When it comes to Neutron Stars Size Matters

The discovery of the largest neutron star to date shines a light on the size limit that exists between these objects and black holes, also answering questions about the most exotic forms of matter.

Neutron stars are some of the most fascinating and mysterious objects in the observable universe. The end result of the death throes of a star, the material that comprises the neutron star exists in an exotic form so dense that it simply couldn’t exist on Earth. The second-densest objects in the universe; a single sugar-cube worth of neutron-star material would weigh 100 million tonnes here on Earth — roughly equivalent to the entire human population.

Of course, we know that when a star reaches the end of its life the process the gives birth to a neutron star — a massive supernova explosion and core-collapse — can also create a black hole. This leads to the obvious question: what determines if a dying star becomes a black hole or a neutron star?

The answer is the size of the star in question and its mass — with more massive cores forming black holes and less massive cores becoming neutron stars. But, what is less clear is where the boundary between the two lies. Fortunately, with the discovery of increasingly larger neutron stars and ever-smaller black holes, that boundary is becoming much less fuzzy.

“Mass and radius are important quantities with neutron stars because we don’t really have a good grip on the physics happening in their interiors,” explains 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. “Each formulation of the possible equation of state predicts a mass at which the neutron star will collapse, so if we keep finding bigger and bigger ones, we’ll rule out certain equations of state.”

Cromartie is part of a team that used the National Science Foundation’s (NSF) Green Bank Telescope (GBT) to discover that the rapidly rotating millisecond pulsar J0740+6620 is actually the most massive neutron star yet to be discovered. It is partnered in a binary system with a white dwarf star, vital in the team’s mass measuring method. The 2.17 solar masses that the neutron star possesses is crammed into a sphere with a diameter of just 30 km. This brings the newly-discovered body close to the hypothesised limit at which a single object should collapse into a black hole.

A size comparison of a typical neutron star and the one discovered by McLaughlin, Cromartie and their team. (B.Saxton, NRAO/AUI/NSF)

“The discovery of a neutron star well over 2 solar masses helps shape the way we think about the physics at play in their interiors,” Cromartie tells me. “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 discovery of such a massive neutron star was a surprise to the team, as Maura McLaughlin, an astrophysics professor at West Virginia University in Morgantown, West Virginia, and co-author of a paper that discusses the discovery explains: “We don’t know the upper mass limits on neutron stars, but prior to this, none had been found over 2 solar masses, so this was definitely unexpected!”

Cromartie elaborates: “We’re only scratching the surface of discovering neutron stars with masses greater than 2 times that of the Sun. We thought for a long time that most were around 1.4 solar masses, so it’s becoming obvious that there are far more massive neutron stars than previously supposed.”

The significance of the discovery of a neutron star of such a mass is that it allows astrophysicists to better define models that describe their origins and how they diverge from black holes. “Neutron stars can probably be heavier than we than previously thought,” McLaughlin continues. “This rules out some of the more exotic equations of state and compositions of neutron stars.”

Shapiro Delay: Measuring the mass of neutron stars

As there are no real observational biases against spotting higher mass neutron stars, even with this discovery suggests we suspect that such objects are less common than their smaller mass compatriots. Despite this, there are still many know neutron stars for which the masses are not well defined.

Artist impression and animation of the Shapiro Delay. As the neutron star sends a steady pulse towards the Earth, the passage of its companion white dwarf star warps the space surrounding it, creating the subtle delay in the pulse signal. (B.Saxton, NRAO/AUI/NSF)

Cromartie goes on to explain the difficulty that astronomers and astrophysicists face with mass measurements of neutron stars. The method that the team employed relies on Shapiro delay, which is only detectable in a small subset of edge-on systems. This is because the pulsar must pass behind the companion for light pulses to be delayed. “That’s the best way we have right now to make mass measurements, though other teams have used optical photometric studies as well,” she adds.

The method the team used relies on the fact that this neutron star is a pulsar or ‘pulsing stars’— an object that emits twin radio wave beams from its magnetic poles much like a lighthouse. J0740+6620 is a millisecond pulsar, one which spins rapidly making hundreds of revolutions per second.

Shapiro delay is a time delay experienced by light as it passes a massive object caused by gravitational time dilation, as put forward by Einstein in his theory of general relativity.

Artist impression of the pulse from a massive neutron star being delayed by the passage of a white dwarf star between the neutron star and Earth. 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. (B. Saxton, NRAO/AUI/NSF)

In this case, the mass of the neutron star’s dense white dwarf companion warps spacetime around it. This distortion causes a delay in the light pulses from the pulsar of tens of millionths of a second as the pulsar passes behind the white dwarf. The length of this delay can be used to calculate the mass of the white dwarf. The team then take this measurement and combine it with observations of the white dwarf and pulsar’s orbits around each other, to give them an estimate of the pulsar’s mass.

“We used pulsar timing, which is accounting for every rotation from a pulsar far into the future in order to detect deviations from expected pulse times of arrival,” Cromartie explains. “Pulsars are brightest at radio wavelengths, so we used the Green Bank radio telescope in WV for this work. The mass measurement was only made possible because this is an edge-on system that exhibits an effect called general relativistic Shapiro delay.

“This is a delay in the arrival of a pulsar’s pulses as it passes behind its companion along our line of sight, whose gravitational field delays the pulses. This allows the measurement of both the pulsar and white dwarf’s companions masses individually.”

Beneath the surface of neutron stars: where quantum phenomena meet gravity

As well as the relative mass limit that separates black holes and neutron stars the team also hope that their observation will help begin to cast a light on the mysteries that lie beneath the surface of neutron stars.

The interior of neutron stars remains something of a mystery, but what is almost certain is the matter found there is some of the most exotic material in the Universe. (NASA)

“Whilst main sequence stars are mostly hydrogens and helium, and white dwarfs are generally helium, neutron stars have no elements at all. Actually, we don’t know what they’re made of,” McLaughlin explains.

“It implies very exotic material that we simply can’t create in a laboratory on Earth.”

Cromartie continues: “Their interiors are made of neutrons held up by neutron degeneracy pressure. In short, they’re just exotic in a way that nothing in our solar system is!”

One of the questions about the interiors of neutron stars is after protons and electrons have been forced together and form a fluid of neutrons, does the tremendous pressure exerted by gravity within these stars cause neutrons to breakdown further into more fundamental particles — quarks?

As neutron degeneracy is a quantum phenomenon, this means neutron stars offer a potential laboratory to study how quantum effects couple and interact with gravity. I ask McLaughlin if this makes the study of neutron stars a likely path towards a quantum theory of gravity.

“Yes, it does,” She replies. “They are the best laboratories, in fact, as we are dealing with relativistic particles in a massive gravitational field with very small separations.”

As for the future, Cromartie says that the team hope to refine the mass measurement with the Canadian Hydrogen Intensity Mapping Experiment telescope (CHIME), and also to use the Neutron star Interior Composition ExploreR (NICER) X-ray telescope to get a measurement of the radius and further constrain the neutron star equation of state.

And after she defends her PhD in the spring of 2020, she will continue to research these little cousins of black holes. “People like to call neutron stars ‘failed black holes’ and they’re not wrong,” she concludes. “Both classes of objects are extremely fascinating, though I love that pulsars serve as a laboratory for so much interesting fundamental physics research, and we can study them directly with pulsar timing!”