Neutron stars, also known as pulsars and pulsars, are one of the few destinations that stars may become after supernova explosion due to gravitational collapse at the end of evolution. When the hydrogen in the core of a star is exhausted in the nuclear fusion reaction and completely converted into iron, it is impossible to obtain energy from nuclear fusion. The peripheral materials without the support of thermal radiation pressure will quickly fall to the core under the traction of gravity, which may lead to the conversion of kinetic energy of the shell into heat energy and the explosion of supernovae, or the whole star will be compressed into white dwarfs, neutron stars and even black holes according to the different mass of local stars. When a white dwarf is compressed into a neutron star, the star is so compressed that the electrons in its constituent materials are merged into protons and converted into neutrons. The diameter is only about ten kilometers, but the top cubic centimeter of material can weigh one billion tons and rotate very fast. Because its magnetic axis and rotating axis do not coincide, the radio waves generated by the rotation of the magnetic field may be transmitted to the earth in a flashing way, so it is also translated as wave burst.
Neutron stars, also known as pulsars, are the densest stars except black holes. Like black holes, they were one of the most important discoveries in the 1960s.
It was August, 1967. Bell, a female graduate student at Cambridge Radio Observatory, noticed a strange "interference" signal on the chaotic recording paper tape. After repeated research, she successfully certified that the earth receives a very regular pulse every 1.33 seconds. Knowing this amazing news, her tutor hewish suspected that it might be Morse code sent by the alien "little green men", and they might be greeting the earth. However, further measurements show that the frequency of the pulse emitted by this celestial body is incredibly accurate and there is no obvious rich code information. Next, Bell found three other similar sources, so he ruled out the alien signals, because it is impossible for the three "little green men" to send stable frequency signals to different directions of the earth at the same time. After careful study, Bell and hewish jointly reported the discovery of a new celestial body, the pulsar, in the British journal Nature in February 1968, and thought that the pulsar is an ultra-dense exotic celestial body predicted by physicists, with a radius of about 10 km, and its density is equivalent to compressing the whole sun into the urban area of Beijing, so it has a super-strong gravitational field. The pulsar material the size of a ping-pong ball is equivalent to the weight of a mountain on the earth. This is an exciting and important discovery in the 20th century, which opened up a new field for human beings to explore nature and had a far-reaching impact on the development of modern physics. It became one of the four major discoveries in astronomy in the 1960s.
[Edit this paragraph] The discovery of neutron stars
1967, astronomers accidentally received a strange radio wave. This radio wave is transmitted every 1-2 seconds, just like a human pulse. People once regarded it as the call of cosmic people, causing a sensation. Later, British scientist hewish finally figured out that this strange electric wave originally came from a special star that was unknown before, namely pulsar. This new discovery won him the Nobel Prize of 1974. So far, more than 300 pulsars have been discovered, all of which are in the Milky Way. There is a pulsar in the center of the crab nebula.
Pulsars are one of the four major astronomical discoveries in 1960s (the other three are quasars, interstellar organic molecules and cosmic 3K microwave radiation). Because it constantly sends out radio pulses, and the interval between two pulses (pulse period) is very stable, the accuracy can be comparable to that of an atomic clock. Various pulsars have different periods, ranging from 4.3 seconds long to 0.3 seconds short.
Pulsars are fast-rotating neutron stars. Neutron stars are very small, generally only 10 km in diameter, but their mass is similar to that of the sun. The lower mass limit is 0. 1 sun, and the upper mass limit is 3.2 (according to Einstein's general relativity, this level can be reached). They are super dense stars with higher density than white dwarfs.
The predecessor of a neutron star is generally a star with a mass greater than that of the sun. The tremendous pressure generated in the process of explosion and collapse greatly changed its material structure. In this case, not only the shell of the atom is crushed, but also the nucleus is crushed. Protons and neutrons in the nucleus are squeezed out, and protons and electrons are squeezed together to form neutrons. Finally, all neutrons gather together to form neutron stars. Obviously, the density of neutron stars, even white dwarfs composed of nuclei, cannot be compared with it. On a neutron star, the mass per cubic centimeter is 65.438 billion tons.
