Abstract
In dense interstellar clouds or in the surroundings of just-formed stars, the larger grains form proto-cores by segregating from the gas under the influence of the gravitational field of the cloud during intervals of the order 108 yr. If the mass of the proto-cores is smaller than a certain limit, the object possesses a negligible atmosphere and remains composed mainly of solid grain material. If the mass of the proto-core exceeds the limiting mass, the proto-core can bind around it an atmosphere. When the temperature of the opaque part of the atmosphere is only fractions of a degree above the temperature of the surrounding tenuous gas, gravitational contraction of the opaque part of the atmosphere sets in, forming gaseous objects up to stellar mass range. Binary and multiple systems originate from neighbouring proto-cores, through gravitational contraction of both their separate and common outer atmospheres. Pycnonuclear reactions are not able to prevent a star with mass ≲0.08M ⊙ from cooling to the black dwarf stadium.
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