Written by: Aditya Gupta - 25027, Grade XI
Posted on: 23 February, 2024
Stars are giant cosmic objects, the glowing balls of gas. They are primarily the mixture of hydrogen and helium held together by the force of their own gravity. The reason behind their shine is the thermonuclear fusion reaction (hydrogen fusion) in their cores that keep stars bright. Whether or not the stars die depends on how we think of 'die'. Stars don't cease to exist in the way that living things do. Instead, they undergo a complex transformation when their fuel runs out. We should imagine it like a caterpillar morphing into a butterfly, but on a cosmic scale.
The entire process is finite due to the limited storage of hydrogen fuel in them. And when a star exhausts all of its nuclear fuel, its fate (end) takes a different turn. The fate of the stars undergoes some complex transformations and gets ended into a cosmic event known as supernovae, resulting in neutron stars or black holes in the case of giant stars, or in the case of smaller stars, a serene white dwarf.
In the case of smaller stars like the sun, they transform into a phase called “Red Giant” when those stars burn all of their hydrogen to helium. This process takes about 10 billion years. After the small stars get changed into a Red Giant, they become bigger and more denser than they are today. At this time it will start burning helium to carbon for a few hundred million years until it runs out of helium. And since it will not be dense enough to form other heavier elements like iron, the fusion process will stop. This will make the star collapse into its core due to inward acting gravity as there will be no fusion energy to stabilize its gravity. Then it will calmly shed its outer layers into space called a “Planetary nebula”. The white dwarfs are formed, an approximately earth-sized inner core. Other white dwarfs in binary systems may explode as novae. But they do not go on to form a neutron star or a black hole like other big stars do.
In case of larger stars, they experience more complex transformations and endings. When their fuel is all used, a “Red Supergiant” star is formed. The biggest stars in the universe are the red supergiant stars, about 1800 times the size of the Sun. Red supergiants are similar to red giants. They form when stars run out of hydrogen fuel in their core. After using up its hydrogen fuel, it becomes a red supergiant. Due to its higher mass when the core collapses, the temperature rises quickly, which speeds up the helium fusion reaction. This fast fusion destabilizes the star. A massive amount of energy pushes the outer layers of the star outward, and turns it into a red supergiant. At this point, the star's pull towards its center is balanced by the strong push outward caused by the intense helium fusion reaction happening in its core. However, the star loses a significant amount of its mass into space. So, even though Red Supergiants are some of the largest stars, they aren't the most massive. Then the Red Supergiants exhausts the helium in their cores within one or two million years and then starts to burn carbon. This continues with fusion of heavier elements until an iron core builds up, which then collapses to produce a supernova.
If the remaining star mass is more than three solar masses, the force is pushed out from the neutrons and that force (called neutron degeneracy pressure) isn't strong enough to stop the collapse. So the core keeps collapsing more and more until it becomes something called a black hole. A black hole is a place in space where gravity is so strong that nothing, not even light, can escape from it. It's like a super squished and dense point in space. And if the remaining star mass is less than three solar masses, a supernova changes into a neutron star. The core crushes down so much that all the protons and electrons get contracted into neutrons. Then these new neutrons stop the collapse and that's when we get a neutron star.
Hence, the stars in the cosmos end into three bodies: White dwarfs, Neutron stars and Black holes. However, stars have this incredible journey in space. They're not like living things that will suddenly “die”, but they go through some amazing changes, transformations and endings. When we look up at the night sky, each star has its own special story to share.
