Speculation about the mysterious origin of the aurora has been around for a long time. But until now, unsubstantiated inferences have been clearly proven. The mystery has been revealed According to a new study published in the journal Nature, a team of physicists from the University of Iowa has finally demonstrated that the aurora is “generated by strong electromagnetic waves during geomagnetic storms.” The Northern Lights appear over a waterfall in Iceland. Photo: Getty Images. Research shows that these phenomena are known as Alfven waves, which launch electrons to Earth, causing the particles to produce a stream of light that scientists call the aurora borealis. “The measurements reveal that the small number of electrons undergo ‘resonant acceleration’ by the electric field of the Alfven wave, similar to a surfer catching a wave and continuing to accelerate while surfing with the wave,” said Deputy Professor Greg Howes, of the Department of Physics and Astronomy at the University of Iowa, is also a co-author of the study, said. The idea that electrons “surf” on an electromagnetic field is a hypothesis first introduced in 1946 by Russian physicist Lev Landau. The theory is named after the physicist who called it Landau damping. After more than 70 years, the Landau damping theory has been proven. Aurora re-creation After decades of research, scientists have understood how the aurora is produced, but only now have they been able to simulate these colorful bands of light. For the first time, artificial auroras have been recreated in the lab using large plasma physics equipment (LPD) at UCLA’s Basic Plasma Science Facility (BaPSF). Auroras observed from the International Space Station. Photo: NASA. According to the CNN , the scientists used a 20-meter-long room to reconstruct the Earth’s magnetic field using powerful magnetic coils on UCLA’s LPD instrument. Inside the room, the scientists created a plasma environment similar to what exists in near-Earth space. “Using a specially designed antenna, we project Alfven waves down the machine, just like when you quickly shake a water hose up and down and watch the waves move along the hose,” Mr Howes said. When they began to experience electrons “surfing” along the wave, they used other specialized instruments to assess how the electrons received energy from the wave. “Although the experiment did not reproduce the range of multicolored light in the sky, our measurements in the laboratory matched predictions from computer simulations and mathematical calculations, demonstrating that the Electrons surfing on Alfven waves can accelerate up to 72 million km / h and create aurora,” Mr Howes said. Study co-author Craig Kletzing added that these experiments allow them to make key measurements to prove the spatial and hypothetical measurements that actually explain how the aurora is produced. The aurora was taken in Alaska. Photo: Getty Images. Many space scientists are also excited by this new news. “I feel so excited! There are very few laboratory experiments that can prove hypotheses and models related to the space environment. Because space is so vast that it can’t be simulated in a lab,” said Patrick Koehn, a scientist in NASA’s Helicopter Physics Division. According to Koehn, understanding the acceleration mechanism of the electrons that produce the aurora will be useful for many future studies. Long road ahead Currently, the theory of how the aurora is generated has been proven, but there is still a long way to go to predict the strength of each upcoming storm. “Predicting the strength of a particular geomagnetic storm, based on observations of the Sun and measurements from spacecraft between the Earth and the Sun, remains a difficult unsolved problem.” Mr Howes said. Howes said they have established the bond of electrons surfing on Alfven waves about more than 16,000 km above the Earth’s surface. And now, they must learn to predict the strength of those Alfven waves using spaceship observatories.

