"Little dream, have you ever heard of asymmetry? Hua Feng asked about Yunmeng after breakfast.

Seeing that Yun Meng was a little confused, Hua Feng immediately explained. The morning passed quickly in such a Q&A.

He knew that massive stars with masses not less than nine times the mass of the Sun had quite complex evolutionary styles. The hydrogen element in the inner core of the star is constantly produced by nuclear fusion to produce helium, and the energy released in it produces an outward radiation pressure, thus ensuring the hydrostatic equilibrium of the inner core and avoiding the collapse caused by the star's own huge gravitational pull.

The collapse of the inner core begins when the hydrogen in the star's inner core is depleted and it can no longer produce enough radiation pressure to balance the gravitational pull, during which the temperature and pressure of the inner core rise dramatically and ignite helium. As a result, the helium in the inner core of the star begins to fuse into carbon, and is able to generate a considerable radiation pressure to stop the collapse.

This causes the core to expand and cool slightly, which at this point has an outer layer of hydrogen fusion and a center of more high-temperature and high-pressure helium fusion. (Other elements such as magnesium, sulfur, and calcium are also produced and, in some cases, burned in subsequent reactions.) ）

This process repeats itself several times, with each core collapse aborted by the fusion process of the next, heavier element, which continues to produce higher temperatures and pressures.

As a result, the star becomes an onion-like layered structure, and the closer the elements are to the outer layer, the more likely they are to fuse. Each layer relies on the thermal energy and radiation pressure generated by the fusion reaction in the next layer inside it to stop collapsing until the fusion fuel in this layer is exhausted, and each layer is hotter and burns faster than its outer layer – the process of burning from silicon to nickel takes only a day or so or so.

Later in this process, an increasing number of heavy elements are involved in nuclear fusion, and the binding energy of the resulting atoms of related elements increases, resulting in a decreasing amount of energy released by the fusion reaction.

At higher energies, photoinduced metamorphosis and electron capture occur in the inner core, which leads to a decrease in the energy of the inner core and generally accelerates the fusion reaction to maintain equilibrium. This continuous synthesis of heavy elements terminates at nickel-56, and no energy is released in this fusion reaction (but iron-56 can be produced through radioactive decay) As a result, the nickel-iron core can no longer produce any outward radiation pressure that can balance the star's own gravity, and the only thing that can play a certain balancing role is the electron degeneracy pressure in the core.

If the mass of the star is large enough, it is possible that the mass of this inner core will eventually exceed the Chandrasekhar limit, so that the electron degeneracy pressure is not enough to balance the gravitational collapse. Eventually, due to the strong gravitational pull of the star itself, the force that separates the nuclei from each other in the innermost layer of the inner core cannot be supported, and the star begins to collapse devastatingly, and there is no fusion reaction that can prevent the collapse from happening.

Kernel collapse:

The supernova core can collapse at a rate of 70,000 kilometers per second (about 0.23 times the speed of light), and when the mass of the original star is less than about 20 times the mass of the Sun (depending on the intensity of the explosion and the total amount of material that falls back after the explosion), the remaining product of the collapse is a neutron star, and for stars above this mass, the remaining mass will continue to collapse into a black hole due to exceeding the Oppenheimer-Volkov limit (this collapse may be one of the causes of gamma-ray bursts, and it is theoretically possible to produce another supernova explosion with the release of a large number of gamma rays) , theoretically the upper limit for this is about 40-50 times the mass of the Sun.

Stars with more than 50 solar masses are thought to skip the supernova explosion process and collapse into black holes, but this limit is difficult to calculate due to the complexity of the model.

However, recent observations have shown that stars with extremely high masses (140-250 solar masses) and low proportions of heavy elements (relative to helium) have the potential to form unstable supernovae without leaving black hole remnants. The formation mechanism of these rather rare supernovae may not be the same (and may be partially similar to a Type Ia supernova explosion), and thus the presence of iron nuclei may not be required. The typical representative of this type of supernova is the type II supernova SN 2006gy, which is estimated to have 150 times the mass of the sun, and observations of it show that the explosion of such a massive star is fundamentally different from previous theoretical predictions.

The process causes a sharp increase in the temperature and density of the core. This process of energy loss in the inner core terminates with the equilibrium of the outward degenerate pressure and the inward gravitational force. Under photoinduced metamorphosis, γ rays break down iron atoms into helium nuclei and release neutrons while absorbing energy, while protons and electrons merge through an electron trapping process (irre β versible decay) to produce neutrons and escaping neutrinos.

In a typical type II supernova, the initial temperature of the newly formed neutron nucleus can reach 100 billion Kelvin, which is 6,000 times the temperature of the Sun's core. Much of this high heat needs to be released to form a stable neutron star, and this process can be done by further neutrino release. These "hot" neutrinos make up neutrino-antineutrino pairs that cover all tastes, and are several times numerically formed by electron trapping.

