According to foreign media reports, October 15 is probably one of the strangest times in the universe. They were born at the time of the death of a giant star, with a very strong gravity, a very high temperature and density, far more than anything we create in the laboratory. Although we have known neutron stars for more than half a century, astrophysicists still don’t know how big they are. < / P > < p > we know that neutron stars are relatively small. Researchers estimate that a neutron star with a mass of about 1.4 times that of the sun has a radius of between 8 and 16 kilometers. By contrast, the radius of the sun is about 696000 kilometers. < / P > < p > with our telescope, even the ordinary star is too small, just a light spot. Therefore, it is impossible to measure the volume of neutron star directly. < / P > < p > however, astrophysicists are very good at making indirect measurements. In their current research, they combine a variety of electromagnetic observation (light based) methods, as well as laboratory analysis and theoretical models. Although the calculated radius range is large (as if the height of human being is between 1.2m and 2.4m), all the calculation results and theoretical conjectures of neutron star structure fall within this range. < / P > < p > but can astrophysicists go any further? The answer may be yes, because now there are more research tools to help: the gravitational wave observatory LIGO and Virgo, and the neutron star internal composition detector (NICE). Among them, nicer is an X-ray observation instrument on the international space station, dedicated to the study of the structure of neutron stars. < / P > < p > “we combine gravitational wave observation with electromagnetic wave observation, using a variety of different technologies.” “This is a very interesting area,” says Anna watts, a neutron star astrophysicist at the University of Amsterdam and a participant in the nice project < / P > < p > in a study published earlier this year, researchers have integrated gravitational wave observation, electromagnetic wave observation and nuclear physics techniques for the colliding twin neutron star system gw170817, which was first observed by gw170817. The study found that a neutron star with a mass of 1.4 times that of the sun has a radius of 10.4 to 11.9 kilometers. Compared with the previous estimation, this is a great improvement. The electromagnetic radiation from gw170817 comes from a “thousand nova”, that is, the high-energy light produced by the nuclear reaction when the neutron stars merge. Using telescopes, astronomers analyzed the Millennium novae in the electromagnetic spectrum from gamma rays to radio rays. Each observation gives us information about different aspects of gw170817. < / P > < p > “when two neutron stars merge, they will emit a lot of material before merging, which is related to what celestial bodies they will form after colliding.” Stephanie M. brown, astrophysicist at Max Planck Institute of gravitational physics. Brown pointed out. Based on the light emitted by the ejected material, the characteristics of gravitational waves, and the results of nuclear physics calculations, brown and his collaborators calculated the radius in accordance with other independent calculations. < / P > < p > due to the complexity of neutron stars, we have to master 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 matter in it will be compressed sharply until the nucleus is compressed into a mixture of nuclear particles. These particles are mainly neutrons, but they may also have protons or even quarks. < / P > < p > “neutron stars may have many different compositions, different forces between particles, and you can come up with all kinds of interesting theories about them.” “You can cross verify these theories by using a variety of observation methods and techniques for different neutron stars,” Watts said < / P > < p > the density and pressure inside the neutron star will increase with the depth, which can be divided into two or more zones, similar to the earth’s mantle and molten core. The mathematical description of the internal state is called “equation of state”, which links the mass and radius together to determine the maximum mass of a neutron star. < / P > < p > 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 change over a lifetime. Under normal circumstances, neutron stars are perfectly spherical, otherwise they will release detectable gravitational waves when they rotate. However, in a collision like gw170817, the strong gravity between the two neutron stars will pull them out of shape. This phenomenon is called tidal deformation, which is also a property determined by the equation of state. < / P > < p > although the super density and pressure inside a neutron star cannot be reproduced in the laboratory, astrophysicists can deduce the interaction between related nuclear particles from low-density nuclear experiments. With the help of a powerful theoretical tool, the effective field model, these experimental results successfully determine the boundary conditions of the equation of state. < / P > < p > “you need to first observe the gravitational waves formed by the twin neutron star system, and then use Bayesian parameter estimation method to get the radius, mass, rotation and tidal deformation of the neutron star.” Brown pointed out. < / P > < p > in scientific research, it is far from enough to draw conclusions only by a set of systems. But so far, nature has not yet provided us with a second neutron star collision event that not only produced gravitational waves, but also released signals of a thousand novae. Fortunately, the nice detector does not need a neutron star collision or even a dual neutron star system. It can measure the X-ray fluctuations and spectral lines emitted by the neutron star system, including the rapidly rotating pulsars. It will produce dense beams, which look like regular flashes with a telescope. < / P > < p > these flashes may be produced when matter falls on the surface of neutron stars, which may provide us with information related to the radius of neutron stars. Flash may also appear in distant binary systems that will not collide temporarily, such as Hulse Taylor double pulsar, which is the first time to reveal the existence of gravitational waves. < / P > < p > the detection results of nice on gw170817 are not completely consistent with the research conclusions of Brown’s team. It’s not a big problem because of the uncertainty in the data of nice, but brown and watts both think it’s better to further study the reasons for the differences. < / P > < p > “if the results of nice 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 speed of the universe, which is also controversial in cosmology. At the same time, Watts suspected that these differences might be related to the observation of the thousand novae. It is not that these observations are wrong, but there may be some unknown systematic problem, that is, different understanding of model bias, which may affect our analysis of the original data, and then affect the measurement results extracted from the complex system. < / P > < p > “you have to be very careful, because what you infer in the end may not be what you first proposed.” “Ultimately, if you want to put together the various measurements, you need to fully understand the nature of the equation of state,” says watts < / P > < p > interestingly, in June 2020, astronomers just announced a gravitational wave system, which may not only make the problem more complicated, but also help us understand something. The system, gw190814, consists of a black hole and an unknown object 2.6 times the mass of the sun. Such a light object is unlikely to be a black hole, and studies of a thousand novae show that neutron stars don’t grow that big. However, Watts pointed out that according to the current detection results of nice, it is possible to have a neutron star with a mass of 2.6 times that of the sun. In this way, the problem of gw190814 system can be solved easily. < / P > < p > regardless of the final truth, astrophysicists have made great progress in measuring very small objects. This is due to their multi messenger and interdisciplinary research methods. If we can get more observations through nice and gravitational waves, the mystery of the size and composition of neutron stars may eventually be solved. (leaf) < a= target=_ blank>Apple extends AppleCare + purchase period: users can decide within 60 days