These word-of-mouth statements are almost consistent with information from the US Senate hearing on a request to fund NASA with a figure of $ 585 million to invest in nuclear-powered propulsion technology in the fiscal year 2022. and other American efforts in this area. This means that Russia’s space nuclear energy program is not only for technical purposes, but also for geopolitical purposes. Russia’s current program has its roots in the Soviet era. The Soviet Union launched a total of 33 military satellites with the function of spying and targeting targets equipped with nuclear reactors into low orbit around the Earth from 1969 to 1988. Most of these satellites used reactors. Buk type nuclear power generation reactor, only 2 of them using advanced thermal electron furnace NPP Topaz with generating capacity from 4.5 to 5.5 kW, however, this project was suspended in 1986 . In the early 1990s, a Russian-American cooperation project aimed at continuing the development of the Topaz furnace, but this project was suspended in 1995. In the period 2000-2007, Russia also tried to find ways to cooperate with China. Nation in this field. Despite the economic decline for a long time, Moscow has been trying to continue its independent efforts in the field of the use of nuclear energy in space since 1998, and during the time of President Dmitry Medvedev in power, the This effort has been identified as a key priority for the Kremlin. The budget for this program of Russia for the period 2010-2018 is 17 billion rubles, divided between Roscosmos 9.8 billion rubles and Rosatom 7.2 billion rubles, equivalent to $560 million in 2010 exchange rates. However, the actual disbursement figure is much smaller. In 2010, only 500 million rubles ($16.5 million) were allocated for this purpose. Over the next decade, total disbursements reached nearly 10 billion rubles ($213 million), according to public information from Roscosmos and Rosatom. The results of these efforts have not been as successful as they initially suggested. For example, the technical requirements of the proposed product are an outer space nuclear reactor with a capacity of 1 MW of electricity and an ion thruster with a capacity of 50 kW. However, the reality shows that Russia is currently only developing nuclear power generation systems YaEU-25M, YaEU-25 and YaEU-50 with a generating capacity of 10-40 kW and propulsion using ionic force. 25 kW. At present, perhaps Russia is just stopping at the computational model run for more powerful reactors and engines. For comparison, NASA is still trying to design a 10 kW outer space nuclear reactor with a Stirling engine for the purpose of increasing efficiency, currently Russia is still revolving around the thermal electron model, and the problem of using Using engines or turbines in combination with reactors is still only a theory. It is hard to believe that Russia will design a nuclear reactor in space with a generating capacity of 1 MW or ion propulsion engines with a larger capacity in the near future. Anyway, Moscow is still trying to turn the results achieved into advanced applications in long-distance space travel or foreign politics. Due to a decline in space research activities in other sub-sectors, coupled with economic weakness, these problems have prompted the Kremlin to look for another trump card. While the development of nuclear reactors for space exploration is far from complete, the Russian government as well as industry is currently looking for the application of nuclear reactors to satellites. military. These satellites can be used for radar reconnaissance or electronic warfare (for example jamming) when they are deployed in low, mid or geostationary orbits. However, no tests of such satellites have yet been conducted, meaning Moscow is not ready to field such satellites in the near future. In addition, the promotion of nuclear-powered spacecraft could be used by Russian space and nuclear industry units as a tool to apply for funding, to promote a slow and risky research program. this. For its part, the Kremlin is still trying to blur its true purpose. They came up with the “Strategy for the development of nuclear energy in space by 2030 in 2019, and issued relevant policies for the first time in 1998. Even if Russian military satellites are used by Russian military satellites. Nuclear power appeared in 2030, it also did not bring about significant changes in the technical and military fields. However, Moscow is still trying to polish it as a tool to help shift political supremacy. First, Russia has consistently supported the ban on placing weapons in outer space. Second, Russia will not be able to stand in the forefront of space technology without cooperating with other countries in the field, so Russian leaders see nuclear technology in space as a way to develop. cooperation, even in times of growing hostility in the West. The Russian SPT-100 series Ion Thruster has been used on satellites since 1994.

