When a star whose mass is 20 times that of the sun dies, it will become a neutron star with incredible density and the size of a city. In the words of Zaven Arzoumanian, an astrophysicist at NASA University, neutron stars are "the strangest objects that most people have never heard of". A neutron star material the size of a ping-pong ball weighs more than 654.38+000 billion tons.
Astronomers believe that most protons and electrons in neutron stars fuse into neutrons under the pressure of gravity-hence the name. But this is not the final conclusion. Astronomers have never observed neutron stars at close range, and ground laboratories cannot produce substances close to their density. Therefore, the internal structure of neutron stars is still one of the main mysteries in the universe.
Neutron stars contain the matter with the strongest known gravity-add a little more mass and they will become black holes, which are not matter in essence, but extremely curved space-time. "What does this critical state look like?" Azu Magnain said, "This is what we are exploring." To answer this question, the researchers put forward several competing theories:
Because we can't cut the neutron star to see what is inside, there is no simple way to judge which of these theories is correct. Nevertheless, scientists have made some progress. A major breakthrough occurred in August 2007+2065438. The researchers detected the gravitational waves produced by the head-on collision of two neutron stars through ground experiments. Gravitational waves are spatio-temporal fluctuations caused by the acceleration of mass objects. The gravitational wave detected this time carries important information such as the mass and size of two colliding neutron stars. Using this information, scientists can further determine the nature and internal composition of neutron stars.
The Neutron Star Internal Composition Detector (NICER), which started operation on the International Space Station from June 2065438 to June 2007, is also helping scientists to collect clues. NICER monitors pulsars, that is, neutron stars with strong magnetic fields and rapid rotation. The light beam emitted by pulsars will constantly sweep across interstellar space. When the earth is in the area swept by the light beam, we will see the pulsar "flashing" at an amazing frequency, and the fastest time 1 second can flash more than 700 times. Through these experiments, scientists are expected to find out what is going on inside neutron stars. If this goal can be achieved, we can not only better understand such strange celestial bodies, but also understand the matter and gravity under extreme conditions.
A supernova explosion may occur when a star runs out of fuel in its core and stops producing energy. Neutron stars were formed by this catastrophic explosion. Suddenly without the opponent's gravity, it will hammer the star like a piston, blow away the outer cladding and smash the core. The star core at this stage is mainly composed of iron. Strong gravity can crush atoms, squeeze electrons into the nucleus, and fuse with protons to produce neutrons. Mark alford, a physicist at Washington University in St. Louis, said: "Pressure from all directions has compressed iron by more than 654.38 million times." "An atom with a diameter of one tenth of a nanometer becomes a neutron with a diameter of several femtometers." It's like compressing the earth into a block. When the star stops collapsing, the number of neutrons inside is about 20 times that of protons.
Physicists believe that the mass of a neutron star is about 1~2.5 times that of the sun, and it may have at least three layers. The outermost layer is a gaseous "atmosphere" composed of hydrogen and helium, with a thickness of several centimeters to several meters. This layer of atmosphere floats on the outer "shell", with a thickness of about 1 km, and consists of atomic nuclei. In this layer, the nuclei are arranged in a lattice structure, and electrons and neutrons are filled in it. The innermost third layer contains most of the mass of neutron stars, and its specific composition is still a mystery. The nuclei here are crowded together, and there is almost no space, reaching the highest density allowed by nuclear physics. The closer to the core of a neutron star, the more neutrons there are in each nucleus. Somewhere, the nucleus will not be able to hold more neutrons, and then the neutrons will overflow. There are no nuclei at this time, only nuclei (protons and neutrons). Eventually, in the deepest part of the neutron star, these particles may also be decomposed.
