The universe has confounded mankind since ancient times. Early man would look up at the night sky and try to decipher the various shapes and figures created by groups of stars clustered together. Of course, now we know these as constellations of stars in our Milky Way galaxy and other galaxies that exist deep in space.
There are literally billions of stars in the universe. But, how did all these stars come to be formed? This question was uppermost in the minds of scientists and astronomers, until eminent scientist Edward Hubble came up with the Big Bang Theory in the early twentieth century. The Big Bang was a cosmic explosion that resulted in an expanding cloud of the two lightest elements, the gases Hydrogen and Helium.
From then on, scientists have rapidly discovered more and more secrets of the stars- their evolution, life and finally death.
So, how exactly are stars formed? Let’s take a look-
Birth of a Star–
The universe consists of millions of galaxies- spiral galaxies, and elliptical galaxies. Our Milky Way is a spiral galaxy. Some of the space in spiral galaxies is taken up by stars and their solar systems. The rest of the space consists of highly dense interstellar medium- dense material that is made up of gas and dust.
In other words, interstellar medium is what space is made of.
The interstellar medium (ISM) is composed of 70 % Hydrogen, while the remaining 30 % is made up of Helium and traces of heavy metals. The Interstellar medium is not of equal density throughout. It is marked by regions of higher densities, and regions of lower densities.
The higher density regions converge to form giant molecular clouds which are composed mainly of Hydrogen. These clouds are so huge and dense, that they can typically measure 100 particles per 〖cm〗^3 in density, 100 light-years (9.5×〖10〗^14 km) in diameter and 6 million times the mass of our sun.
Because the Hydrogen present in these clouds is in the molecular form (H_2), they are known as molecular clouds. These clouds, also called nebulae, are cold and dark. More than half the mass of the ISM of a galaxy is made up of these nebulae. Scientists estimate that the Milky Way itself contains about 6000 such nebulae.
All stars are formed in these nebulae. Cooler nebulae generally give birth to stars of lower masses, while warmer nebulae give rise to stars of all masses, giant ones, as well as medium sized ones. The molecular clouds have an internal gravitational force that binds the gases to it. The kinetic energy of the gases is balanced by the potential energy of this internal gravitational force of the cloud.
Sometimes, however, this balance is disturbed by events such as galactic explosions, supernova explosions, collision with another passing molecular cloud etc. At such time, the gravitational force of the cloud exerts a force on its contents, pulling the contents inside, triggering something called a gravitational collapse. This gravitational collapse compresses the cloud, resulting in an increase in its density as well as temperature. The centre of the cloud becomes the most compressed region. The gravity of the cloud pulls more and more hydrogen atoms to fall into the centre of the cloud. The temperature gradually rises and as it reaches 2000 K, the centre becomes hotter and denser and forms an opaque core.
Gas is still being gravitationally pulled towards the centre, and as the gas molecules fall into the core, they collide with it and send shock waves, which further heats up the core.
At some point, the temperature reaches 10 million degrees Celsius, triggering nuclear fusion in the cloud, now called a Protostar. Hydrogen molecules are converted to Helium, along with the release of vast amounts of energy. This energy makes the protostar turn into a glowing ball of fire, and a new star is born.