When a star shrinks into a neutron star, its rotation will accelerate, reaching several to dozens of revolutions per second. At the same time, the contraction makes the neutron star become a very powerful "magnet", which emits electric waves in a certain part of it. When it rotates rapidly, it regularly emits electric waves to the earth like a searchlight on a lighthouse. When the part that emits radio waves is facing the earth, we receive radio waves; When this part deflects with the rotation of the star, we can't receive radio waves. Therefore, the radio waves we receive are intermittent. This phenomenon is also called "lighthouse effect".
Neutron stars are extremely massive. A neutron matchbox-sized substance needs 96,000 locomotives to pull! So the mass of neutron stars can't be ignored.
The energy radiation of neutron stars is 6.5438+0 million times that of the sun. According to the current situation of electricity consumption in the world, if all the energy radiated by it in one second is converted into electric energy, it will take billions of years for our earth.
Neutron star is not the final state of a star, it has to evolve further. Due to high temperature and fast energy consumption, the service life is only several hundred million years. When its energy is exhausted, the neutron star will become a black dwarf without light.
[Edit this paragraph] Characteristics of pulsars
Pulsar, as a fast rotating neutron star, has many unique properties, which makes us open our eyes. Because they can never be realized in the earth laboratory, so that we can have a deeper understanding of the nature of stars. To sum up, these attributes are:
(1) Pulsars are very small, surprisingly small. Its typical diameter is only 10 km, that is to say, the "waist circumference" of a small neutron star is only over 30 km, which is equivalent to the distance a car travels at the ordinary speed of 1 hour. However, it is amazing that such a small star has so many extreme physical conditions!
(2) The pulse period is so short that it is almost unimaginable. The longest pulse period observed is only 4.3 seconds, and the shortest is about 2 milliseconds, which is two thousandths of a second. In other words, the pulsar rotates very fast, from 4.3 seconds to 1 second to 500 times! The duration of the transmitting pulse is about110 to1100 of its period. Many millisecond pulsars have been discovered in recent years. Will pulsars with shorter or longer pulse periods be found in the future? It's hard to say yet.
(3) the density is amazing. Density is generally expressed in grams of 1 cubic centimeter. The density of water is 1 g per cubic centimeter, iron is 7.9 g, and mercury is 13.6 g. If we remove 1 cubic centimeter from the pulsar, its weight can exceed1billion tons, or even reach1billion tons. Assuming that the density of our earth has reached this alarming level, its average diameter is not 12740 km, but below 100 m.
(4) The temperature is surprisingly high. It is estimated that the surface temperature of pulsars can reach 6.5438+million degrees, and the center is millions of times higher, for example, reaching 6 billion degrees. Compared with the sun, we can have a slightly specific concept: the surface temperature of the sun is less than 6000 degrees Celsius, the deeper the temperature, the higher the central temperature is about150,000 degrees Celsius.
(5) The pressure is staggering. The atmospheric pressure in the center of our earth is about 3 million atmospheres, which is more than 3 million times of the standard atmospheric pressure 1. It is believed that the central pressure of pulsars can reach 1000 billion atmospheres, which is 3 trillion times stronger than the geocentric pressure and 300 million times stronger than the center of the sun.
(6) Extremely strong radiation. The sun radiates amazing energy all the time, and only 2.2 billion of it reaches the earth. Even so, we humans have benefited a lot. The radiation energy of pulsars is millions of times that of the sun on average.
(7) Particularly strong magnetic field. On the earth, the magnetic field intensity of the earth's magnetic pole is the largest, but it is only 0.7 gauss (gauss is the unit of magnetic field intensity). The magnetic field of sunspots is extremely strong, about 1000 ~ 4000 gauss. However, the magnetic field intensity in the polar region of most pulsars is as high as 65.438+000 billion gauss or even 20 trillion gauss.
Pulsars are celestial bodies in our galaxy, usually thousands of light years away from us, and the farthest is about 55,000 light years. According to some scholars' estimates, the total number of pulsars in the Milky Way should be at least 200,000. By the end of 1980s, the number of discoveries was less than five thousandths of the estimated number. The task of observation and research in the future is still very arduous.
