Birth of Stars

Space is an incredible field to research. From child to adult, everyone loves to explore Space. You can see so many stars at the night sky. Have you ever thought that where were the stars born? It is very dramatic to say that stars are made from the dust. Amazingly, it is true! Much of our bodies, and our planet, are made of elements that were created in the explosions of massive stars. 

Gigantic, turbulent clouds of gas and dust where tens of thousands of stars are bursting to life. The star birth is a violent process, producing intense ultraviolet radiation and shock fronts. The radiation clears out cavities in stellar nursery clouds and erodes material from giant gas pillars that are incubators for fledging stars. Energetic jets of glowing gas from young stars are a byproduct of gas swirling into newly forming stars, some of which gets channeled by magnetic fields and shot from the poles of the spinning stars at supersonic speeds in opposing directions. 

Stars are born inside great clouds of gas and dust called nebulas. Nebula starts to give birth to stars. The process begins when a nebula starts to shrink, then divides into smaller, swirling clumps. Each clump becomes ball-shaped, and as it continues to shrink the material in it gets hotter and hotter. When the temperature in the core reaches about 18 million oF (10 million oC), massive explosions called nuclear fusion begin and a new star begins to shine. Around 90 percent of a star’s life is spent in the main sequence phase – the middle age of a star. The bright shining from the center of the dust disk signals the birth of a new star. The baby star is surrounded by a disk of dust and gas. The disk has a mass like that of the young star. Stars do not live forever. After billions of years, they burn out, and can end their lives in several different ways. 

Like animals, star also has life cycle. A star’s life cycle is determined by its mass. The larger its mass, the shorter its life cycle. A star’s mass is determined by the amount of matter that is available in its nebula. Over time, the hydrogen gas in the nebula is pulled together by gravity and it begins to spin. As the gas spins faster, it heats up and becomes as a protostar. Eventually, the temperature reaches 15, 000, 000 degrees and nuclear fusion occurs in the cloud’s core. The cloud begins to glow brightly, contracts a little, and becomes stable. It is now a main sequence star and will remain in this stage, shining for millions to billions of years to come. This is the stage our Sun is right now. 

As the main sequence star glows, hydrogen in its core is converted into helium by nuclear fusion. When the hydrogen supply in the core begins to run out, and the star is no longer generating heat by nuclear fusion, the core becomes unstable and contracts. The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and glows red. The star has now reached the red giant phase. It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward. In the core of the red giant, helium fuses into carbon. All stars evolve the same way up to the red giant phase. The amount of mass a star has determines which of the following life cycle paths it will take from there. 

Figure: The life cycle of a low mass star (left oval) and a high mass star (right oval)

The illustration above compares the different evolutionary paths low-mass stars and high-mass stars take after the red giant phase. For low-mass stars, after the helium has fused into carbon, the core collapses again. As the core collapses, the outer layers of the star are expelled. A planetary nebula is formed by the outer layers. The core remains as a white dwarf and eventually cools to become a black dwarf. 

On the right of the illustration is the life cycle of a massive star (10 times or more the size of our Sun). like low-mass stars, high-mass stars are born in nebulae and evolve and live in the main sequence. However, their life cycles start to differ after the red giant phase. A massive star will undergo a supernova explosion. If the remnant of the explosion is 1.4 to about 3 times as massive as our Sun, it will become a neutron star. The core of a massive star that has more than roughly 3 times the mass of our Sun after the explosion will do something quite different. The force of gravity overcomes the nuclear forces which keep protons and neutrons from combining. The core is thus swallowed by its own gravity. It has now become a black hole which readily attracts any matter and energy that comes near it.