The bright pearl in the universe

 Stars also have "birth, old age, sickness and death". Massive stars emit bright light when they die-supernova explosions, and ancient Chinese records on supernovae are also the most perfect in the world.

　　 Today, supernovae have become one of the research hotspots of astronomers. So, what exactly is a supernova? How do scientists interpret supernovae?

　　What is a supernova?

　　 Supernova is not a “superstar or a newly born star”, but a violent explosion that occurs when a particular type of star evolves to the end of its life. It is a “spectacular show” at the moment of star death. This type of explosion generally destroys the star completely, accompanied by extremely high energy release (the luminosity emitted per second is equivalent to the sum of the total energy released by the 10 billion to 100 billion suns), so it is one of the most dazzling astronomical wonders in the universe. One. After the supernova exploded, the brightness continued to increase for a period of time, and the supernova closer to the earth even became visible to the naked eye.

　　 In ancient my country, people called these newly-emerged stars in the sky "guest stars". Since people did not know about supernovae in ancient times, these guest stars include not only supernovae, but also novae and even comets. Now we know that the intensity of nova explosions is more than 10,000 times weaker than supernova explosions. They may erupt again after experiencing an explosion, and supernova explosions will basically destroy the entire star.

　　 There have been observations and records of supernova explosions very early in the history of our country. For example, on December 7, 185 AD, astronomers in the Eastern Han Dynasty in my country observed the explosion of supernova SN 185 near Nanmen 2 (between the constellations of Compasses and Centauri), which is a record in human history. The first supernova. "The Book of the Later Han Dynasty" has a more detailed record of the supernova SN 185. The original text reads: "In October of the second year of Zhongping, the guest star went out of the South Gate. It was as big as a half-banquet, with five colors of joy and anger, and a little smaller. Monthly consumption". The supernova was visible two years after the explosion, and astronomers can still find an obvious radio source at this location.

　　 Another example, SN 1006 is a relatively bright supernova recorded in the history of our country. This supernova was discovered in the Song Dynasty and was recorded in the "Song History·Astronomical History". The article reads: "Wuyin in April of the third year of Jingde, Zhou Boxing saw him, went out of Dinan, once riding the official west, shaped like a half-moon, with awning horns, bright and bright, you can look at things in the library. August, with the sky. Turn into turbidity. See you again in Die in November. Freely, I often see the east in November and turn into turbidity in the southwest in August.” This supernova explosion in the Jingde period was divined as auspicious by the astronomical officer Zhou Keming at the time. This is also the origin of "Jingxing Gaozhao". For another example, the "Song Hui Yao" records: "In May of the first year of Hehe, the morning will go out of the east and guard the sky. The day is like white, the horns appear everywhere, the color is red and white, and you see it on the 23rd." Supernova is a supernova that exploded in 1054 and later evolved into the famous Crab Nebula.

Why study supernova?

　　 Supernova is closely related to a series of important astrophysics research, so it occupies a very important position in astronomy research. In the process of supernova explosion, we can detect many extreme physical processes, such as thermonuclear combustion, shock wave heating, interaction between projectiles and surrounding matter, and radioactive element decay. When a supernova explodes, it ejects the heavy elements produced by the nuclear fusion inside the star, and at the moment of the explosion, many elements heavier than iron are formed through the process of neutron capture, which affects the evolution of galaxies and the universe's metal abundance and the formation of life To the vital role. The predecessor of our solar system is the second generation of metal-rich stars formed by recondensing the debris of supernova explosions.

　　Secondly, supernovae are a powerful tool for testing theories of stellar evolution. White dwarf hot nuclear explosions (also known as type Ia supernovae, see the next section for details) and massive star gravitational collapse explosions are two ways of star death. Observation and research on supernovae will help us understand the evolution of binary stars and massive stars. process.

　　 Finally, supernovae are also important probes for astronomers to study the history of the expansion of the universe. In the late 1990s, astronomers observed and studied the luminosity of type Ia supernovae in the distant universe and the nearby universe. They discovered for the first time the astonishing result that the universe is accelerating and predicting the existence of dark energy in the universe. This discovery was also made in 2011. Nobel Prize in Physics

　　 The center of a supernova explosion produced by the collapse of a massive star core generally forms a compact celestial body, such as a neutron star or a black hole, and is accompanied by a large number of neutrinos. The former is the main target of high-energy astrophysics research and has important research value, while the latter is a detection research object of great interest to particle physicists. The first neutrinos detected by humans in the universe were released by supernova explosions. The remnants (remains) of supernova explosions are also important radio sources, X-ray sources, and gamma-ray sources in the Milky Way, so they are also important objects for high-energy physics research. Physicists are also concerned about supernovae, because supernova explosions provide an experiment for nuclear fusion and high-energy particle interaction under extreme conditions, which are impossible to achieve on Earth.

  various kinds(hermes outlet)!

　　To understand a star, we must first look at its spectrum (disperse the energy radiation along the wavelength direction). Therefore, spectroscopy is also the main tool for studying the nature of supernova explosions. We can classify supernovae based on the spectral characteristics of supernovae near the maximum light maximum. The earliest classification is based on whether there are hydrogen lines in the supernova spectrum and supernovae are divided into types I and II: Type I supernovae have no hydrogen lines in their spectra; Type II supernovae have hydrogen lines.