"Our understanding of this substance at abnormally high pressure and high density is still in the hypothetical stage." Alford said, "We think that neutrons may actually have been crushed and overlapped with each other, so you can't regard it as a neutron fluid, but should call it a quark fluid." The exact form of this fluid remains an open question. One possibility is that quarks form a kind of "superfluid", which has no viscosity and theoretically will never stop once it moves. This strange state of matter may exist in neutron stars, because the correlation between quarks makes it possible for them to form bound "Cooper pairs" if they are close enough.
Quarks are fermions themselves-their spin quantum numbers are semi-integers. When two quarks are paired, they behave as bosons as a whole-their spins are integers. This change means that particles will follow new laws. Fermions obey the Pauli exclusion principle, that is, two identical fermions cannot occupy the same state-but bosons are not so limited. In a crowded neutron star, as fermions, quarks must have higher and higher energy to occupy a higher energy level than other quarks. However, after becoming a boson, everyone can stay in the lowest energy state. When quark pairs are in this state, superfluids are formed.
Outside the highest density core region, neutrons remain intact, and they can also be paired to form superfluids. In fact, scientists are convinced that there is superfluid in the shell of neutron stars, and the evidence comes from the "periodic jump" of pulsars, that is, the rotation of neutron stars suddenly becomes faster for a period of time. The rotation of neutron stars will naturally slow down, while the superfluid flow without friction will not slow down. When their speed difference becomes too large, the superfluid will transfer angular momentum to the shell. "It's like an earthquake," said astronomer James Latimer of the State University of New York at Stony Brook. "Neutron stars burp, suddenly release some energy, the rotation frequency increases for a short time, and then recovers."
In 20 1 1 year, Latimer and his colleagues claimed that they had found evidence of superfluid in the core of neutron stars, but he admitted that it was still controversial. Latimer's team, led by Dany Page of the National Autonomous University of Mexico, studied the X-ray observation data of Cassiopeia A. They found that the pulsar in the center of the nebula cooled faster than expected by traditional theory. One explanation is that some neutrons in neutron stars are paired into superfluids. When the neutron pairs disperse and recombine, they emit neutrinos, which makes the neutron stars lose energy and cool down.
Superfluid is just a possibility hidden behind the mysterious gate of neutron stars. Neutron stars may also be the home of rare "strange quarks". Quarks have six types, or more precisely, six tastes-up, down, down, odd, top and bottom. The lightest atoms have only the top and bottom. The rest of the taste is too unstable, and often only appears briefly in the high-energy particle collision experiment of particle accelerators (such as the Large Hadron Collider).
However, in neutron stars with extremely high density, some upper quarks and lower quarks in neutrons may become strange quarks (the remaining rare flavors-charm, top quarks and bottom quarks are too heavy to form even here). If strange quarks appear and bind with other quarks, they will form a "variant" of neutrons-hyperons. It is also possible that these quarks do not form other particles at all, but roam freely in the "quark soup".
Every possible state of matter will significantly affect the size of neutron stars. In Azur Magnain's words, neutrons "form a hard solid core like marbles". The solid core will support the outer layer and make the neutron star bigger. On the other hand, if these neutrons are decomposed into a pot of quark-gluon soup, a "soft, mushy" core will be formed, and the radius of neutron stars will also become smaller. The purpose of a better experiment is to determine which explanation is correct. Azu Magnain, one of the project's chief researchers in charge of scientific affairs, said: "A key goal of NICER is to measure the mass and radius of neutron stars, thus helping us to choose or exclude some theories about dense matter."
Even better is a box the size of a washing machine, which is installed outside the International Space Station. It continuously monitors dozens of pulsars in the sky and detects X-ray photons emitted by them. NICER can detect the energy and arrival time of photons and the bending degree of light under the gravitational field of neutron stars, thus helping scientists to calculate the mass and radius of these pulsars and compare them.
