at these mind-boggling densities," says Ransom. 

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