About 1,046 joules of gravitational energy – about 10% of the star's remaining mass – translate into neutrino bursts that last about 10 seconds, the main product of the event. Neutrino bursts take away the energy from the inner core and accelerate the collapse process, while some neutrinos may also be absorbed by the star's outer material to provide energy for subsequent supernova explosions.

The inner core eventually collapses into a sphere about 30 kilometers in diameter, with a density comparable to that of an atomic nucleus, after which the collapse is abruptly terminated by strong nucleon interactions and neutron degeneracy pressure. The movement of the material that collapses inward is suddenly stopped, and the matter rebounds to a certain extent, which will trigger a shock wave that propagates outward. The results of computer simulations show that this outward spreading shock wave is not the direct cause of the supernova explosion, and that in fact, the energy consumption in the outer region of the inner core due to the disintegration of heavy elements exists for a time of only milliseconds.

This necessitates the existence of an ununderstood process that allows the outer region of the inner core to regain about 1044 joules of energy, resulting in a visible burst. Current research focuses on the combination of neutrino reheating, spin and magnetic field effects that underlie this process.

Due to the presence of the Barmo absorption line in the hydrogen spectrum, the photometric curve of type II supernovae is distinctive: compared with the photometric curve of type I supernovae, the photometric curve of type II supernovae decreases by an average of 0.008 magnitude per day, which is much lower than the former.

According to the characteristics of the photometric curve, type II supernovae can be divided into two subclasses, one with a flat plateau region on the photometric curve (type II-P) and the other with only linear attenuation (type II-L) in the photometric curve.

In this way, the overall attenuation rate of type II-L supernovae is 0.012 magnitude per day, which is higher than that of type II-P supernovae of magnitude 0.0075 per day. In the case of type II-L supernovae, the reason for this difference is that most of the outer layers of hydrogen in the primordial star were ejected.

The plateau zone in the photometric curve of a type II-P supernova is due to a change in the opacity of its outer layer. The shock wave generated by the explosion ionizes the hydrogen atoms in the outer layer, preventing the photons from the internal explosion from escaping through the outer layer, thus significantly increasing the opacity of the outer layer. When the hydrogen ions in the outer layer are cooled and recombined into atoms, the transparency of the outer region returns.

Among the many anomalous properties of the type II supernovae spectrum, II

It is possible that a type supernova is born from the interaction of ejecta with the material surrounding the star, while a type IIb supernova is likely to be a massive star that has lost most, but not all, of its hydrogen outer layer under the tidal forces of its companion star. As the Type IIb supernova ejecta expands, the remaining outer layer of hydrogen quickly becomes transparent, revealing the inner structure.

Asymmetry:

A long-standing mystery surrounding supernova research is how to explain the high velocity of the remaining dense matter produced after the explosion relative to the inner core. (Pulsars as neutron stars have been observed to have high velocities, and black holes can theoretically have very high velocities, but it is difficult to confirm them through isolated observations.) ）

In any case, the force that can propel matter into such a velocity should be considerable, as it can cause an object with a mass greater than the Sun to produce a velocity of 500 km/s or more. Some explanations suggest that this impetus includes convection when the star collapses and jets that occur when neutron stars form.

This composite image of X-rays and visible light depicts the electromagnetic radiation emitted from the core region of the Crab Nebula. The velocity of particles released from pulsars near the center can be close to the speed of light. The neutron star has a velocity of about 375 km/s, and in particular, the large-scale convection generated above the core is capable of causing local changes in elemental abundance, resulting in unevenly distributed nuclear reactions during the collapse that bounce back and then explode.

The jet interpretation suggests that the accretion of gas by the neutron star at the center will form an accretion disk and produce a highly directional jet, which will eject the material at a very high speed and generate a transverse shock wave to completely destroy the star. These jets may be an important factor in supernova explosions. (A similar model has been used to explain the generation of long gamma-ray bursts.) ）

Over time, however, the explosion becomes more symmetrical. This asymmetry can be detected by measuring the polarization of the outgoing light in the initial state.

Type Ia nuclear collapse:

Because Ib, Ic, and a variety of type II supernovae have similar mechanistic models, they are collectively referred to as nuclear collapse supernovae. The basic difference between a type Ia supernovae and a nuclear collapse supernovae is the source of the energy of the radiation released near the peak of the photometric curve. The original stars of the nucleus collapse supernovae all have an extended outer layer, and this outer layer requires less expansion to achieve a certain level of transparency. Most of the energy required for the optical radiation at the peak of the photometric curve comes from the shock wave that heats and ejects the outer layer of matter.