Sizing Up Neutron Stars

A neutron star is the lingering leftovers of a massive star that is the brilliant and fatal fireworks of a supernova explosion has ended its nuclear-fusing “life” These extremely dense city-sized objects are in fact the collapsed cores of dead stars that weighed-in between 10-29 times the mass of our Sun before their violent “deaths.” These bizarre, lingering relics of heavy stars are so extremely dense that a teaspoon full of material from neutron stars can weigh as much as an elephant herd. An international research team of astronomers announced in March 2020 that they’ve obtained new measurements of how large these oddball stars are. They also noticed that neutron stars that are unfortunate enough to collide with voracious black holes are likely to be eaten whole — unless the black hole is also small and/or rotating hard.

Led by members of the Max Planck Institute for Gravitational Physics (Einstein Institute: AEI) in Germany, the international research team obtained their new measurements by combining a general first-principles description of the mysterious behavior of neutron star material with multi-messenger observations of the binary merger of a duo of neutron stars dubbed GW170817. Their findings, published in the journal Nature Astronomy’s March 10, 2020 issue, are more stringent by a factor of two than earlier limits and show that a typical neutron star has a radius of nearly 11 kilometers. Furthermore, they found that because such unfortunate stars are swallowed entirely during a catastrophic merging with a black hole, these mergers might not be observed as sources of gravitational waves, and would also be invisible in the electromagnetic spectrum. Theoretical work in physics and other sciences, if it originates directly at the level of established science and does not make assumptions such as empirical model and parameter fitting, is said to be of first principles (ab initio).

Gravitational waves are ripples in the Spacetime fabric. Imagine the ripples that propagate to a pond surface after throwing a pebble into the water. Gravitational waves are perturbations in the Spacetime curvature. They are generated by accelerated masses that propagate at the speed of light as waves outward from their source. Gravitational waves are providing astronomers with a new and important tool to use, as they reveal phenomena that observations using the electromagnetic spectrum can not. Nevertheless, neither gravitational-wave measurements nor experiments utilizing the electromagnetic spectrum can be seen in the case of neutron star / black hole mergers. This is why such mergers may not be observable.

“Binary neutron star mergers are a gold mine of information. Neutron stars contain the densest matter in the observable universe. In fact, they are so dense and compact, that you can think of the whole star as a single atomic nucleus, scaled up to the size of a city. By measuring the properties of these objects, we learn about the fundamental physics that govern matter at the subatomic level,” he explains. Dr. Capano is a Hannover-based AEI scholar.

“We consider that the average neutron star, approximately 1.4 times heavier than our Planet, has a distance of around 11 kilometers. Our findings restrict the range to about 10.4 and 11.9 kilometers, which is a factor of two more strict than previous tests,” Dr. Badri Krishnan noted in the same Max Planck Institute Press Release. Dr. Krishnan is in charge of the AEI research team.

Strange Beasts In The Stellar Zoo

Neutron stars are formed as a consequence of a giant star ‘s catastrophic supernova eruption, coupled with gravitational collapse, which compresses the center to the atomic nucleus level. How the extremely dense, neutron-rich matter is behaving is a scientific mystery. That is because the requisite conditions can not be produced in any laboratory on Earth. Although physicists have proposed various models (state equations), it remains unknown which (if any) of these models actually describes star matter in the neutron.

Once the neutron star is born from its progenitor star’s wreckage that has gone supernova, it can’t actively churn out heat anymore. Consequently, as time goes on, these celestial oddballs sweet. They do have the ability, however, to develop further through collision or accretion. Most basic models suggest that neutron stars are almost entirely constituted of neutrons. The nuclei of atoms are made of neutrons coupled with protons. Neutrons do not have a net electrical charge and a slightly greater mass than protons. In regular atomic matter, the electrons and protons fuse to produce neutrons at the neutron star conditions.

The detected neutron stars are searing-hot and usually have a surface temperature of 600,000 K. They are so incredibly dense that about 2 billion tons will weigh in a matchbox containing its content. These dead stars’ magnetic fields are about 100 million to 1 quadrillion times more potent than the Earth’s magnetic field. At the bizarre surface of a neutron star, the gravitational field is about 200 billion times that of the gravitational field of our own planet.

As the heart of the doomed massive star collapses, its rate of rotation is increasing. This is a product of angular momentum conservation, and for this purpose, the infant neutron star — called a pulsar — can spin as many hundred times a second. Some pulsars emit regular electromagnetic radiation beams as they rotate rapidly and that is what makes them detectable. The electromagnetic radiation beams emitted by the pulsar are so regular that they are often likened to Earth’s lighthouse beacons.

The first observational indication that neutron stars exist was the discovery of pulsars by Dr. Jocelyn Bell Burnell and Dr. Antony Hewish in 1967. It is believed that the radiation from pulsars is mainly emitted from areas close to their magnetic poles. Unless the magnetic poles will not suit the neutron star ‘s rotational axis the light wave can cover the sky. When observed from a distance, if the observer is located somewhere in the beam ‘s path, it will appear as regular radiation pulses emitted from a fixed point in space — thus the “lighthouse effect.” PSR J1748-2446ad is currently the fastest spinning pulsar known, and it rotates at a breathtaking rate of 716 times a second, or 43,000 revolutions per minute, giving a linear speed.

Around 100 million neutron stars are thought to be in our Milky Way. Scientists have derived this number, estimating the number of stars in our Galaxy that have gone supernova. The problem is that most neutron stars are not young, wildly spinning pulsars, and under certain conditions, neutron stars can only be easily spotted — for instance, if they are members of a binary system, or if they are young pulsars.