As noted by Fox News, NASA’s rocket launch window opened at 20:04 and the rocket launched at 20:36 on the same day. About 10 minutes after launch, the Black Brant XII rocket ejected barium vapor at an altitude of about 350-400km above the Atlantic Ocean, just north of Bermuda and about 870-900km from Wallops. The barium vapor from the NASA rocket is not harmful to the environment and has formed two blue-violet clouds that can be seen across the East Coast for about 30 seconds. However, clouds prevented the launch from being viewed with the naked eye. “This is the last date NASA intends to authorize a rocket launch from Wallops. The vocal rocket was originally scheduled to launch on May 8 but has been postponed several times a week,” Fox News wrote and reported. For further information, the Black Brant XII rocket launched during NASA’s KiNet-X mission – designed to study how energy and momentum is transported between different regions of space with a magnetic connection. The Black Brant XII missile is a 4-stage rocket. Peter Delamere, KiNET-X principal investigator from the University of Alaska Fairbanks said: “The aurorae – also known as the north pole or pole light, is formed when particles in the ‘near universe’ of the Earth’s ‘near space’. Earth interacts with the atmosphere Electrons in Earth’s space environment and in the solar wind are relatively low in energy, however, the aurora is produced by very high energy electrons. Vapors emitted from the rocket’s payload will generate magnetic interference and potentially energized electrons.KiNET-X consists of a single rocket launch carrying seven separable payloads – equipment primary diagnostic device, along with four small sideloads and clouds of barium vapor set to release from two additional, larger payloads”. Beautiful performance Analyzing further, meteorologist Katie McGraw told News 5 that when the launch happened, people in Northeast Ohio could see it thanks to clear skies. NASA called the May 16 launch of the Black Brant XII rocket a “beautiful display” and couldn’t have written a better story. Katie McGraw further revealed that the Black Brant XII rocket was used for the mission to explore energy transport in space. Black Brant is the result of research at the Canadian Arms Research and Development Facility (CARDE) in the 1950s into the nature of the upper atmosphere as part of ongoing research into anti-missile systems. ballistic fire and long-range communications. The launch took place on May 16 after a week of waiting. In 1957, CARDE contracted with Bristol to produce a simple rocket fuselage, called the Propulsion Test Vehicle, for high-powered solid fuel studies. Albert Fia’s design is quite heavy, as it is designed to accommodate a wide range of engine burn times, propellant loads and launch angles well suited to its role as a test vehicle for development. ABM system. The first test flight took place just two years later. CARDE’s attention then turned to long-distance communications, and they found the propulsion test vehicle system useful as a locating missile. To better suit this role, Bristol modified the design to be lighter and more suitable for the Black Brant missile role. CARDE has launched several Black Brant rockets over the years with the original Black Brant I design able to carry a payload of 68 kg to an altitude of 150 km and fly for the first time in October 1960. The missile’s design emphasizes payload reliability and range. There are 12 versions of the Black Brant and the Black Brant XII is a four-stage sonic launcher manufactured in 1995. The missile was first launched from the Andoya rocket range off the northwest coast of Norway and caused an explosion also known as the fear of Black Brant. This incident has put Russia’s nuclear forces on high alert, fearing a high-altitude nuclear strike could blind Russia’s radar, and Russia’s Cheget “nuclear briefcase” has been put on high alert. sent to Russian President Boris Yeltsin, who must then decide whether to launch a retaliatory nuclear strike against the United States. This is the first and only known incident to date where any nuclear-weapon country has activated its nuclear briefcase and prepared for an attack. On September 19, 2009, a Black Brant XII rocket launched to study the clouds caused calls from eastern North America reporting “strange lights in the sky”. NASA reported that the light came from an artificial noctilucent cloud formed by the exhaust particles of the rocket’s fourth stage at an altitude of about 278 km.

