Zero Genie

Neutron stars are the end-state of massive stars that have spent their fuel and exploded as supernovae. There’s an upper limit to their mass, because a massive enough star won’t become a neutron star; it’ll become a black hole. But finding that upper mass limit, or tipping point, between a star that becomes a black hole and one that becomes a neutron star, is something astronomers are still working on.

Now a new discovery from astronomers using the National Science Foundation’s (NSF) Green Bank Telescope (GBT) have found the most massive neutron star yet, putting some solid data in place about the so-called tipping point.

Neutron stars are made of ultra-dense matter. They’re tightly-compressed, and they’re the densest objects in the Universe that are made of normal matter. (Black holes are actually denser, but they’re not normal.) They’re so dense, in fact, that a sugar-cube of neutron star would weigh about a hundred million tons on Earth. And they’re plentiful: astronomers think there are about 100 million of them in the Milky Way alone.

The researchers behind this discovery are members of the NANOGrav Physics Frontiers Center. The star they discovered is a rapidly-rotating pulsar, the most massive one ever measured. It’s called J0740+6620, and it’s 2.17 times as massive as our Sun. And all that mass is jammed into a tiny sphere only about 30 km in diameter.

According to our understanding of these types of stars, this neutron star is about as massive, and as compact, as a star can be before it collapses into a black hole. According to LIGO, (Laser Interferometer Gravitational Wave Observatory) and the gravitational waves it observed from merging neutron stars, 2.17 times the mass of the Sun might be the upper limit.

“Neutron stars are as mysterious as they are fascinating,” said Thankful Cromartie, a graduate student at the University of Virginia and Grote Reber pre-doctoral fellow at the National Radio Astronomy Observatory in Charlottesville, Virginia. “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.”

The neutron star they discovered is in a binary pair, and it’s also a pulsar. Both of these facts made it possible for the team to measure it’s mass.

Pulsars emit beams of electromagnetic radiation from their magnetic poles. These radio waves sweep through space as the star spins. Some of them can rotate very quickly, hundreds of times per second. Because of their speed and their predictive regularity, they can be used like atomic clocks. Astronomers can use them to measure the masses of objects in space.

The other star in this binary pair is a white dwarf. The two stars are almost edge-on as seen from Earth, which created a kind of natural laboratory to measure the mass of J0740+6620.

First they measure the mass of the companion white dwarf star. The white dwarf and the pulsar orbit a common center of gravity. As the pulsar emits its radio waves, and the white dwarf moves between Earth and the pulsar, the gravitational pull of the white dwarf exerts a tiny gravitational force on the radio waves, forcing them to travel a little farther. This is called the Shapiro delay.

By measuring that delay, they can find the mass of the companion white dwarf. Astronomers can measure the overall mass of the binary pair, so subtracting the mass of the white dwarf companion yields the mass of the neutron star.

“The orientation of this binary star system created a fantastic cosmic laboratory,” said Scott Ransom, an astronomer at NRAO and coauthor on the paper. “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.”

The tipping point, or maximum mass that a neutron star can have, is only one of the unknowns in neutron stars. Astrophysicists also have questions about the exact nature of the matter in those stars. Scientists think that as a neutron star is compressed, the electrons and protons are crushed together into neutrons and neutrinos. The neutrinos travel off into space, leaving only neutrons.

There are likely structured layers in a neutron star, due to the extreme gravity. Those layers probably have different compositions and densities. Scientists think that the strongest material in the Universe, what they call “nuclear pasta,” is deep inside the crust of neutron stars. But for now, anyway, there’s really no way of knowing. All scientists can do is chip away at neutron stars and see where the evidence takes them.

These results are in a new paper published in the scientific journal Nature Astronomy. The title of the paper is “Relativistic Shapiro delay measurements of an extremely massive millisecond pulsar.”