Many of the neutron stars in our Milky Way, though, are elderly — and dark. Non-accreting neutron stars that spin gradually are nearly undetectable. However, after RX J185635-3754 was discovered by the Hubble Space Telescope, a small number of nearby neutron stars have been spotted which obviously emit only thermal radiation. Soft gamma repeaters have been proposed to be a type of neutron star that possesses particularly powerful magnetic fields, called magnetars. Some scientists, however, believe soft gamma repeaters are actually neutron stars with old, fossil disks surrounding them.

Any main-sequence (hydrogen-burning) star on Stellar Evolution’s Hertzsprung-Russell diagram that sports an initial mass exceeding 8 times our Sun’s, has the potential to become a neutron star ‘s stellar progenitor. As the aging star evolves away from the main sequence, further nuclear burning results in a core rich in iron. The center will be protected by degeneration pressure alone until all nuclear fuel in the heart has been used up. Stars on the hydrogen-burning main-sequence stay bouncy because they experience a very delicate balance between their own gravity squeeze and radiation pressure push. When radiation pressure can no longer be produced by nuclear fuel burning, gravity crushes the dying star.

Additional deposits from the burning of shell fuel cause the core of the doomed star to exceed what’s called the Chandrasekhar limit. As a result, the dying, doomed massive star temperatures soar to more than 5X10 at ninth power K. Photodisintegration (the splitting up of iron nuclei into alpha particles by high-energy gamma rays) happens at these very hot temperatures. As the temperature rises ever higher, electrons and protons merge through electron capture to create neutrons. These release a neutrino flood. A combination of strong nuclear force repulsion and neutron degeneration pressure stops further contraction when the densities reach a nuclear density of 4 X 10 to the seventeenth power kg / m cubed. The doomed old star’s infallible outer envelope is halted and hurled outward by a flux of neutrinos produced in neutron creation. The elderly star has reached the end of this long, stellar road, and it’s going supernova. If the stellar ghost sports a mass exceeding about 3 solar masses, it will collapse further and turn into a black hole.

As during a Type II (core-collapse) supernova (or a Type Ib or Type Ic supernova) the center of a massive star is squeezed, it collapses into a neutron star. The stellar relic retains most of its angular momentum — but because it possesses only a small percentage of the radius of its progenitor star, a neutron star is born with a very high speed of rotation. This incredible oddball is winding down over a very long time period.

Sizing Up A Dense Stellar Oddball

Mergers of a duo of binary neutron stars, such as GW 170817, provide a treasure trove of information about the behavior of matter under such extreme conditions and the underlying nuclear physics behind it. In August 2017, GW 170817 was first observed in gravity waves and throughout the electromagnetic spectrum. Scientists can go on to determine the physical properties of these odd-ball stars, including their radius and mass, from this type of important astrophysical event.

The AEI research team used a model focused on a summary of the first principles of how subatomic particles move together at the incredibly large densities contained within neutron stars. Remarkably, as the team of scientists found, length-scale numerical measurements of less than a trillionth of a millimeter can be contrasted with Earth observations of an astrophysical body over a hundred million light-years.

“It’s a little mind-boggling. GW 170817 was triggered by the interaction between two city-sized bodies 120 million years ago, while dinosaurs were wandering about here on Earth. That occurred in a world a billion trillion kilometers away. Through this, we obtained insight into subatomic physics,” Dr. Capano reflected in the March 10, 2020, Max Planck Institute Press Release.

The scientists’ first-principle descriptions predict numerous potential state equations for neutron stars which are derived directly from nuclear physics. The researchers chose only those from these possible equations of state that are most likely to explain different astrophysical observations, which agree with GW 170817 gravitational-wave observations. The team used observations derived from public LIGO and Virgo results, which as a result of the merger created a brief hyper-massive neutron star, and which agreed with established constraints on the maximum neutron star mass from GW 170817 electromagnetic counterpart observations. This approach enabled scientists not only to derive new information on dense-matter physics but also to obtain the most stringent limitations on the size of neutron stars to date.

“These findings are promising not just because we have been able to significantly increase the measurements of neutron star radii, but also because it offers us a glimpse into the eventual fate of neutron stars in binaries merging,” Stephanie Brown noted on March 10, 2020, Max Planck Institute Press Release. Ms. Brown is the co-author of the publication and an AEI Hannover doctoral student.

The new results suggest that with an event such as GW 170817, the design sensitivity detectors LIGO and Virgo will be able to distinguish, from gravitational waves alone, whether the neutron star duo or the black holes duo have fused. Observations in the electromagnetic spectrum were central for GW 170817 in making that important distinction.

The Laser Interferometer for Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and develop astronomical level gravitational waves, observers. The interferometer Virgo is a large interferometer designed for the detection of gravitational waves.

The team of scientists has noticed that gravitational wave collision measurements alone would have a tough time separating such phenomena from binary black holes with mixed binaries (a neutron star fusing with a black hole). To discern between the two, measurements in the electromagnetic spectrum or gravitational waves from after the fusion would be essential.

It turns out, though, that the new results also suggest that mixed binary mergers multimessenger observations are unlikely to occur. “We have seen that in nearly all situations the neutron star won’t be ripped down by the black hole and eaten whole. Only when the black hole is very tiny or rotating fast will it destroy the neutron star before consuming it; only then do we hope to see something other than gravitational waves,” Dr. Capano elaborated in the March 10, 2020, Max Planck Institute Press Release.

The existing gravitational wave detectors will become even more sensitive in the next decade, and further detectors will begin to observe. The research team expects more gravitational wave detections from the merging of binary neutron stars and possible multimessenger observations. One of these fusions will give great opportunities to know more about neutron stars and nuclear physics.

Neutron Stars – The Most Extreme Things that are not Black Holes