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Author: Wang Qiru

Proofreading: Mufu Astronomy Proofreading Team

Post-production: Kutliavka Li Ziqi

The Parker Solar Probe was launched on August 12, 2018

Credit: NASA/Bill Ingalls

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Regardless Are you a senior astronomy enthusiast or someone who doesn’t know much about the starry sky? When you look up at the stars on a clear night, the first thing that catches your eye will definitely be the stars dancing in the dark sky. The stars above - they are either as bright as torches, or dim, or red or yellow - make the originally single and empty night sky colorful. Almost all of them are stars. It’s also interesting that stars, like life on earth, have their own lives: some of them have just been born, with immature cores sprouting in the hazy hotbed of molecular clusters; some are in their prime, emitting light outward through violent fusion reactions. some have entered middle age, they have begun to gain weight, and their size has become huge, but their energy has not diminished; while some have grown old, emitting dim and weak starlight, waiting for the end to come. The reincarnation of Under the standard cosmological framework of cold dark matter (ΛCDM), the structure of the universe is formed from the bottom up: due to the instability of gravity, the dark matter in the universe slowly gathers into increasingly larger dark haloes. Baryon matter collapses to form galaxies after cooling. Take the famous M31 (Andromeda Galaxy) as an example. Its interior contains a large amount of dark matter, which provides the most primitive and powerful driving force for the birth of stars. Its core is a. A dense and compact star cluster that contains the entire process from the birth of new stars to the death of old stars

M31, Andromeda Galaxy

Credit: printerset

Delivery Room - Giant Molecular Clusters

During the above-mentioned process of the collapse of baryonic matter to form galaxies, cold gas condenses into clusters (giant molecular clouds) on a small scale, and stars are born in them. The process of star formation from molecular gas will be affected. The influence of factors such as the temperature and density of molecular gas, the metallicity of galaxies, and the internal structure of galaxies; at the same time, a large amount of theoretical and observational work has found that the interaction between galaxies can collapse the gas on the galactic disk and promote the growth of stars. form. Here we take M42 (the Orion Nebula) as an example. M42 is a very young object with a large number of young stars inside it, as well as a lot of prestellar objects. It has to be said that M42 is one of the most suitable delivery rooms for star birth in the universe.

M42, Orion Nebula

Credit: Messier Club

Stellar "embryos" - protostars

The above-mentioned giant molecular clusters It will be affected by external forces such as shock waves generated by the explosion of nearby stars, thereby breaking the balance of pressure and gravity in its clouds, and the clouds will begin to shrink; or as charged particles slowly drift through the magnetic field lines that limit them, supporting the gas The magnetic field begins to weaken, causing the giant molecular clusters to begin to shrink. When collapse begins, the gas cloud naturally breaks apart into smaller and smaller clumps of material due to the ongoing effects of gravitational instability. In this process, fragments with a total mass of matter between 0.25 and 8 times the mass of the sun will eventually form sun-like main sequence stars; fragments with a total mass of matter between 0.08 and 0.25 times the mass of the sun will eventually form Low-mass dwarfs; fragments with a total mass of less than 0.08 times the mass of the sun will eventually form extremely low-mass brown dwarfs; fragments with a total mass of more than 8 times the mass of the sun will eventually form a massive giant or supergiant. As the cloud fragments continue to shrink, their density increases, eventually making it difficult for photons to escape. The trapped radiation then causes the cloud to rise in temperature and pressure, and eventually the fragmentation will stop. At this point, the fragments of the giant gas cloud began to look like stars. The dense, opaque central region is called a protostar. Here we take the T Tauri star (T Tauri) as an example. It was discovered near the NGC 1555 molecular cloud. It has a large radius and extremely low temperature, so that it is not enough to trigger hydrogen fusion. Using the gravitational energy generated by the contraction to move toward the main sequence, it will become a main sequence star in about 100 million years.

