Shulou(Shulou.com)11/24 Report--

On August 6, 1967, Josephine Bell (Jocelyn Bell) was observing the wavy lines drawn by a red pen on moving paper-data from her doctoral project using radio telescopes to observe distant galaxies. She noticed that a wavy line looked strange. Bell, now a visiting professor of astrophysics at Oxford University, told me in her office that it was a "tiny miscellaneous peak". This "stray peak" is a series of sharp pulses with an interval of 1.3 seconds. Bell watched it for the next few nights.

The paper tape recording the pulse signals over the next few months, Bell, her doctoral mentor, Antony Hewish Huish, and some colleagues closely guarded the news of the discovery, while checking all possible options, especially whether it might be a signal from extraterrestrial intelligence. Bell recalled half-jokingly that she was not excited about the possibility that a group of aliens contacted our planet and hijacked her doctoral program six months before her thesis defense.

Images of neutron star collisions astronomers are struck by the symmetry of neutron star collisions. On December 21, before going on vacation, Bell went to check the data again. She found another wavy line, the same part of our Milky way as the first signal. This made Bell breathe a sigh of relief: it was impossible for a second group of aliens to signal Earth from another part of the sky at the same time. These pulses must come from a new, unknown celestial body.

A schematic diagram of the rotation of a neutron star, however, this understanding is no more reliable than the interpretation of "Discovery of Aliens". A very short pulse means a small celestial body, about 1/10 light-seconds, which is not much bigger than Earth. However, the extreme regularity of the pulse means a huge energy reserve, which means that the object is supposed to be huge. As soon as their discovery was published, Anthony Michaelis, a science journalist who described it, gave the new object a well-known nickname: pulsar.

Its small radius and huge mass make Bell, Huish and colleagues think it is a body called a neutron star by theorists. Decades later, astrophysicists still don't know what's going on inside these bodies. But last summer, in an eye-catching article in the Astrophysical Journal KuaiBao, Bell and others reported on a neutron star that is 2.35 times heavier than our sun, the heaviest neutron star ever reported. Although not everyone agrees with this observation, it is still within a reasonable range. The heaviest neutron stars are considered to have 2.08 solar masses, and several have more than 2 solar masses-heavier than some theorists think possible. This makes them rethink what happens when matter is pushed to its limits.

A neutron star that contains a lot of energy will get a neutron star by stuffing the mass of the sun, which is 1.4 million kilometers in diameter, into a volume only 20 kilometers in diameter. It is the densest object we know of made up of ordinary matter, only a little worse than a black hole. There may be hundreds of millions of neutron stars in our Milky way.

It is not easy to compress a star to the size of a city, even for the basic forces of nature. Matter tends to resist compression, which is why planets and stars usually do not collapse under their own weight. A neutron star is born when an ordinary star is big enough, 8 to 15 times heavier than the sun, uses up all its nuclear fuel and is compressed to extreme density. The outer layer of the star is ejected into space during the supernova explosion, while the core remains as a neutron star.

A schematic diagram of the evolution of stars physicists believe that neutron stars are a bit like eggs, with shells (shells), outer cores (egg whites) and cores (yolks). The shell consists of an iron core because the element iron is the end point of the nuclear fusion process. If you go to the inner layer of the neutron star, the pressure is increasing. Nucleons (that is, protons and neutrons) are pressed tightly together, making them into strange shapes. Physicists call this area "nuclear dough".

In the outer nucleus, the iron nucleus breaks down into protons and neutrons. The protons themselves do not last. They fuse with electrons to form more neutrons. This process produces a liquid made up of neutrons, known as neutron soup. It is not an ordinary liquid, but a superfluid that violates many of our intuitions about fluid flow. If you put some superfluid in a beaker on earth, it will climb up the wall of the cup!

Schematic diagram of matter density distribution in neutron stars up to now, although the material composition of neutron stars is strange, it is completely within the range of conditions that physicists often study in the laboratory. Go a little deeper into the kernel, and there is a complete mystery. The core of a neutron star is denser than the nucleus. Theorists do not know whether the neutrons there are still intact, or whether they are further broken down into smaller particles-quarks. Extremely low temperature and ultra-high pressure may theoretically lead to the formation of a quark jelly state.

It is hard to imagine how to study this extreme matter, which is by definition on the verge of collapsing into a black hole. But you only need to consider two numbers: the size and mass of neutron stars, and you can make amazing progress. These two numbers reflect the compressibility of matter forms in the kernel. To describe this compressibility, physicists have proposed a so-called equation of state, which connects density with pressure. There are many different models that propose different components, and each model-- each equation of state-- predicts a specific relationship between the size and mass of neutron stars. For a given density, the heavier the neutron star, the higher the pressure.