Pulsars were discovered only twenty or thirty years ago. Nevertheless, it provides scientists with very rich and rare observation data, and contributes to the study of celestial evolution and the study of physical processes and changing laws of matter under extreme conditions. At the same time, it also puts forward a series of problems and mysteries to people in this new field.
[Edit this paragraph] Astronomical information
Bright on March 20, 2007. Com- Guangming Daily: Scientists from the European Space Agency recently announced that they have discovered the fastest neutron star so far with the help of the powerful "integral" astronomical telescope, rotating 1 122 times per second, which is 1 100 million times faster than the earth's rotation.
Kukerle, a Spanish astronomer who first observed this star, said that this neutron star, code-named J 1739-285, was discovered as early as 1999, but its rotation speed was only recently calculated through a telescope.
The neutron star is about 10 km in diameter, but its mass is close to the sun, and its density is amazing, reaching 65438+ 1 100 million tons per cubic centimeter. Its huge gravity constantly captures a large amount of hot gas from nearby stars, and constantly induces thermonuclear explosions.
It is through this phenomenon that astronomers discovered it. In the past, the record of neutron star rotation was 7 16 revolutions per second, and the star speed was generally 270-7 15 revolutions per second. 700 revolutions was once considered as the limit of celestial rotation. According to the current physics theory, if the rotation speed exceeds this limit, the star will be destroyed by strong centrifugal force or become a black hole. But the latest findings deny this view.
Theoretically, 1 122 revolutions per second is not the rotation limit, and the rotation speed of a large neutron star may be as high as 3000 revolutions. What puzzles astronomers is why celestial bodies shrink constantly under the strong centrifugal force of high-speed rotation, but their own materials will not be lost.
However, the ownership of honor is controversial. 1974 the nobel prize in physics was only worn on the head of the tutor hervish, completely ignoring the contribution of the student bell, and public opinion was in an uproar. Sir Huo Yier, a famous British astronomer, gave a speech in The Times of London. He believes that Bell should share the Nobel Prize with Hervish, and criticizes the investigation conducted by the Nobel Prize Committee before the award. He even thought it was a scandal and sexist case in the history of the Nobel Prize. Huo Yier also thinks that Bell's discovery is very important, but her tutor has concealed this discovery for half a year, which is objectively a kind of theft. Some scholars even pointed out that "Miss Bell's outstanding discovery made her tutor Hervish win the Nobel Prize in Physics". The title page of the book Pulsar written by famous astronomers Manchester and Taylor reads: "Dedicated to jocelyn Bell. Without her cleverness and persistence, we can't get the joy of pulsars."
It has been 40 years since the debate about the real discoverer of pulsars and the questioning of the Nobel Prize Committee. Today, 40 years later, it has once again become a topic of concern. Looking back, it is understandable that Hervish won the Nobel Prize as a mentor, but it is a pity that Bell lost his honor. Without Bell's careful investigation of "interference" signals, they might have missed the discovery of pulsars. If the Nobel Prize competition is compared to the Scientific Olympic Games, then the "judges" 40 years ago obviously blew the "black whistle", or at least misjudged it, which tarnished the scientific justice and authority of the Nobel Prize.
Recently, during Bell's visit to Beijing, the author talked with her about the discovery experience of pulsars and her views on the Nobel Prize. She said that she was forced to leave Cambridge University shortly after the pulsar was discovered. She was silent for a while and said bluntly that in the 1960s, there was a general tendency to ignore students' contributions in scientific institutions, especially female students. Tutors often pretend to be "superior leaders", steal students' grades for themselves, and then try to kick students away. However, in 1993, when two American astronomers won the Nobel Prize for discovering pulsar binary stars, the Nobel Committee was extra careful and invited Bell to attend the award ceremony, which was considered as compensation. From 65438 to 0968, after leaving Cambridge, she and hewish stopped working together. It was not until the 1980s that they met and shook hands at an international conference. Since the discovery of pulsars, in addition to the Nobel Prize, she has won more than a dozen world-class scientific awards and become a scientific ambassador.