Dark matter is thought to be six times more abundant than normal matter. What is dark matter? First and perhaps most confusingly, researchers are still unsure what exactly dark matter is. Initially, some scientists surmised, the missing mass in the universe was made up of faint small stars and black holes. However, detailed observations have not shown enough evidence to explain the effects of dark matter. The current leading hypothesis is a hypothetical particle known as the weakly interacting mass particle, or WIMP. This particle would behave like a neutron and be 10-100 times heavier than a proton. However, this conjecture has opened a series of other unanswered questions. Is it possible to detect dark matter? In the 1930s, Swiss astronomer Fritz Zwicky discovered that galaxies in a distant cluster were orbiting each other much faster than they should have been visible. At the time, he suggested, an invisible substance, dark matter, could be pulling gravity on these galaxies. Since then, researchers have confirmed, this mysterious matter can be found throughout the universe. They are six times more abundant than ordinary matter. If dark matter were produced from WIMPs, they would be all around us, invisible and virtually undetectable. While they won’t interact much with normal matter, there’s always some small chance that a dark matter particle could bump into an ordinary particle like a proton or electron as it travels through space. So the researchers carried out a series of experiments to study the huge numbers of ordinary particles deep underground, where they are shielded from interfering radiation. These particles can behave like a collision between dark matter particles. After decades of searching, none of these detectors have made a reliable discovery. Earlier this year, China’s PandaX experiment reported failing to detect WIMP. According to physicist Hai-Bo Yu at the University of California, Riverside, it seems that dark matter particles are much smaller than WIMP, or lack properties that make them easy to study. Does it include more than one particle? Ordinary matter is made up of particles like protons and electrons, as well as more exotic particles like neutrinos, muons, and pions. So some researchers have wondered, could dark matter, which makes up 85 percent of the matter in the universe, be just as complex? “There is no good reason to assume that all dark matter in the universe is made up of one type of particle,” said Harvard physicist Andrey Katz. According to Katz, dark protons can combine with dark electrons to form dark matter. From there, create configurations as varied and interesting as those found in the physical world. However, this problem has not been elucidated so far by scientists. Does the dark force exist? Some researchers have been looking for “dark photons,” analogous to photons exchanged between ordinary particles that generate the electromagnetic force. However, dark photons will only be sensed by dark matter particles. Physicists in Italy are preparing to break a beam of electrons and their antiparticles, called positrons, into a diamond. If dark photons exist, electron-positron pairs could annihilate and create one of the strange force carriers. From there, it is possible to open up a whole new realm of the universe. Is it made up of axions? As physicists pay more and more attention to understanding WIMPs, other dark matter particles are starting to gain favor. One of the top substitutes is the axion. This hypothetical particle is very light, smaller than a proton. Axion is currently being searched for in a few trials. Recent computer simulations have raised the possibility that these axions can form star-like objects. At the same time, they can produce radiation that is easily detected, similar to the mysterious phenomenon known as fast radio bursts. What are the properties of dark matter? Astronomers have discovered dark matter through its gravitational interaction with ordinary matter. This is also an indication that it is the main way that the presence of dark matter is known in the universe. According to some theories, dark matter particles must be their own antiparticles. This means that the two dark matter particles will annihilate each other when they meet. The Alpha Magnetic Spectrometer (AMS) experiment aboard the International Space Station has been looking for amazing signs of this destruction since 2011. As a result, AMS detected hundreds of thousands of events. Scientists are still unsure if these come from dark matter, and have yet to determine exactly what dark matter is. Does it exist in every galaxy? Because of its greater mass than normal matter, dark matter is often thought to be the driving force that organizes large structures such as galaxies and galaxy clusters. So it was strange that earlier this year, astronomers announced that they had found a galaxy called NGC 1052-DF2. Surprisingly, this galaxy barely contains any dark matter. At the time, Pieter van Dokkum of Yale University said: “Dark matter does not seem to be a necessary element to form a galaxy.” However, another team of researchers analyzed and showed that van Dokkum and colleagues miscalculated the distance to the galaxy. This means that visible matter is much fainter and lighter than first detected.