The neutron star diagram cramps the mass of our sun into a volume with a diameter of 20 kilometers. This is a neutron star. Astronomers have a series of techniques to measure the mass of neutron stars. One of the best ways is through pulsar timing: measuring the regularity of pulses on a scale that lasts for years or even decades. The radius of a neutron star is more difficult to measure accurately.

Scientists solve the problem in many ways. They combine nuclear theory with experimental observations of gravitational waves, radio pulses and X-rays. X-ray data is a particularly important new development from the NICER (Neutron Star Interior Composition Explorer) instrument installed by NASA on the International Space Station in 2017. "if there is material in the core that is different from neutrons and protons, observing heavy neutron stars is the best chance to see signs of it," said Achim Schwink, a researcher at Darmstadt University of Technology, analyzing the NICER data.

When a neutron star is in a binary system, the motion of the neutron star and its companion star is very sensitive to the mass of both objects. One object is the weight scale of the other, and vice versa. Another method is to study the degree of deformation of neutron stars during collision. Deformability tells us how difficult it is for gravitational tidal forces to compress a neutron star when another neutron star approaches. In 2017, two gravitational wave detectors, LIGO in the United States and Virgo-- in Italy, detected tiny ripples in time and space, making history. These ripples are caused by the collision of two neutron stars that disturb the structure of the universe. Just last week, astronomers studied the aftermath of the event and found that early fragments, "a fireball rich in heavy metals", were more symmetrical than expected.

The gravitational waves generated by double neutron stars indicate that through various techniques, theorists gradually rule out the candidate equation of state. The discovery of a neutron star twice the mass of the sun suggests that the material inside the core is not much like jelly-it has to be very hard to support such a mass. However, the deformability measured by LIGO and Virgo shows that the equation of state is not very accurate.

However, astronomical observation alone is not enough. As Jorge Picarevich, a researcher at Florida State University, puts it, the density of the core of a neutron star ranges from about half the density of the nucleus to five to six times that of the nucleus-creating a "density ladder" inside the star. He and others must use different theoretical methods to describe the different levels of neutron stars: shells, cores, etc. No single technique can determine the entire equation of state. Therefore, the research must be interdisciplinary. "this provides unique synergies in many areas aimed at understanding the structure of matter under conditions that cannot be replicated in the Earth Laboratory," Pikarevich said. "

Neutron star imagery nuclear physics experiments can be close to repeating these conditions. One way is to use particle accelerators to collide with heavy nuclei such as gold-for example, the Schvionen Synchrotron 18 at the GSI Heimholtz heavy Ion Research Center in Germany. Collision simulates the merging process of neutron stars on the flying meter scale. They compress matter to several times the density of the nucleus, simulating the conditions of the outer core and the core. Schwink said that the information about the equation of state in these collisions is very consistent with the constraints of astrophysics.

At this density, the details of subatomic particles will be very different. Neutrons and protons are generally thought to be the same size, but in fact, there is a slight difference in nuclei where the number of neutrons is greater than the number of protons-neutrons have an additional shell, or "skin" in jargon. Pikarevich and his collaborators believe that the thicker the skin, the greater the pressure on neutrons, and the greater the neutron star for a given mass. A team led by Kent Paschke of the University of Virginia measured neutron skin at the Jefferson Laboratory in Newport News, Va., to test the theory.

The substance in the nucleus hinted, however, and the result brought a new surprise. Experiments at Jefferson's laboratory show that neutron star matter is very hard, harder than gravitational wave observations suggest. Assuming that both are correct, it presents a paradox. This could mean something new happened inside the neutron star-perhaps an unexpected state change that turned quark jelly into something weirder. "if this hard-soft-hard result can be confirmed, it suggests that a phase transition may take place inside the neutron star." "it's too early to tell what the phase transition is-quarks, hyperons or something else," Pikarevich said. "

The strange "fragments" that Josephine Bell discovered that night in the summer of 1967 changed astronomy forever. It opens a window for us to see the most extreme matter in the universe. Neutron stars may not be aliens, but looking for their composition is equally intriguing.

Author: KATIA MOSKVITCH

The cloud opens and the leaves fall.

Revision: Xiao Cong

Original link: Giant Zombie Atoms of the Cosmos

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: KATIA MOSKVITCH

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