Measuring the radius of neutron star can effectively simplify the candidate theory about the internal material state of neutron star. Scientists once thought that half of the neutrons in neutron stars would be transformed into hyperons containing strange quarks. Theoretical calculations show that this neutron star rich in hyperons cannot exceed 1.5 times the mass of the sun. However, in 20 10, astronomers led by Paul de Maurette of the National Radio Observatory of the United States measured that the mass of a neutron star was 1.97 times that of the sun. This discovery excludes many theories about the interior of neutron stars. Now physicists estimate that the hyperon content of neutron stars will not exceed 10%.
We have gained a lot from studying a single neutron star, but it is more valuable to study the collision between two neutron stars. For many years, astronomers have observed some intense flash phenomena called gamma ray bursts through telescopes, and they have always suspected that this event originated from the collision of two neutron stars. Through the gravitational wave detected in August 2065438+2007, astronomers finally saw the first case of neutron star merger. 20 17, 17, two experimental groups in Europe-LIGO and Virgo-simultaneously detected the gravitational ripple produced when two neutron stars precessed with each other and then merged into a neutron star or a black hole.
This is not the first time that scientists have detected gravitational waves, but the previous gravitational waves all came from the collision of two black holes. Not only that, this time, while detecting gravitational waves, scientists also observed electromagnetic waves from the same position in the sky with telescopes. The combination of electromagnetic wave and gravitational wave provides a lot of information about the collision location and process, which is of great benefit to the study of neutron star physics. Astrophysicists tracked gravitational waves and found a pair of neutron stars at a distance of 0/0.30 billion light years from the Earth. The details of gravitational waves, that is, the changes of frequency, intensity and mode with time, allow researchers to estimate that the mass of two stars before collision is about 1.4 times that of the sun, and the radius is1~12km.
This information can help scientists construct a key equation describing the properties of neutron stars, that is, the equation of state. This equation describes the density of matter at different pressures and temperatures and should be applicable to all neutron stars in the universe. For different internal states of neutron stars, theorists have put forward several possible equations of state, some of which can be ruled out by new observations. For example, this observation found that the radius of neutron stars is relatively small, which is quite surprising. If we try to describe these dense neutron stars and known massive neutron stars with the same equation of state (such as 1.97 times the mass of the sun), some theories will get into trouble.
If we can improve the sensitivity of gravitational wave detector, we will get great returns. For example, one way to test the state of neutron stars is to look for gravitational waves emitted by internal rotating fluids. If the viscosity of a fluid is very low or zero-just like superfluid-it will flow in a special way called R mode and emit gravitational waves. "This gravitational wave is much weaker than the gravitational wave emitted by the merger." Alford said, "Matter is shaking quietly, not being torn." Alford and his collaborators confirmed that the advanced LIGO detectors currently in operation can't see this gravitational wave, but it is possible to see an upgraded LIGO and some planned observatories in the future, such as the ground-based Einstein telescope being considered in Europe.
Solving the mystery of neutron stars can help us understand matter in extreme cases that we can't understand. This substance is very different from the atoms that make up our world and can expand our cognitive boundaries. It may turn some strange ideas into reality, such as quark matter similar to fluid, superfluid neutrons and unusual hyperon stars. Moreover, understanding neutron stars is of greater significance: physicists' deeper goal is to use these dense stars to solve more important unknown problems, such as the law of internal interaction of nuclei and the biggest unsolved mystery in physics-the nature of gravity. Neutron stars are just a way to study nuclear forces, and particle accelerators all over the world are doing this kind of research, which can spy on the nucleus like a microscope.
When most nuclear physics problems are solved, scientists can turn their attention to gravity. "Neutron stars combine gravitational physics with nuclear physics," said Or Hen of MIT. "Now we use neutron stars as laboratories to study nuclear physics. Because we can use the nuclei on the earth, we hope to study the problems of nuclear physics very thoroughly in the end. Then we can use neutron stars to study gravity, which is also one of the most challenging physical problems. " Our current theory of gravity is Einstein's general theory of relativity, which is difficult to be compatible with quantum mechanics. One of these two theories will eventually make concessions, and physicists don't know which one. "We will know," said Heng. "This prospect is exciting."