Illustration. If the universe is filled with matter but there is no force of interaction between them, there would be no formation of nuclei, atoms, molecules… and galaxies, stars and planets. Basic interactions of nature Electromagnetic interaction or electromagnetic force is the interaction between charged particles, for example protons with the same positive charge repel each other, but attract electrons because electrons have a negative charge. This interaction is not only a matter of attraction or repulsion of the magnet poles due to the excess or lack of electrons, but the most important thing is that it is the attractive properties of this proton and electron that the electron can keep its orbit around the atomic nucleus. , and thus the atoms, molecules and, more generally, all the matter we see every day, that make us up. Electromagnetic interactions are transmitted by bosons or photons, or as we know them as light-transmitting particles. Light is precisely an electromagnetic wave of the right wavelength to produce images on the retina of the human eye. The photon itself has neither mass nor charge, it only plays the role of transmitting electromagnetic interactions through its oscillation frequency. The frictional force generated when one object slides over another is due to the electromagnetic interaction of the atomic particles of the contact surface, the elastic force of a spring or the tension of a string is also an electromagnetic interaction due to the change in the surface area. changing the distances of the atoms from each other leads to a change in the magnitude of the interaction from the initial steady state, the force produced by muscles when you lift a heavy object or any other movement that is elastic muscle bundles, so of course there’s also an electromagnetic interaction… Strong interaction (also known as strong nuclear force). The interaction is caused by bosons called gluons – a type of particle with no mass and no charge. This is the bonding interaction between quarks, the main components of the two types of baryons, protons and neutrons, which as we know are the particles that make up the nucleus of an atom. It is thanks to this type of interaction that new baryons are formed and also bind the protons and neutrons in the atomic nucleus together (protons carry the same charge, so they generate an electromagnetic force that repels each other, thanks to the presence of electrons). neutrons, so the nucleus of an atom can exist). The strong interaction is the strongest of the basic interactions of nature, but it also has the shortest range of effects. Simulation of forces in the universe. Outside the radius of the atomic nucleus, the strong interaction does not work and of course that is also the reason why matter can exist today because with the magnitude of this interaction it can act. As far as the electromagnetic interaction goes, there will be no existence of atoms with electron shells because they will be crushed by the attraction between the nuclei themselves (because this force is much stronger than the electromagnetic repulsion). between nuclei). The weak interaction (or weak nuclear force) plays a role in causing beta decay of neutrons, thereby causing nuclear decay such as radiation and fission. The neutrons themselves are unstable in isolation, they are stable only when bound to protons. Stand-alone, neutrons can absorb or emit W or Z bosons and undergo beta decay to form a proton, an electron, and a neutron fraction. The nuclei of heavy elements have many protons and, respectively, require many neutrons to keep the protons from repelling each other. But at the same time when there are many neutrons, there will be neutrons isolated from the proton, out of the range of the strong interaction. Then the weak interaction causes them to decay and make the nucleus unstable. Elements whose nuclei have this phenomenon are called radioactive elements. The heavier the nucleus, the more unstable it is, and so the weak interaction is what keeps the number of elements in the universe finite, not infinite. Standard model of particle physics These three types of interactions are now uniformly described in the standard model of particle physics. According to the standard model, at sufficiently high energies, these interactions are consistent, although as already stated they appear to be very different in mechanism. In the cosmological model based on the Big Bang theory, the first stage of the universe from the age of the universe is 10–43s to the time of 20-36s the universe undergoes a period known as the great unification epoch. , in which the three types of interactions mentioned above have not been separated from each other. Immediately after the great unification era is the electroweak era, when the strong interaction has separated but the remaining two interactions are still united, called the electroweak interaction. It wasn’t until the quark era, when the age of the universe was 10-12s, that the universe cooled down enough for the electromagnetic and weak interactions to separate. Illustration. Gravitational interaction (or gravity) Interactions tend to pull objects and particles of mass toward each other. This is the weakest of the four basic interactions of nature on a certain object, but it is the one with the furthest range of effects. It plays a major role in forming the great structures of the universe, from stars, planets, asteroids, satellites to galaxies, clusters and superclusters of galaxies. The Earth and the planets revolve around the Sun also due to the effect of this type of interaction. Despite being the weakest force, gravity has not only the longest range, but also an unstoppable force, every man-made weightless environment or gravitation-isolating material in sci-fi movies. thought is unscientific. It cannot happen not because of technology but because of the general principle of the universe. For the same reason, gravity is also the force that causes the most violent phenomena in the universe, typically the collapse of matter to form neutron stars or black holes and supermassive black holes. In this phenomenon, thanks to the large amount of matter, the gravitational force is enough to overcome the electromagnetic force and the strong interaction causes the structure of matter to be destroyed. Gravity is very common to everyday life as we can stand on the ground and objects that are thrown high fall because of gravity. Some so-called forces, such as the resistance of the ground when you are standing on it or the Archimedes earth force in a liquid, are just indirect manifestations of gravity. The reason this universal force is mentioned last in the article is because it is the only one of the four fundamental interactions of nature that is not included in the standard model. Is there a fifth fundamental interaction? The existence of a fifth interaction of nature is still believed by many scientists, especially since the presence of dark matter has been confirmed. To date, there have been a number of experiments showing the involvement of a strange variation that was thought to be such a small interaction that has never been known. But so far, the fifth interaction has not been described in any way. Thus, the number of fundamental interactions of nature is still only four, with three unified in the standard model and gravitational interactions standing independently of them.

