Did you know that everything around you including you was made in a star? Originally only Hydrogen and Helium were there in our universe. All other elements were formed by the fusion of these two elements in a star. Hence, the life cycle of stars becomes very important to understand. Especially how stars are formed which is the first step in their life cycle.
The dusty nurseries of stars
A galaxy not only consists of billions of stars but has dust and gases in between the stars. If the galaxy was a neighborhood then the houses would be the stars and the roads in between them would be gases and dust.
The space between the stars is known as the interstellar medium. This interstellar medium consists mainly of hydrogen (about 75%) and some amount of helium (about 24%) as well. The remaining consists of traces of other elements released during the death of a star.
The places in the interstellar medium where the density is high form clouds. These clouds are called nebulae. Star formation takes place in a nebula. In these nebulae, hydrogen is present as a molecule. Hence these nebulae are also called molecular clouds.
These gigantic molecular clouds have typical diameters of 100 light-years (9.5x10^14 km), a mass of up to six million Suns (1.2x10^37 kg), and an average interior temperature of 10K (-263°C).
Eagle Nebula captured by the Hubble
The collapse of the molecular cloud
The gas and dust in these nebulae aren't stationary, they keep moving. Hence, the mass and density are different in different places. When the mass of a certain region reaches a value called jeans mass then that region begins to collapse under its gravity.
There is a law in physics, that tells us that when matter is compressed the density increases, and the temperature increases. The observations in a collapsing region are in accordance with this law. The temperature of the core slowly increases to room temperature (27°C).
When the collapsing region has reached a size of nearly 10,000 AU (1.5x10^15 m), it is called a pre-stellar core. Pre-stellar means, “before becoming a star.” Core refers to the fact that this region will become the central region of the new star.
For the next 50,000 years, the pre-stellar core keeps reducing in size. That may sound like an eternity but on an astronomical scale, it is a quick process compared, for instance, to the age of the sun which is around 5 billion years.
This pre-stellar core keeps contracting until it is about 1000AU (1.5x10^14 m). The amount of gas and dust is still the same so the density is increasing as the size shrinks to about 1/10th of the original size of the collapsing region.
50,000 years later, a disk-like structure will be seen around the central core, and the excess materials will be ejected from the poles of the star as seen in the picture below.
The random motion of the gas and dust that we described earlier, combined with the system's contraction as the pre-stellar core forms, will cause the whole system to rotate.
This process causes a flat disk to form around the pre-stellar core. This is similar to the way a dress forms a flat disk around a spinning figure skater. If the skater wasn't spinning then the dress would hang along her sides and wouldn't for a flat disk.
This system is now known as a proto-star which is like a baby. It has to develop more to become an actual star.
Protostar to star
The disk around the proto-star is mainly composed of gas, which rotates with the proto-star and slowly approaches the surface of the proto-star. This disk is vital for the proto-star to grow to a proper size. When the gas of the disk comes close to the star, it gets pulled towards the surface of the proto-star because of its gravity, and the proto-star grows.
The proto-star is said to accrete or accumulate mass from the disk.
Over the next 800-1200 years, the matter from the disk is accreted by the star. As the proto-star keeps increasing in mass the temperature also rises. After a while, the star has acquired a high enough temperature for the central region to initiate a nuclear reaction, which causes the star to shine, like the Sun. Now the system is known as a T-Tauri star, and this is the first time that the star can be observed visually as it is shining.
The core eventually stops accumulating matter from the disk, but the remaining material around the star is still in a disk-like shape. The disk no longer feeds the star.
The disk is now just a rotating plane of matter. The matter slowly aggregates slowly into spheres. These spheres orbit around the star in the plane of the disk. These small spheres, made from left-over material from the star's creation, will form new planets.
The final system is finished when the disk is completely exhausted, and all the planets are formed. The Giant shining ball of plasma held by its own gravity in the center of the system is what we call a star.
Our star - The sun (Sol)
The process described above is how stars of low masses are formed. The mystery lies in the formation of larger stars.
Scientists have observed large stars, so we know that large stars exist. Since they exist we also know that they should have been formed in some way. But, how it happens is still a big question for people all over the world.