[edit] The birth of stars
The Horsehead Nebula, the bright spot at the base is the IC 434 star that is forming new stars. The evolution of the star begins in the giant molecular cloud. The density of most void space in a galaxy is about 0.1 to 1 atom per cubic centimeter, but the density of giant molecular clouds is millions of atoms per cubic centimeter. A giant molecular cloud contains hundreds of thousands to tens of millions of solar masses and is 50 to 300 light-years in diameter.
As the giant molecular cloud rotates around the galaxy, some events may cause its gravitational collapse. Giant molecular clouds may collide with each other or pass through dense parts of spiral arms. High-velocity material ejected from nearby supernova explosions may also be a trigger. Finally, the compression and disturbance of nebulae caused by galaxy collisions may also form large numbers of stars.
The conservation of angular momentum during the collapse process will cause the giant molecular cloud fragments to continuously break down into smaller fragments. Debris with a mass less than about 50 solar masses will form stars. During this process, the gas is heated by the released potential energy, and the conservation of angular momentum will also cause the nebula to begin to rotate and then form a protostar.
The initial stages of star formation are almost completely obscured by dense nebular gas and dust. Typically, star-forming sources are observed by casting shadows on surrounding clouds of bright gas, known as Pocket spheres.
Protostars with very small masses cannot reach a temperature high enough to start the nuclear fusion reaction of hydrogen, and they will become brown dwarfs. The exact mass limit for stars and brown dwarfs depends on their chemical composition, with more metals (compared to elements heavier than helium) lowering the limit. The limit for protostars with metal compositions similar to those of the Sun is approximately 0.075 solar masses. Brown dwarfs with a mass greater than 13 Jupiter masses (MJ) will undergo deuterium fusion reactions. Some astronomers believe that such stars can be called brown dwarfs. Objects larger than planets but smaller than brown dwarfs are classified as substellar objects. . For both types, regardless of whether they can burn deuterium, their luminosity dims and gradually cools over hundreds of millions of years, slowly heading toward death.
For higher-mass protostars, the core temperature can reach 10 million K, and the proton-proton chain reaction can begin to fuse hydrogen first into deuterium and then into helium. In stars slightly larger than the mass of the Sun, the carbon-nitrogen-oxygen cycle contributes a considerable amount to energy production. The onset of nuclear fusion will cause a brief loss of hydrostatic equilibrium, which is the balance between the "radiation pressure" outward from the core and the "gravitational pressure" caused by the star's mass to prevent further "gravitational collapse" of the star, but The star rapidly evolves to a stable state.
LH 95 is a stellar nursery in the Large Magellanic Cloud. Newly born stars come in a variety of sizes and colors. Spectral types range from hot blue to cold red, and masses range from a minimum of 0.085 solar masses to more than 20 solar masses. The brightness and color of a star depend on its surface temperature, which in turn is determined by its mass.
Newly born stars fall at a specific point on the main sequence of the Hertz-Russell diagram. Small, cold red dwarfs burn hydrogen at a slow incoming rate and can stay on the main sequence for tens of billions of years, while massive and hot supergiants can only stay on the main sequence for millions of years. A star of average size like the sun spends about 10 billion years on the main sequence. The Sun is thought to be at the midpoint of its lifetime, so it is still on the main sequence. Once a star has consumed most of the hydrogen in its core, it leaves the main sequence.
Sagittarius is a star field that gathers a large number of stars.
[edit] Maturation of stars
Depending on the mass of the star at birth, after millions to billions of years, the nuclear fusion reaction that continues in the core accumulates in the core A lot of helium. More massive and hotter stars produce helium faster than less massive and cooler stars.
The accumulated helium has a higher density than hydrogen and gradually increases due to its own compression and the continued nuclear reaction. Stable equilibrium must be maintained by higher temperatures resisting the increased gravity due to compression.
Eventually, the hydrogen supplied by the core will be exhausted, and there will be no outward pressure generated by the nuclear fusion of hydrogen to resist gravity.
It will shrink until the electrons become degenerate enough to resist gravity, or the core becomes hot enough (100 million degrees K) to burn helium, whichever happens first depends on the mass of the star.