Exploring the stars is man’s way to the universe. Some people think that each star represents a destiny, others say that the stars are small angels tasked with lighting up the night. Today, science has been able to give us a more precise answer. What is a star? Stars are all celestial bodies that are capable of emitting their own light. All of them are giant air spheres. They are tens to hundreds of thousands of times more massive than Earth. Only thanks to such a large mass can they create their own light. An object to be able to emit its own light needs to have a mass of at least 70 times the mass of Jupiter – the largest planet in the Solar System, that is, about 7% of the mass of the Sun. Why do we see the stars? The stars in the sky have always been a mystery to the human imagination. Our Earth has a mass of about 6x1024kg (6 million billion billion tons). The Sun is 330,000 times heavier than the Earth. That is, a star with a mass of 7% of the mass of the Sun would be about 23,000 times heavier than the Earth. Every object has a gravitational force that directs the center of it to its heart. Normally no one notices but we ourselves are always attracted to our own. Because each part of the body is attracted to each other and the sum of them all form a gravitational force directed towards a center of mass in our body (the center of gravity of the object). The table, the chair, the Earth, are always gravitating to itself by a force called centripetal gravity. But why doesn’t it all burn brightly? That’s because the mass of the objects we come into contact with every day just can’t afford that. Because gravity is a force proportional to mass, gravity in everyday objects is so small that they don’t cause any significant effects. With very large objects such as planets, Earth, gravity is also negligible because it creates a clear attraction that pulls everything towards it. For example, when you jump high, you will fall very quickly because of the pull from the Earth. As for the aforementioned massive objects (tens of thousands of times heavier than the Earth), the great gravity makes the pressure at the center of the celestial body very high, this pressure provides a great acceleration for the celestial bodies. gaseous atoms (mostly hydrogen). They collide strongly with each other at high velocities, breaking the electron shells, separating electrons from the atomic nucleus. At the core of the star is no longer ordinary gas but a state of chaotically moving nuclei and electrons. This state is called plasma. In the plasma state, the hydrogen nuclei have a chance to collide directly with each other at high velocities, which causes what we call fusion reactions, fusing hydrogen nuclei into heavy hydrogen and finally is the helium nucleus. This reaction is known on Earth in hydrogen bombs – bombs capable of releasing thousands of times more energy than atomic bombs of the same mass. The fusion reaction at the core of a star releases a lot of energy in the form of radiation, some of which is visible light. This radiation is transferred to the star’s surface and causes the star to glow. Stars are composed mainly of hydrogen (over 70%), with a large part helium remaining, and an insignificant fraction of heavier gases. The surface temperature of a star is usually in the range of 3,000 to 50,000K, and the temperature at the center is in the range of several million to several tens of millions of K. It can be as high as 100 million K for red giants and several billion K. with red supergiant stars. Star classification Graphic image. By mass, stars are divided into two types, dwarfs and giants. Today, modern division is based on spectral charts. In which, the star with the obtained spectrum of which position on the chart will be determined to belong to which group with specific characteristics of mass and temperature. The most widely used spectrogram today is the Hertzsprung-Russell chart. This graph represents the luminosity, size, and temperature of any star when its spectrum is obtained. According to temperature, the chart is divided into 7 levels with the symbols O, B, A, F, G, K, M respectively. In which, the star closer to O is hotter and closer to M is cold. Each level itself is divided into several sub-levels. Through the chart, it can be seen that most of the stars in the universe are concentrated in the main sequence of the chart. This sequence is a sequence of dwarfs and subgiant stars. Our sun is also on this sequence. It is located in the G group, has the detailed spectral designation G2V (yellow dwarf/Yellow dwarf). Below the sequence are groups of white dwarfs, and above are giants and supergiant, supergiant stars. Star