Neutron stars and black holes are two mysterious celestial bodies with the strongest density and gravity in the universe. Neutron stars alone are incredible enough, but it is necessary to add a black hole. It is a death trap and bottomless abyss in the universe, and nothing can get rid of its powerful gravity, including light. Near it, all the laws of physics seem to be inapplicable today.
We know that when a star ends its long life, a small and medium-mass star will become a white dwarf, and a large and super-mass star will lead to a supernova explosion. How a star evolves after a supernova explosion will depend on the mass of the remaining core. Indian astrophysicist Chandraseka discovered in the late 1930s that when the mass of the left nucleus reaches 1.4 times that of the sun, its gravity will be large enough to compress the atoms in the nucleus, so that electrons and protons can combine into neutrons. At this time, the star core becomes a neutron star, and its density is equivalent to stuffing the mass of one and a half suns into a core with a diameter of about 24 kilometers.
This is a single neutron star with a surface temperature of over1200,000 degrees and a diameter of only 28 kilometers. (HST)
A neutron star moving at a high speed of 200 times the speed of sound is about 200 light years away from the earth. It will have a slight impact on the earth in 300 thousand years. (HST)
A single star-level black hole floating in a galaxy will cause a gravitational lens phenomenon, which will make the star behind it produce two images. (HST)
There may be a giant black hole emitting intense ultraviolet radiation in the dust disk in the center of NGC625 1. (HST)
There may be a super black hole with a mass 300 million times that of the sun in the dust disk at the center of the elliptical galaxy NGC7052. (HST)
The dust disk in the center of Sagittarius A(NGC5 128) galaxy contains a giant superblack hole. (HST)
[Edit this paragraph] Characteristics of neutron stars
The surface temperature of neutron stars is about1100,000 degrees, and they radiate χ-rays, γ-rays and visible light. Neutron stars have a very strong magnetic field, which makes them emit beam-like radio waves (electric waves) along the magnetic pole direction. Neutron stars rotate very fast, reaching hundreds of revolutions per second. The magnetic poles of neutron stars are usually inconsistent with the poles, so if the magnetic poles of neutron stars are just facing the earth, the radio beams emitted by neutron stars will sweep the earth again and again like rotating lighthouses as they rotate, forming radio pulses. People also call such celestial bodies "pulsars".
After the supernova explosion, if the mass of the star core exceeds two or three times that of the sun, it will continue to collapse and finally become an infinitesimal singularity with infinite density and disappear from people's sight. Around this singularity is an area that cannot be returned. The boundary of this area is called "horizon" or "event horizon", and the radius of this area is called "schwarzschild radius". Any matter entering this region, including light, can't escape the great gravity of this singularity. It's like they fell into the abyss and never came back.
Astronomers call this celestial body formed by the death of a star a stellar black hole. It is generally believed that most black holes in the universe are formed by the collapse of stars. In addition, there is a supermassive black hole in the center of many star systems, such as the center of quasar galaxies. In the early universe, there may have been many miniature black holes (primitive black holes). These black holes are very small and the mass is equivalent to a mountain.
Although the black hole itself is invisible, it can be detected in at least two ways. When a black hole attracts dust, gas or stars, its strong gravity will tear these substances into atomic particles, which will fall from the edge of the black hole to the center along the spiral line, and the speed will be faster and faster until it reaches more than 900 kilometers per second. When an object is swallowed by a black hole, the temperature will rise to several million degrees due to collision, and X-rays and γ-rays will be emitted. In the universe, only black holes can accelerate objects to such a high speed in dense orbits; Only black holes emit χ rays and γ rays in this way.
Any matter or radiation reaches the edge of a black hole and disappears out of its horizon forever. Near the singularity of a black hole, any existing physical laws are not applicable. The singularity of a black hole is completely different from all the states of matter in the universe as we know it now. So far, there is no scientific method to measure black holes. Now we say that we have found a black hole, which is calculated by indirect means.
One particle of a neutron star is 65.438+billion tons.