Star Formation

By Author: Pierce Brosnan
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Introduction on star formation:

The process of star formation involves collapse of dense molecular clouds into a denser ball of plasma to form a star. Star Formation as a subject includes a study of interstellar medium and giant molecular clouds that precede star formation along with a study of young stellar objects including planets of stars.

Precursors to Star Formation

Empty Space, Interstellar Clouds and Cloud Collapse

Typically the space between interstellar objects, both within galaxies like our Milky Way and between galaxies situated far apart, is not an absolute void or vacuum and contains a diffused interstellar medium (ISM) of gas and dust. ISM has a very low density and about one hundred thousand to one million particles per cubic meter. Its composition by mass is approximately 70% hydrogen and the rest being made up mainly by helium with traces of heavier molecules. Higher density parts of ISM form interstellar clouds whose collapse leads to formation of stars.

Interstellar clouds contain a major part of Hydrogen in the molecular form and are hence referred to as molecular ...
... clouds too. Dense giant molecular clouds can often have densities of 100 million particles per cubic meter with very large diameters of 100 light-years (a million trillion km) and a total mass of up to a million times that of our Sun. The process of cloud collapse leads to a rise in temperature.

This internal cloud of gas remains in a stable equilibrium with the two forces of gravitational attraction and kinetic energy of particles working against each other. When the cloud gets sufficiently large and massive and the forces of gravity overcome the kinetic energy, then the process of cloud collapse begins. This may happen on its own or sometimes may be triggered by other stellar events such as collision of molecular clouds, a nearby supernova explosion and galactic collisions. Sometimes, an extremely heavy black hole at the core of a galaxy may also play a role in triggering or preventing star formation.

During the process of collapse Interstellar Cloud breaks into smaller pieces until its fragments reach stellar mass with each fragment radiating energy gained by the release of gravitational potential energy. The process of collapse leads to an increase in density restricting energy radiation and causing a rise in the temperature of the cloud. Rising gravitational force also acts to limit further fragmentation leading to formation of rotating spheres of gas called stellar embryos.

History of Protostar:

A wide range of forces caused by turbulence, spin, magnetic fields formed due to spinning and macroscopic flows come into play and are affected by and also affect the cloud geometry. These influences can hinder or accelerate the process of collapse. If the process of collapse continues the dust within the cloud becomes heated leading to a rise in temperature to around 60,000 –100,000 degrees Celsius with its particles emitting radiations of far infrared wavelengths promoting further collapse of the cloud and rise of temperature in the core.

Rising core temperature and declining density of the surrounding gases create conditions congenial to let the energy escape. This allows the core temperature to rise further causing dissociation of hydrogen molecules. Resulting ionization of hydrogen and helium atoms absorbs energy of contraction. The process of collapse continues until a new equilibrium is reached between the internal pressure of hot gases and gravitational forces. The object so formed is called a protostar.

Star Formation

Protostar continues to grow by attracting material and finally when the conditions are just right the process of fusion begins. Resulting radiation further slows the process of collapse. Finally the surrounding gas and dust envelope is eliminated through absorption into protostar or dispersal and further accretion of mass stops though the process of collapse continues.

At this stage the main source of energy continues to be gravitational contraction and the object is called a pre-main sequence (PMS) star. Further collapse stops at a point and fusion process begins in the core replacing gravity as the main source of energy. The object then begins a main sequence star. Further life cycle of the star thus formed depends on its size.

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