Star formation: A Brief Introduction Stars like our Sun also have their "Life" like us. Stars are born in the cloud of interstellar gas and dust. The dense part of interstellar gas cloud is called "molecular cloud" because the major gas content, hydrogen, is in the molecular form (H2). The molecular clouds have typical densities of 100~1000 particles per cm3, which is almost "vacuum" in our daily life, but is not totally empty. The temperature inside the molecular cloud is very low; it is only 10 K (-263 °C) on average. In the images of visible light, molecular clouds are identified as "dark clouds" because the light from the stars behind the clouds is obscured by dust particles in the clouds. Since star formation proceeds inside of dark clouds, observations with visible light are not suitable; instead, longer wavelengths such as infrared, submillimeter, and millimeter play important roles in studying the star forming regions. The Figure illustrates how stars like our Sun are formed. First, density fluctuation in the molecular cloud grows by means of self-gravity (panels a and b). Usually, the gravitational collapse does not go spherically symmetrically. As described in the "low-mass star formation section", forming star is surrounded by the rotating and flattened structure, and ejecting the gas to the polar direction (panels c and d). In these stages, the newly born star grows by acquiring mass from the accretion disk. When the mass accretion stops, the central star starts to contract by means of its own gravity, and raises its temperature. The disk surrounding the star also changes its structure and starts forming planets (panel e). Finally, when the temperature and density of the star becomes high enough, the star ignites the nuclear fusion and starts its main-sequence life (panel f).
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If the star does not acquire more than 0.08 Msun, its temperature and density cannot be high enough to ignite hydrogen burning. Such a very low-mass star becomes "brown dwarf". If is still unknown whether all brown dwarfs are formed in the same process as the low-mass stars. Formation of massive stars (larger than 8 Msun) are also a matter of issue. Recent observational results have shown that some of the massive stars with spectral type of B are formed through mass accretion, which is similar to the process of forming low-mass stars. However, it is an open question how to form more massive O-type stars. (Author/Naomi Hirano)
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