[edit] Low-mass stars
What will happen after low-mass stars stop producing energy through nuclear reactions is not yet directly known: the currently known age of the universe is only 13.7 billion years old, which is shorter than the time it takes for low-mass stars to cease nuclear reactions (in some cases, orders of magnitude shorter), so current theories are shaped by computer simulations.
For stars with a mass less than 0.5 solar mass, after the hydrogen fusion in the core stops, it is simply because there is not enough mass to generate enough pressure in the core, so the helium core fusion reaction cannot be carried out. They will become red dwarfs like Proxima Centauri, some of which will live thousands of times longer than the Sun. Current astrophysics models believe that a star with a mass of 0.1 solar mass can stay on the main sequence for up to 6 trillion years, and it will take hundreds of billions of years or more to slowly collapse. Become a white dwarf[1]. If a star's core becomes stagnant (thought to be a bit like our sun today), it will always be surrounded by several outer layers of hydrogen, perhaps created during evolution. However, if the star has complete convection (the idea is thought to be the protagonist of low-mass stars), there will be no separation of layers around it. If so, it will, like the intermediate-mass stars described below, develop into a red giant without causing helium fusion; in other words, it will simply shrink until electron degeneracy pressure prevents the collapse of gravity, and then directly Transform into a white dwarf star.
[edit] Intermediate-scale stars
When stars with a mass similar to the sun die, they become planetary nebulae, like the Cat's Eye Nebula. In another case, several shells containing hydrogen around the core immediately cause the star to expand due to the acceleration of the nuclear fusion reaction. Because these are the layers outside the core, they are subject to lower gravity and will expand faster than the increase in energy, thus causing a drop in temperature and making them even hotter than in the main sequence. Reddish. Stars like this are called red giants.
According to the HR diagram, red giants are huge stars that are not on the main sequence. The star classification is K or M. Aldebaran in the constellation Taurus and Arcturus in the constellation Bo?tes are both red giants. .
Under the support of electron degeneracy pressure, stars with masses within several solar masses will develop a helium core still surrounded by hydrogen. Its gravity squeezes several layers of hydrogen directly onto the helium core, causing the hydrogen fusion reaction to occur faster than in stars of the same mass on the main sequence. This instead causes the star to become brighter (by a factor of 1,000 to 10,000) and expand; the expansion exceeds the increase in luminosity, thus causing a decrease in effective temperature.
The star expands in the outer convective layer, which carries material from the area close to nuclear fusion to the star's surface, where it mixes with surface material through turbulence. In all stars except the lowest mass stars, the material undergoing nuclear fusion inside is buried deep inside the star before this point. Through the action of convection, the products of nuclear fusion can be seen on the surface of the star for the first time. . At this stage of evolution, the results are very subtle. The largest effects are changes in the isotopes of hydrogen and helium, but these have not yet been observed. What contributes is the carbon-nitrogen-oxygen cycle that occurs at the surface, lower 12C/13C ratios and changing carbon to nitrogen ratios. These were discovered spectroscopically and measured on many evolving stars.
A simplified diagram demonstrating the evolution of a star with a mass similar to that of the Sun. Stars are born from collapsing gas clouds (1), go through a contraction phase to become protostars (2), and then enter the main sequence (3). Once the hydrogen in the core is depleted, it expands into a red giant (4), then its outer shell escapes into a planetary nebula and the core metamorphoses into a white dwarf (5).
When the hydrogen surrounding the core is consumed, the core absorbs the produced helium, further causing the core to shrink and causing the remaining hydrogen to undergo nuclear fusion faster, which will eventually lead to helium fusion (including the 3-helium process) in the core . In stars larger than 0.5 solar masses, electron degeneracy pressure may delay helium fusion by millions to tens of millions of years; in heavier stars, the total mass of the helium core and the stacked outer layers of gas, The electron degeneracy pressure will be insufficient to delay the process of helium fusion.
Helium flash will occur when the temperature and pressure of the core are sufficient to ignite helium fusion in the core, and if electron degeneracy pressure is the main force supporting the core. In a core with greater mass, electron degeneracy pressure is not the main force supporting the core, and the combustion of helium fusion proceeds relatively quietly. Even if a helium flash occurs, the rapid release of energy (on the order of 108 solar energy) is short-lived, so the surface layer that can be observed outside the star will not be affected [2]. The energy generated by helium fusion will cause the core to expand, so the fusion rate of hydrogen superimposed on the outer layer of the core will slow down, reducing the total energy production. So, the star will shrink, although not all will return to the main sequence, it will migrate on the horizontal branch of the HR diagram, gradually shrinking in radius and increasing the surface temperature.
