Neutron Stars: Why study them? What makes them so fascinating?

Over the last several months, Universe Today has explored a plethora of scientific disciplines, including impact craters, planetary surfaces, exoplanets, astrobiology, solar physics, comets, planetary atmospheres, planetary geophysics, cosmochemistry, meteorites, radio astronomy, extremophiles, organic chemistry, black holes, cryovolcanism, planetary protection, dark matter, and supernovae, and how each of these unique disciplines continue to teach is about the cosmos and our place throughout its vastness.

Here, Universe Today discusses the field of neutron stars with Dr. Stuart Shapiro, who is a Professor of Physics and Astronomy and NCSA Senior Research Scientist at the University of Illinois at Urbana-Champaign, regarding the importance of studying neutron stars, the benefits and challenges, the most intriguing aspect about neutron stars he’s studied throughout his career, and any advice he can offer upcoming students who wish to pursue studying neutron stars. Therefore, what is the importance of studying neutron stars?

“Neutron stars are fundamental constituents of the universe,” Dr. Shapiro tells Universe Today. “They are detected throughout our Galaxy as isolated radio pulsars and as X-ray sources accreting gas from normal stars that serve as their binary companions. Neutron stars are also observed in distant galaxies as gravitational wave and gamma-ray emitters during the merger of two neutron stars in a binary system. The interior of neutron stars has the density of an atomic nucleus, some 14 orders of magnitude larger than typical materials on Earth. Such high nuclear densities cannot be reached in a lab on Earth, neutron stars provide an effective lab for studying matter and the laws of physics at extreme densities.”

Animation depicting a rapidly-spinning neutron star, also called a pulsar. (NASA’s Goddard Space Flight Center/Conceptual Image Lab)

The potential existence of neutron stars was first proposed by Fritz Zwicky and Walter Baade in 1933—which was also less than two years after the neutron was officially discovered—at a meeting of the American Physical Society. The goal of these discussions was to ascertain how supernovae were created, but they instead deduced that neutron stars resulted from supernovae, with the original star becoming ultra-dense with neutrons after the explosion.

However, research interest in neutron stars did not occur until several decades later in 1967 due to scientists deducing that they were far too small to be observed with the available technology, and only after neutron stars were found to exhibit large magnetic fields due to their rapid spin rates. Since then, neutron star research has gradually expanded, including using neutron stars to make the first detection of gravitational waves in 2017. Therefore, given their unique characteristics, what are some of the benefits and challenges of studying neutron stars?

Dr. Shapiro tells Universe Today, “We can’t collide neutron stars in an accelerator, as we do for, say, high energy protons and electrons, to study elementary particles. But nature provides us with neutron star collisions when binary neutron stars collide. We have already detected a couple of collision events when LIGO [Laser Interferometer Gravitational-Wave Observatory] observed the radiated gravitational waves, and more detections are expected in the near future.”

Given their extreme density, this means the size of neutron stars are incredibly small, averaging only 20 kilometers (12 miles) in diameter, or the size of a small city, with a mass of 1.4 times the Sun, meaning one teaspoon of a neutron star weighs approximately one billion tons on Earth. Henceforth, Dr. Shapiro notes these results are extremely difficult to replicate in a laboratory setting. Additionally, their spin rates have been found to be as high as 716 rotations per second, or approximately 0.24 the speed of light if an observer was standing on its surface, with an unconfirmed finding indicating a neutron star exhibiting 1,122 revolutions per second. There are also different types of neutron stars, including pulsars which Dr. Shapiro mentioned, and magnetars which are highly magnetized neutron stars.

Size comparison between a neutron star and Manhattan. (Credit: NASA’s Goddard Space Flight Center)

While neutron stars don’t get as much publicity as other stars, it is currently hypothesized that approximately one billion neutron stars currently exist within the Milky Way Galaxy. This might seem like a large number, except it is estimated there are approximately 100 billion stars in the Milky Galaxy, meaning neutron stars could potentially comprise only one percent of our galaxy’s star population. Therefore, what are some of the most intriguing aspects about neutron stars that Dr. Shapiro has studied throughout his career?

Dr. Shapiro tells Universe Today, “One of the properties my collaborators and I uncovered was the ability of rotation to support neutron stars of higher mass than nonrotating spherical stars. It is well known that nonrotating neutron stars have a maximum mass of a couple of times the mass of the sun, the precise value depending on the equation of state, i.e. the precise nature of the pressure law for nuclear matter that supports the star against gravitational collapse.”

Dr. Shapiro continues, “However, we found that if the star is spinning, then it can support at larger mass. The maximum mass increases by about 20 per cent if it rotates like a rigid body (i.e. uniform rotation) but can increase much more if it rotates differentially, with its spin rate very high at the center and decreasing toward the surface. Stars rotating uniformly above the nonrotating mass limit we called ‘supramassive’, while stars rotating differentially above the supramassive mass limit we called ‘hypermassive’. Supramassive and hypermassive stars are likely formed when binary neutron stars merge, at least until they shed their angular momentum (i.e. Spin) via gravitational radiation and magnetic fields.”

Like black holes or other celestial objects that we rarely observe directly, the study of neutron stars involves a lot of theoretical research where researchers use computer models to simulate their hypotheses and use powerful instruments like LIGO to confirm these hypotheses down the road. Therefore, the study of neutron stars involves several scientific backgrounds, including theoretical astrophysics theory of general relativity, computational astrophysics, computer science, among others. Additionally, one exciting aspect of science is coining new terms, as the terms supramassive and hypermassive were coined by Dr. Shapiro and his colleagues. Therefore, what advice can Dr. Shapiro offer upcoming students who wish to pursue studying neutron stars?

Dr. Shapiro tells Universe Today, “Neutron stars have properties that deal with all four of the fundamental forces of nature: gravitation, electromagnetism, strong and weak particle interactions. To best study neutron stars, one should thus acquire a strong and broad background in physics. Since the equations describing neutron stars in various states are often very complicated, they must be solved numerically on supercomputers. So aspiring students should also acquire a good background in computational physics if they want to work at the cutting edge.”

How will neutron stars teach us about our place in the universe in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!