T Tauri star

Credit: Wikipedia

Formation of Sun-like stars

Protostars are born from the fragmentation of shrinking gas clouds In the block, although the "embryo" form of the star in its early stages of birth has been formed, the surrounding fragments are still shrinking and fragmenting. The outer material is collapsing inward more and more violently. The mass of the protostar continues to increase, and the radius is under the influence of gravity. keeps decreasing. As the cloud fragments shrink, their rotation speed continues to increase and become flattened, eventually evolving into a rotating protostar disk with a diameter of about 100 AU (1 AU is the average distance between the sun and the earth), surrounding the protostar run. The heat inside the protostar gradually diffuses from the hot center to the cooler surface, and is radiated from the surface into the surrounding space. The effect of this is that the overall speed of contraction is decreasing, the surface temperature of the protostar remains almost unchanged, and the luminosity decreases with the contraction. This stage of evolution typically exhibits intense surface activity, producing extremely violent protostellar winds that are much denser than the solar wind flowing from the Sun. Finally, when the mass of the protostar becomes 0.25~0.8 times the mass of the sun and the radius shrinks to one million kilometers, the temperature at the center of the protostar reaches 10 million Kelvin, which is enough to trigger a nuclear reaction. The protons in the core begin to fuse into helium nuclei. A sun-like star is born. Let’s take Alpha Centauri A (Alpha Centauri) as an example. It is a sun-like star with a mass similar to that of the sun and is located on the main sequence.

Comparison of Alpha Centauri and the Sun

Credit: Wikipedia

Formation of low-mass stars

For masses between 0.08~0.25 The formation process of stars with twice the mass of the sun is similar, except that the protostars of low-mass stars are formed from the aggregation of small-mass gas cloud fragments. Since the time it takes for an interstellar cloud to form a main-sequence star depends on its mass, for prestellar objects with a mass less than the mass of the Sun, it would take nearly 1 billion years to form a low-mass star. Proxima Centauri is the third star of the above-mentioned Alpha Centauri triple star. It is a red dwarf star with a mass of 0.12 times that of the sun and is 4.22 light years away from the earth.

Proxima Centauri

Credit: universe sandbox

"Failed" star - brown dwarf

According to the basic theoretical model, it is necessary to For the core temperature to be high enough to ignite nuclear combustion, the minimum mass of gas required is 0.08 solar masses (80 Jupiter masses). In giant molecular clusters, there are always some small-mass gas fragments that are difficult to reach the lower mass limit required to ignite nuclear combustion. They are not transformed into stars, but are further cooled and eventually become dense, dark "slag chunks" - - Cold fragments of unburned material - they orbit stars or wander in interstellar space. This type of object is named "brown dwarf". The lower mass limit of a brown dwarf is 13 times the mass of Jupiter. Above this lower limit, deuterium fusion reactions can occur in the core of the star. The resulting energy can be temporarily used to resist further collapse of the star, but the deuterium will soon be consumed. If the mass of the star exceeds 60 times the mass of Jupiter, the core temperature after collapse can cause the core to undergo a reaction in which lithium nuclei and protons fuse to produce helium nuclei (lithium burning). Similarly, the original small amount of lithium in the star will soon be exhausted. Brown dwarfs approaching the mass limit (80 times the mass of Jupiter) may ignite hydrogen in their cores, but because their own gravity is still not strong enough, the energy generated by hydrogen burning is "splashed" away, causing the core temperature to drop and the hydrogen burning to be quickly extinguished. .

Formation of massive stars

Now we focus on massive stars that are more than 8 times the mass of the sun. It is known that all stars evolve from protostars, and giant molecular clouds are the birthplaces of protostars. The more massive fragments in interstellar clouds tend to produce more massive protostars, and eventually form more massive stars. . No matter how massive it is, the protostar’s foothold cannot escape the main sequence range. Antares (α Scotpii) is a red supergiant star on the main sequence. Its mass is 12.4 times that of the sun. It is 500 light-years away from the earth and its radius is 680~800 times that of the sun. It is the tenth largest star in the entire sky. Five bright stars, it and Mars are the two reddest celestial bodies in the sky.

The proportional relationship between Antares and the Sun

A star is born! The sun nurtures all things on earth, and the stars decorate the night sky!

Reference materials:

[1] Wang Youfen; Shao Zhengyi. Observational characteristics and search of brown dwarfs. Progress in Astronomy. 2013 (01): 19-38

< p> [2] Wang Hongyan. Massive neutron stars can contain hyperons. Journal of Jilin University (Science Edition). 2020 (03): 236-240

[3] Xu Lanping. Post-main sequence evolution of stars . Progress in Astronomy. 1989 (04): 50-58

[4] Gao Yang; Xiao Ting. Research progress on molecular gas and star formation in galaxies. Progress in Astronomy. 2020 (02): 4- 21