evolution All stars form from large clouds of dust and gas called protostar nebulae. Due to gravity they gather together and shrink until they form a dense mass. As we all know, all objects that carry mass carry gravity. The same object itself also has a force of attraction between different parts of it. However, the gravitational force between small masses is negligible and we hardly notice it. Only significant forces, such as Earth’s gravity acting on people and objects, are enough to be noticed. In stars, gravity is very strong (due to its high mass). When the force of gravity is too great for the atoms to bear, they break the atomic shells and accelerate their nuclei. Hydrogen nuclei (consisting of 1 proton) when collided at high velocity, combine to form heavy hydrogen, and then helium. This reaction releases energy that causes the star to burn brightly. This is a fusion reaction (also known as a nuclear explosion. This reaction is used in the hydrogen bomb (H bomb) – the most destructive destructive weapon that mankind has built). Thanks to the great energy released from nuclear fusion in the star’s core, the gravitational contraction is halted as the released energy balances the gravitational force. The star burns so brightly for several tens, hundreds of millions or billions of years. The lower the mass of the stars, the longer the lifespan. For example, our Sun is a dwarf, medium mass, it can live for about 10 billion years. Meanwhile, stars are much larger, sometimes only living a few hundred or even tens of millions of years because the high mass creates greater pressure towards the center. It causes nuclear fusion to happen faster and the star to deplete energy faster. After burning out all of its hydrogen energy, the star no longer produces energy against centripetal gravity. It will once again shrink. At this time, the helium nuclei combine to form nuclei of heavier elements such as carbon, oxygen and heavier elements up to iron. This process releases an energy that inflates the star’s crust while the star’s core continues to contract. This is the red giant stage. For medium-sized stars (with a mass between 0.5 and 10 times the mass of the Sun), the red giant shell, when inflated sufficiently large, will explode and break up to form a planetary nebula. Meanwhile, high-mass stars have massively inflated stellar shells, becoming red supergiant stars. During this stage, the stellar core continues to contract due to gravity, temperature and pressure both increase many times compared to the previous stage, allowing nuclei of heavier elements to be synthesized (from familiar metals). from copper, silver, and gold to radioactive elements). Up to a certain limit, the energy released from the core creates a large explosion that breaks the outer shell. This is a supervova explosion. After the shell is broken, the star’s core remains for both massive stars as well as light stars. For low- and medium-mass stars like the Sun, the core will stop shrinking, becoming a white dwarf, emitting a very faint light. After billions or tens of billions of years, the generation of radiation ends, stars no longer emit light. It’s called a black dwarf, a dark, dead mass of matter. In fact, the process for a white dwarf to become a black dwarf is so long that so far a black dwarf is only a theoretical prediction. No white dwarf in the universe has been around long enough to become a black dwarf. For massive stars whose core remains after the supervova are at least 1.4 times more massive than the Sun, the mass is so great that they continue to shrink. The nuclei react with each other to form heavy nuclei. The contractions are not over yet, they cause the free electrons to be squeezed tightly against the protons, combining to form neutrons. The star becomes a solid mass of matter, composed entirely of neutrons. Therefore, it has extremely high density and extremely fast rotation speed. This object is called a neutron star. Previously, when this object was first observed, astronomers saw that it emitted a very strong amount of electromagnetic pulses, so they called them pulsars. Even more massive stars with a core mass at least 2 or 3 times that of the Sun, have not stopped after reaching the neutron star stage. They squeeze all matter together to an infinitely large density, concentrated at a location called a singularity. This singularity warps the space around it, a region of space that is bent to an infinite (closed) curvature. The boundary of this space is called the event horizon. Because the space is bent inward, anything that goes in can’t get out, not even light. This entire region of space bounded by the event horizon is called a black hole.