The formation of a star

 by

      Spoorthi .S


The first place a star’s life cycle begins is in the molecular cloud. Stars are formed as a result of some gravitational imbalance in a molecular cloud. These molecular clouds are made of relatively high concentration of interstellar gas and dust. These molecular clouds are very cold and have an internal temperature of around 7 – 15 K.  At these temperatures, gases start to clump together. When the density reaches a certain point, a star is formed.

Sometimes, shock waves from a supernova can hit a molecular cloud and compress it. This can provide some help. Even the rotation of the milky way can cause gas to pile up therefore speeding up the process.

The denser parts in the core of the cloud collapse under their own gravity. A protostar now forms at this core. These pre main sequence stars are often surrounded by a proto-planetary disk.

Once the star is a Hydrogen burning furnace, a stellar wind forms. Very young stars have a flow of gas from its poles. These kinds of stars are the T-Tauri stars. In this T-Tauri phase, a star loses around half of its mass and turn into main sequence stars. T Tauri stars are protostars with a mass of 3 solar masses or less

Protostars are large.  Around 20 times the diameter of the Sun

Due to this, they have a very high luminosity. These are pre main sequence stars and have around 100 times the luminosity they have when they are a main sequence one! Now many of you might be familiar with the Hertsprung Russell diagram. This shows the relation between a star’s temperature and luminosity. These protostars are now giving away or a better word would be, radiating energy. Due to this, the star’s gravitational collapse is pulling it inward. This is going to affect the luminosity of the star . So, it takes a drop in the HR diagram though its temperature is pretty high. Flash forward, the temperature of the star is around 10 million K now. This starts nuclear fusion in its core. Just to make things clear, in nuclear fusion, two nuclei collide to form a heavier nucleus. At the beginning, the energy released by this fusion reaction is not enough to balance the star’s gravity, but eventually, as the core’s temperature gets higher, the rate of fusion too increases and the two forces in the star balance each other out to form a thermal /hydrostatic equilibrium which is what tells us that a star is a main sequence one. This whole process of turning a protostar / a pre main sequence star to a main sequence star takes a few million years.

The environment in which a star forms also affects its outcomes. Example -gas, which does not contain metals will be less able to cool as its density increases making it harder to form stars

The first stars to form after the big bang would not contain any metals and as a result may have a different mass distribution.

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