Formation of Stars in the Interstellar Medium

The dark places between the stars or interstellar space are not empty spaces but filled with interstellar medium which is composed of gas and dust. Gas and dust are the raw materials from which stars are formed. The most abundant element in the interstellar medium is hydrogen with approximately 90% of gas in the cloud. Helium is the next most abundant component with another 9% of the gas, and the remainder 1% is composed of other elements. The material contained in the interstellar molecular clouds is the material from which stars are formed.

Composition of the interstellar cloud

Giant molecular clouds are big clusters of cold, ranging from 10-100 K, gas most of which consists of molecular hydrogen, a small percentage of helium and traces of other elements. Higher density interstellar medium forms giant molecular clouds where stars are formed. These giant molecular clouds may have densities of a hundred particles per cubic centimeter and diameters of one hundred light years with masses of more than six million solar masses. The Orion Nebula contains interstellar clouds of ionized hydrogen caused by the radiation of young hot stars, and is the nearest place to the solar system where massive stars are being formed.

Gravity and pressure

A giant molecular cloud may remain undisturbed as its two opposing forces, gravity pressing matter inward and the kinetic energy of gas pressure pushing outward, are in balance or maintain hydrostatic equilibrium.  When the gas pressure is insufficient to maintain hydrostatic equilibrium, the cloud will collapse under its own weight. The mass that a molecular cloud must attain to undergo gravitational collapse must be of tens to thousands of solar masses. The collapse may also be triggered by a number of interstellar events.

Cloud collapse

A giant molecular cloud may undergo one of several stellar events so as to initiate its gravitational collapse. A molecular cloud could collide with another molecular cloud, causing its collapse; the explosion of a supernova can produce a powerful expanding shock wave which can reach a molecular cloud and trigger its collapse; or two galaxies may collide to one another producing massive star formations as the gases in each galaxy collide, producing star formations. Once the cloud begins to collapse, it breaks into hundreds or even thousands of fragments which will be the new stars of distinct masses, depending on the size of the fragment.

Over millions of years, a fragment of collapsing cloud material contracts, radiating gravitational energy which escapes into outer space. At the center of the collapsing cloud of gas and dust, the densities are higher, and more radiating energy is trapped, raising the temperature. As the cloud fragment continues to contract, photons are unable to move out of the cloud and temperature is increased even higher. Due to contraction, the fragment has now become into a spinning sphere of gas and dust of what will become a star.


The star continues to contract and at the same time increasing in temperature, although not sufficiently hot to start nuclear fusion reactions in its core. Over ten million years, the protostar´s core temperature increases from one million to five million K. At some point in the star formation jets of outflowing material known as Herbig-Haro objects appear on the poles of the star. Once the temperature in the core of the star reaches ten million K, the star begins to fuse hydrogen into helium and hydrostatic equilibrium stops further collapse of the star, as the two forces, gravity and heat pressure, acting on the star are balanced.

When a fragment of interstellar gas and dust fails to accrue sufficient mass, its core temperature will not rise sufficiently to start nuclear fusion and will usually become a dwarf star. Astronomers can analyze the content of clouds of interstellar gas by observing their spectral absorption lines. According to, 90% of interestellar matter is composed mainly of hydrogen, and 10% of helium. One out of one thousand particles in the interstellar cloud contains other elements, such as nitrogen, carbon and oxygen, and the dust particles in the cloud contain silicates and graphite and are often surrounded by a layer of ice.