neutron stars are probably one of the strangest celestial bodies in the universe. They were born when huge stars died. They have both extremely strong gravitational force and extremely high temperature and density, far surpassing any matter created in the laboratory.
Although we have known neutron stars for more than half a century, astrophysicists still don’t know how big they are. There are two unsolved mysteries of neutron stars: What is the center of it? How big can their volume grow?
We know that the volume of neutron stars is relatively small. According to estimates by researchers, the radius of a neutron star with a mass approximately 1.4 times that of the sun is between 8 and 16 kilometers. In contrast, the radius of the sun is approximately 696,000 kilometers.
Looking through our telescope, even an ordinary star is too small, just a spot of light. Therefore, it is impossible to directly measure the volume of neutron stars.
However, astrophysicists are very good at making indirect measurements. In the current research, they combine a variety of electromagnetic observation (light-based) methods, as well as laboratory analysis and theoretical models. Although the calculated radius is relatively large (as if the height of a human being is between 1.2 meters and 2.4 meters), all calculation results and theoretical speculations on the structure of neutron stars fall within this range.
But can astrophysicists go further? The answer may be yes, because there are now more research tools to help: such as the gravitational wave observatory LIGO and Virgo, and the neutron star internal composition detector (NICER). Among them, NICER is an X-ray observer located on the International Space Station, dedicated to studying the structure of neutron stars.
“We have combined gravitational wave observations and electromagnetic wave observations, using a variety of very different technologies.” Anna Watts, a neutron star astrophysicist at the University of Amsterdam and a participant in the NICER project, said, “This is a Very interesting area.”
The “Mentality” of Neutron Stars
In a study published earlier this year, researchers integrated gravitational wave observations, electromagnetic wave observations, and nuclear physics technology for the colliding binary neutron star system GW170817 (first observed in GW170817). The study found that a neutron star with a mass equivalent to 1.4 times the sun has a radius between 10.4 and 11.9 kilometers. Compared with the previous estimation results, this is a great improvement.
The electromagnetic radiation emitted by GW170817 comes from a “thousand nova”, which is the high-energy light produced by the nuclear reaction when neutron stars merge. Astronomers use telescopes to analyze thousands of new stars in the electromagnetic spectrum from gamma rays to radio rays. Each observation provides us with information about different aspects of GW170817.
“When two neutron stars merge, they will emit a lot of matter before they merge. This has something to do with what celestial bodies will be formed after they collide.” Astrophysicist Stephanie M Brown (Stephanie M .Brown) pointed out. Based on the light emitted by the ejected material, the characteristics of gravitational waves, and the results of nuclear physics calculations, the radius calculated by Brown and co-researchers is consistent with other independent calculation results.
Because neutron stars are too complex, we must have a lot of data. According to the current understanding of neutron stars, when a large star becomes a supernova, its core will collapse under the action of gravity, and the material in it will be sharply compressed until the nucleus is compressed into a mixture of nuclear particles. These particles are mainly neutrons, but there may also be protons and even quarks.
“Neutron stars may have many different compositions and different inter-particle forces. You can put forward a variety of interesting theories about these.” Watts pointed out, “You can use a variety of observation methods for different neutron stars, and use many applications. Different observation techniques to cross-validate these theories.”
The density and pressure inside a neutron star will continue to increase with depth, which can be divided into two or more regions, similar to the Earth’s mantle and molten core. The mathematical description of the internal state is called the “state equation”, which links mass and radius together, and can determine the maximum mass of a neutron star.
Astrophysicists have not yet come up with a complete equation of state, but they are not ignorant. The size of a neutron star is completely determined by gravity and nuclear force, while the size of an ordinary star such as the sun will continue to change during a lifetime. Under normal circumstances, neutron stars are perfectly spherical, otherwise they would release detectable gravitational waves when they rotate. However, when a collision like GW170817 occurs, the strong gravitational force between the two neutron stars will deform them. This phenomenon is called tidal deformation, and it is also a property determined by the equation of state.
Although it is impossible to reproduce the super-high density and pressure inside a neutron star in the laboratory, astrophysicists can deduce the interaction between related nuclear particles from low-density nuclear experiments. Coupled with a powerful theoretical tool-hand-scaled effective field model, these experimental results successfully determined the boundary conditions of the equation of state.
“You must first observe the gravitational waves formed by the binary neutron star system, and then use Bayesian parameter estimation to obtain the radius, mass, rotation, and tidal deformation of the neutron star.” Brown pointed out.
Using this method, the most accurate estimation result of the radius of a neutron star under a given mass is obtained.
NICER shows off
In scientific research, it is far from enough to draw conclusions based on a set of systems. But so far, nature has not yet provided us with the second neutron star collision event that not only generated gravitational waves, but also released thousands of nova signals.
Fortunately, the NICER detector does not need neutron stars to collide, or even a binary neutron star system. It can measure the X-ray fluctuations and spectral lines emitted by the neutron star system, including fast-rotating pulsars, which will produce dense beams, which look like regular flashes of light through a telescope.
These flashes may be produced when matter falls on the surface of the neutron star, which may provide us with information related to the radius of the neutron star. Flashes may also appear in binary star systems that are far away and will not collide temporarily, such as the Hulse-Taylor double pulsar that first revealed the existence of gravitational waves to the world.
NICER’s detection results of GW170817 are not completely consistent with Brown’s team’s research conclusions. Due to the uncertainty in NICER’s data, this is not a big problem, but both Brown and Watts believe that it is best to further study the reasons for the difference.
“If NICER’s results are consistent with ours, that would be great.” Brown pointed out. She believes that the difference between the two studies is similar to the calculation of the expansion rate of the universe, which is also divided in cosmological circles.
At the same time, Watts suspected that these differences may be related to the observation of Thousand Novas. It is not that these observations are wrong, but that there may be some unknown systemic problems, that is, different understandings of model deviations, which may affect our analysis of the original data, and then affect our extraction from complex systems. Measurement results.
“You have to be very careful, because what you end up inferring may not be what you put forward at the beginning.” Watts said, “Finally, if you want to put together various measurement results, you need to fully understand the state. The nature of the equation.”
The mission of the NICER probe has just begun. Both Watts and Brown will continue to monitor whether there are new results.
Interestingly, astronomers just announced a gravitational wave system in June 2020, which may complicate the problem and help us figure out some things. The system called GW190814 consists of a black hole and an unknown celestial body with a mass 2.6 times the sun. A celestial body with such a light mass is unlikely to be a black hole, and studies on thousands of stars have shown that neutron stars will not grow to such a large size. But Watts pointed out that based on the current NICER detection results, neutron stars with a mass 2.6 times the sun are possible. In this way, the problem of the GW190814 system is easily solved.
Regardless of the final truth, astrophysicists have made tremendous progress in measuring extremely small celestial bodies. This is all due to the multi-messenger and cross-professional research methods they use. If we can obtain more observations through NICER and gravitational waves, the mystery of the size and composition of neutron stars may eventually be solved.
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