After the star consumes the helium in its core, fusion continues near the hot core, which contains carbon and oxygen. As the star enters the asymptotic giant branch on the HR diagram, it evolves parallel to the original red giant, but the energy is generated faster (and therefore lasts shorter) [3].
Changes in energy output cause periodic changes in the star's size and temperature. The energy output itself reduces the frequency of energy emission, accompanied by an increase in the rate of mass loss through strong stellar winds and violent pulsations. Stars in this stage are called late-type stars, OH-IR stars, or Mira-type stars, depending on the distinctive characteristics they exhibit. The expelled gas comes from the interior of the star and is also relatively rich in created elements. In particular, the abundance of carbon and oxygen is related to the type of star. The expanding shell of gas is called a circumstellar envelope and gradually cools as it moves away from the star, allowing dust and molecules to form. Under ideal conditions, high-energy infrared light from the core After entering the ring star package, it will stimulate the formation of a ray.
The rate of helium burning is extremely sensitive to temperature, which will lead to a huge instability combination, which will eventually give the star enough kinetic energy. Several layers of gas shells are ejected to form a potential planetary nebula. The temperature of the stellar core that remains in the center of the nebula will gradually decrease and become a small and dense white dwarf star.
[edit] Massive star
The Crab Nebula is the scattered remnant of a supernova that exploded about 1,000 years ago. In massive stars, the core was large enough to be produced by the fusion of hydrogen before electron degeneracy pressure could become dominant. Helium ignites. So as these stars expand and cool, they won't be much brighter than the lower-mass stars; but they will be much brighter than when the lower-mass stars first formed, and they will be much brighter than the lower-mass stars they formed. Red giants are bright, so these stars are called supergiants.
Extremely massive stars (approximately more than 40 times the mass of the sun) will be very bright and have very long and high-speed stellar winds as they expand. Before red giants, due to the strong radiation pressure, they tended to peel off the outer gas shell first, so their mass loss was also very fast, which caused them to maintain high surface temperatures (blue and white color) during the main sequence stage. . Because the star's outer shell will be stripped off by extremely strong radiation pressure, the mass of the star cannot exceed 120 solar masses. Although a lower mass can slow down the speed of the outer shell being stripped off, if they are close enough binary stars. , when it expands and the outer shell is peeled off, it will combine with the companion star; or because their rotation is fast enough, convection will bring all the material to the surface, causing complete mixing, and there is no core and outer shell that can be separated. Avoid becoming a red giant or red supergiant [4].
As hydrogen is obtained from the base of the shell and fused into helium, the core also gradually becomes hotter and denser. In massive stars, electron degeneracy pressure is not enough to prevent gravitational collapse alone. Each element consumed in the core, igniting the fusion fire of heavier elements, can also temporarily prevent gravitational collapse. If the core of the star is not too heavy (about 1.4 times less than the mass of the sun, considering that a lot of mass has been lost before this), it may be able to form a white dwarf like a lower-mass star as mentioned above (outside May be surrounded by planetary nebulae), the difference is that this white dwarf is mainly composed of oxygen, neon and magnesium.
Before core collapse, the core structure of a massive star is arranged in layers like an onion (not to scale). Above some mass (estimated to be 2.5 times the solar mass, the mass of the original star is about 10 times the solar mass), the temperature of the core can reach the temperature of local destruction (about 1.1GK) and start to form oxygen and helium, and helium and It immediately fuses with residual neon to form magnesium; oxygen then fuses to form sulfur, silicon and small amounts of other elements. Finally, when the temperature reaches such a high level that any element will be partially destroyed, alpha particles (helium nuclei) are usually released and immediately fuse with other nuclei, so a small number of nuclei will become heavier nuclei after being sorted, and The net energy released is increased because the energy released by breaking the parent nucleus is greater than the energy required to fuse into the daughter nuclei.
