Star formation

The dense region in the molecular cloud collapses to form stars

Stars are born in molecular clouds. The dense region in the molecular cloud collapses to form stars. Infrared and radio telescope With its wavelength advantage, it can detect the internal structure of the molecular cloud, especially the Spectral line The observation can detect the dynamic process inside the molecular cloud, which is very helpful for us to understand the process of star formation.

By the end of 2019, for small quality and Medium mass star The formation process of High quality The process of star formation is still very controversial.

Chinese name: Star formation
Foreign name: the formation of star
Alias: Star birth
Material source: Nebula

Reaction: Fusion reaction
Basic conditions: hydrogen 、 gravitation And a long time
Formation stage: Diffuse cloud - dense cloud core - pre stellar cloud core - protostar - main sequence star (or dwarf star, planet)

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one Star forming region
two probe
three Star formation with different masses

Star forming region

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Orion Nebula

In the dense star producing nebulae, most hydrogen exists in the form of molecules, so these nebulae are called molecular clouds. Observations show that the coldest molecular clouds tend to form low mass stars, first in the infrared And then observed on the surface of the cloud layer when it dissipates visible light And usually warmer Giant molecular cloud Produce stars of various masses. The typical density of these giant molecular clouds is 100 particles per cubic centimeter, the diameter is 100 light years, and the mass is up to 6 million Solar mass The average internal temperature is 10 K.

Molecular cloud mass accounts for Galaxy Half of the total mass. In the Milky Way, there are an estimated 6000 molecular clouds, each with a mass of more than 100000 solar masses. The closest nebula to the sun that is forming massive stars is Orion Nebula , 1300 light years away from the sun, the formation of low mass stars occurs about 400-450 light years away from ρ Ophiuchi Cloud system Medium.

A denser place for star formation is composed of dense gas and dust Opaque Clouds (dark clouds), called Bok globule , named after astronomer Bartlebock. These clouds can be formed together with collapsed molecular clouds or independently^[1] 。 Bok globules usually have a diameter of one light year and a mass of several solar masses. They are easily observed in contrast to bright nebulae or background stars. More than half of the known Bok globules have been found to contain newly formed stars.

Formation of massive stars (more than 8 solar masses) Protostar Will have a strong Ultraviolet radiation These ultraviolet photons will ionize Peristellar matter （ Photodissociation ）。 Because most of the molecular clouds are hydrogen atom And hydrogen molecules, so this ionized region is also called Ionized hydrogen region 。

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Tricolor diagram of the universe observed by SDSS

Since stars are born in dense regions in molecular clouds, they can only be used visible light Observation of external wavelengths. 1997 2 micron all day sky survey （The two Micro All Sky Survey；2MASS^[2] ）Is the earliest large scale Near infrared Celestial survey , revealing the near infrared image of the universe. However, even infrared observation has encountered difficulties, atmosphere It is almost completely opaque from 20 μ m to 850 μ m, and there are narrow windows only in 200 μ m and 450 μ m. Must be used outside this range extinction Technical observation. Well known extinction observations include Digital Sky Survey Digital Sky Survey I; DSS^[3] ）And Sloan Digital Sky Survey The Sloan Digital Sky Survey; SDSS^[4] ）。

Dynamic Structure of Intragalactic Molecular Clouds Detected by CO

In addition to near-infrared extinction, dust continuum radiation and molecular rotational transition radiation can also directly detect the internal structure of molecular clouds, especially molecular transitions Radiation spectrum , can accurately measure the kinematics Features（ Gravitational collapse , outward flow, etc.). The dynamics of molecular clouds in the Milky Way can refer to the CO molecular survey of Columbia University^[5] 。

Star formation with different masses

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Depending on the mass of the star, its formation mechanism is slightly different. The theory of low mass star formation is strongly supported by observations, that is, low mass stars are increased by the rotational density in the molecular cloud Gravitational collapse Formed. Collapse of a rotating cloud composed of gas and dust Accretion disk And the matter is guided to the center through the accretion disk Protostar On. To maintain Conservation of angular momentum There will be material outflow at the poles of the vertical disk axis of the protostar. The formation mechanism of massive stars is still unknown.

Massive stars release a lot of radiation These radiation will produce Reaction force 。 In the past, it was thought that the radiation pressure might be large enough to prevent high mass Protostar And prevent the formation of stars whose masses exceed dozens of solar masses. Recent theoretical research shows that, Jet And outflow cleared a cavity through which most of the radiation of the massive protostar can escape without hindering the accretion through the disk and protostar^[6] 。 High mass stars may therefore be able to form through a mechanism similar to that of low mass stars. More and more evidence shows that at least some massive protostars are indeed surrounded by accretion disks. Other theories of massive star formation still need to be observed and tested. Among them, perhaps the most prominent is competition Accretion theory It shows that the massive protostars are "seeded" by the low-quality protostars, which compete with other protostars and accumulate material from the whole parent molecular cloud, rather than just from a small local area^[7-8] 。

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