Based on other spectral line characteristics, type I and type II supernovae can be further classified in a more detailed manner: In addition to the absence of hydrogen spectral lines, type Ia supernovae also exhibit the absorption characteristics of ionized silicon in the early stage and near the optical maximum. , There are significant W-type ionized sulfur characteristics near the optical maximum (the product of thermonuclear combustion); there are no obvious silicon lines in the spectrum of Ib supernovae, but strong helium lines can be seen in the early spectra; For type Ic supernovae, the characteristics of silicon and helium lines are not obvious.

The classification of 　　 Type II supernovae is more complicated and can be divided into IIP, IIL, IIb and IIn. Generally speaking, people classify type II supernovae into II-P type (with platform) and II-L type (without platform) when there is a "platform" in the descending stage of the light curve. Type IIb supernovae are considered to be an intermediate type between Type II and Type Ib. The predecessor star lost more hydrogen shell material before the explosion, so the hydrogen characteristics in the spectrum are in the period after the explosion. It disappears over time and the characteristics of helium gradually become stronger. Research on the historical archive objects before the supernova explosion shows that the precursor stars of the above different types of type II supernovae are generally red supergiant stars. From the type IIP-IIb, the mass of the precursor stars has a tendency to gradually increase.

In contrast, IIn-type supernovae are a brighter subclass, and their early spectra will show narrow or medium-width hydrogen and helium emission lines. This type of feature comes from supernovae exploding projectiles and nearby dense surrounding matter. Produced by interaction. The predecessor stars of this type of supernova are generally large (greater than 30-40 times the mass of the sun), and are generally considered to be bright blue variable stars. Due to their huge mass, this type of star will experience several violent explosions during the evolution process and cause a lot of material loss. The famous Eta Carinae (Eta Carinae) belongs to this type of star. It produced a violent explosion in 1837 and produced energy equivalent to that of an ordinary supernova (but the star is still alive). At its brightest, the magnitude is- Class 1 became the second brightest extrasolar celestial body observed on Earth at that time.

The different observational characteristics of supernovae indicate that the environment before they exploded and the explosion process are likely to be different. Stellar evolution studies have shown that for stars with a mass of more than 8-10 times the mass of the sun, their interior can undergo all nuclear reactions from hydrogen to iron. That is, when the star evolves to an advanced stage, the center will form an iron nucleus. Fusion reaction is an endothermic process, so the appearance of iron will herald the cessation of nuclear reactions inside the star. As the mass of the iron core continues to increase, when it exceeds the Chandrasekka mass limit, the electron degeneracy pressure provided by the core will not be able to support the star's own weight, so a dynamic nuclear collapse process begins.

The 　　 collapse process causes the star center to form a "neutron ball" composed of neutrons, and then the material falling from a larger radius will hit the surface of the neutron ball. Since the interior of the neutron star core can provide much stronger pressure than the degenerate electron gas, this "neutron ball" is almost incompressible in front of falling matter. This causes the falling outer material to hit the surface of the inner core to generate a rebound shock wave, carrying part of the energy of the neutrinos generated when the neutron ball is formed. The shock wave pushes the outer material away, and a supernova explosion occurs. .

　　From the perspective of explosion mechanism, this type of supernova is a nuclear collapse supernova. Theoretical studies have shown that not only type II supernovae, but also type Ib/Ic supernovae belong to this type of nuclear collapse supernovae. However, since Ib/Ic supernovae do not have hydrogen spectral lines in observations, their predecessor stars are likely to lose their hydrogen or helium envelopes through stellar wind or binary star material interactions. Such stars are called wo Wolf-Rayet stars usually have extremely high temperatures.

　　 Some supernovae have become "famous" because of detailed research. For example, a Type II supernova, SN 1987A, discovered in the Large Magellanic Cloud on February 23, 1987, has received considerable attention. This supernova is only 150,000 light-years away. The neutrino signal generated by its burst was successfully detected by ground-based detectors. This result won the 2002 Nobel Prize in Physics. SN 1987A is the brightest supernova recorded since the Kepler supernova (SN1604), and it can even be observed with the naked eye. It is also the first supernova to be observed and studied across the entire spectrum.

　　 Shortly after this "historical moment" occurred, the Hubble Space Telescope successfully launched and took a high-resolution image of the supernova, which provided tremendous help for people to study the evolution of supernova explosions. Using the image data before the explosion, it was discovered that the predecessor of SN 1987A was a blue supergiant star with a mass of about 20 times the sun. This changed the traditional understanding that type II supernovae only derive energy from red supergiant stars. An important feature of the explosion evolution of SN 1987A is that its projectile collides with surrounding matter at a high speed to evolve into a "tricyclic" nebula that rings stars (Figure 3). Inferring from the expansion speed of the rings, they should be formed at the same time. Currently, many theories try to explain the origin of these "rings". For example, stellar wind interacts with predecessor stars at different stages of evolution, binary star mergers, bipolar jets, and protostar disk interactions. However, the explanation of this phenomenon is still controversial, so it is still a very interesting research object.