Stars whose cores are too massive to form white dwarfs, but not large enough to withstand the conversion of neon into oxygen and magnesium, will undergo a process of gravitational collapse (due to electron capture) before fusing into heavier elements. )[5]. Regardless of the increase or decrease in temperature caused by electron capture, smaller nuclei (such as aluminum and sodium) will be formed before gravitational collapse, which can have a significant impact on the total energy production before gravitational collapse [6]. This may have an impact on the abundance of elements and isotopes ejected in subsequent dramatic supernova explosions.
Once the process of stellar nucleosynthesis produces iron-56, all subsequent processes consume energy (the energy released by combining the fragments into nuclei is less than the energy required to break the parent nuclei). If the mass of the core is greater than the Chandrasekhar limit, the electron degeneracy pressure will not be enough to support and resist the gravity generated by the mass, and the core will suddenly collapse, and the catastrophic collapse will form a neutron star or a black hole (in the mass of the core exceeding the Tolman-Oppenheimer-Wakov limit). Although the process is not fully understood, certain transformations in gravitational potential energy cause these cores to collapse and convert into Type Ib, Ic or II supernovae. What is known is that when the core collapses, as observed in supernova 1987 A, a huge surge of neutrinos is produced. Extremely high-energy neutrinos will destroy some atomic nuclei. Some of their energy will be consumed in releasing nuclei, including neutrons, and some of the energy will be converted into thermal energy and kinetic energy, thus causing the shock wave to merge with some material from the core collapse. rebound. The electron capture that occurs in very dense converging matter creates additional neutrons. Some of the rebounding matter is bombarded by neutrons, which in turn induces some nuclear capture, creating a series of radioactive materials, including uranium, which is heavier than iron. elements[7]. Although the neutrons released by non-explosive red giant stars in early reactions and secondary reactions can also create a certain amount of elements heavier than iron, the abundance of elements heavier than iron produced in this reaction (in particular, Some stable and long-lived isotopes (and some isotopes) differ significantly from supernova explosions. We found that the abundance of heavy elements in the solar system is different from both, so neither supernovae nor red giants alone can be used to explain the observed abundances of heavy elements and isotopes.
The energy transferred from core collapse to rebounding matter not only creates the heavy elements, but also provides the escape velocities needed for their acceleration and escape (a mechanism that is not fully understood), thus leading to Ib, Generation of type Ic or type II supernovae.
The current understanding of these energy transfer processes is still unsatisfactory. Although current computer simulations can provide a partial explanation for the energy transfer of type Ib, Ic or II supernovae, they are still insufficient to explain the observed energy carried by the material ejection. [8]. Some evidence obtained from analysis of the orbital parameters and masses of neutron star binaries (requiring two similar supernovae) suggests that supernovae resulting from the collapse of oxygen-neon-magnesium cores may differ (except in size) from those observed from the collapse of iron cores. There are other differences) [9].
The most massive stars may be completely destroyed in a supernova explosion because their energy exceeds their gravitational binding energy. This rare event causes the pair to be unstable, and the wreckage afterwards is not even a black hole [10].
[edit] Stellar debris
After a star exhausts its fuel, based on its mass during its life, if the hypothetical strange stars are not counted, its debris will It is one of the following three types.
[edit] White dwarf
Main article: White dwarf
A star with a mass of 1 solar mass, after evolving into a white dwarf, has a mass of about 0.6 solar mass, compressed The volume is approximately the size of the Earth. A white dwarf is a very stable object because its inward gravity is balanced by the electron degeneracy pressure generated by the electrons in the core (which is a result of the Bao exclusive principle). Electron degeneracy pressure provides a rather loose limit against further compression by gravity; therefore, for different chemical elements, the greater the mass of the white dwarf, the smaller the volume. Without fuel to continue burning, the star's residual heat can continue to radiate outward for billions of years.
The chemical composition of a white dwarf depends on its mass. Stars with only a few solar masses can fuse carbon to produce magnesium, neon and a small amount of other elements, creating a white dwarf star whose main components are oxygen, neon and magnesium. Under the condition that enough mass is thrown away, its mass will not exceed the Chandrasekhar limit (see below); and under the condition that the carbon burning is not violent enough, it will avoid becoming a supernova [11]. Stars of the same order of magnitude as the Sun are unable to ignite the nuclear reactions of carbon fusion, and the resulting white dwarfs are mainly composed of carbon and oxygen and are too low in mass to produce gravitational collapse unless the mass can be increased at a later stage (see below). Stars with masses less than 0.5 solar masses cannot even ignite helium combustion (see above), so the main component after being compressed to become a white dwarf is helium.
