How stars are formed ?

Stars are born from compact knots within dark molecular clouds that are refrigerated by dust that blocks heating starlight. If the random knots, compressed by supernovae or other means, are dense enough, they can contract under their own gravity.

Conservation of angular momentum demands that as they collapse they must spin faster. Star formation requires that angular momentum be removed such that the new “young stellar objects” do not completely tear themselves apart before they can become fully developing protostars. High-speed particles (cosmic rays) from exploding stars partially ionize the dusty knots.

The ions grab onto the weak magnetic field of the Galaxy and, as a result of their physical interaction with neutral atoms and molecules, provide the initial means to slow the rotation. If the rotation is still too fast, the contracting body may split into a double (or more complex) star, though the origins of doubles are not clearly solved. A contracting protostar still indeed rotates progressively faster until the part of its mass not accreted by the star itself is spun out into a dusty disk, from which planets might later accumulate. From the disk shoot powerful molecular flows that slow the star still more .

When the protostar’s interior reaches about 106 K (1.8 × 106 ◦F), it can fuse its internal deuterium. That and convection, which brings in fresh deuterium from outside the nuclear-burning zone, bring some stability, and a star can now be said to be born. Stars like the Sun shrink at constant temperature until deuterium fusion dies down. Then they heat at roughly constant luminosity until the full proton-proton chain begins, which provides the stars’ luminosity and stops the contraction. The stars settle onto the zero-age main sequence (from which they will later evolve). At the same time, the surrounding dusty cloud is clearing, allowing new, accreting, and highly active T Tauri stars to be seen flocking around their birthclouds . The whole process takes only 10 or so million years, the mature stars then leaving their birthplaces, destined both to age and to orbit the Galaxy. High-mass stars proceed similarly, but at such a great pace that the death process begins even as the birth process is ending.


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