In the end, all white dwarfs will turn into cold, dark objects, and some people call them black dwarfs. But the current universe is not old enough to produce objects like black dwarfs.
If the mass of the white dwarf can be increased beyond the Chandrasekhar limit - 1.4 solar masses for a white dwarf whose main components are carbon, oxygen, neon, and/or magnesium, the electron degeneracy pressure will not be able to Against gravity, the star will collapse due to electron capture. Depending on the chemical composition and core temperature before collapse, the core may collapse into a neutron star or runaway by igniting the combustion of carbon and oxygen. Heavier elements are more prone to star collapse because higher temperatures are needed to re-ignite the fuel in the core. Therefore, the electron capture process that can lighten the nucleus can make nuclear reactions easier to proceed; however, the higher the core temperature, the more likely it is that the star will collapse. It is easy to cause the star's nuclear reaction to go out of control, which will cause the star to collapse into a Type Ia supernova [12]. Even if a Type II supernova produced by the death of a massive star releases more total energy, this type of supernova will be several times brighter than a Type II supernova. This collapse-causing instability makes it impossible for white dwarfs above or even close to 1.4 solar masses to exist (the only possible exception would be super-rapidly rotating white dwarfs, since the effect of centrifugal force offsets the mass problem). Mass transfer between binaries may cause the mass of the white dwarf to approach the Chandrasekhar limit, causing instability.
If there is a white dwarf and an ordinary star in a close binary system, the hydrogen from the larger companion will form an accretion disk around the white dwarf and increase the mass of the white dwarf until the temperature of the white dwarf Increased triggering of runaway nuclear reactions.
Before the mass of the white dwarf reaches the Chandrasekhar limit, this explosion will only form new stars.
[edit] Neutron stars
The bubble-like image is the shock wave produced by a supernova that exploded 15,000 years ago and is still expanding. (view larger image).Main article: Neutron stars
When a star's core collapses, the pressure causes electrons to be trapped, causing most of the hydrogen to turn into neutrons. After the electromagnetic force that originally separated the atomic nuclei disappeared (in proportion, if the nucleus was the size of dust, the atom would be the size of a 600-foot-long football field), the core of the star becomes a dense sphere containing only neutrons (like It is a huge atomic nucleus), with several outer shells composed of degenerate matter (mainly thin layers of iron and substances produced by subsequent reactions). Neutrons also obey the Baoli exclusion principle, using a force similar to electron degeneracy pressure but stronger to resist the compression of gravity.
Stars like this, called neutron stars, are extremely small—on the order of only 10 kilometers in diameter and no larger than a large city—and have extremely high densities. Their rotation periods are dramatically shortened by the star's contraction (due to conservation of angular momentum), some as high as 600 rotations per second. As these stars rotate at high speed, the Earth receives a pulse of radiation every time the star's magnetic pole points toward the Earth. Neutron stars like this are called boshas, ??and the first neutron star discovered was of this type.
[edit] Black hole
Main article: Black hole
If the remnant of the star is massive enough, the neutron degeneracy pressure will not be enough to prevent the star from collapsing When it shrinks below the Schwarzschild radius, the star's remnant will become a black hole. It is not yet known how much mass is required for this to occur, but current estimates place it at 2 to 3 solar masses.
Black holes are celestial objects predicted by general relativity, and astronomical observations and theories also support the existence of black holes. According to the tradition of general relativity, no matter or information can be transferred from the interior of a black hole to an observer on the outside, although quantum effects allow errors in this strict law.
Although the mechanism of stars collapsing to produce supernovae is not fully understood, it is also unknown whether stars can directly collapse to form black holes without a visible supernova explosion, or whether a neutron star must first be formed after a supernova explosion. , and then continue to collapse into a black hole; the correlation between the initial stellar mass and the final debris mass is not entirely reliable. To resolve these uncertainties, more supernovae and supernova remnants will need to be analyzed.