Stellar Evolution: The Life Cycle of Stars

Introduction

Throughout history, humans have turned their eyes to the night sky, using stars for practical navigation and early explorations in astronomy. Initially seen as celestial jewels, we later realised that stars were giant balls of gas like the Sun. The Sun, a constant in our lives, did not start as the radiant ball we know. Originating as a protostar, it underwent a gradual evolution and will ultimately end in a stellar demise. Stellar evolution encompasses the intricate processes through which a star changes over time. This essay delves into the detailed exploration of these stages, shedding light on the fascinating journey of stars in the cosmic expanse.

A Star Is Born

The first stage of stellar evolution is the formation of a protostar. Protostars are formed when gas and dust accumulate due to gravitational forces and heat up. This process takes place in regions within galaxies called Nebulas. A protostar is not a fully developed star yet, it is on its way to becoming one. The more dust and gas get attracted to the protostar, the denser it gets. That means that particles collide more often with each other, hence raising the temperature. Once a protostar has collected enough material and reached a crucial temperature, it enters the main sequence phase, which is the present stage of our Sun.

Main Sequence Star

In the main sequence phase, the pressure and temperature of the protostar are high enough for nuclear fusion to happen. Here, two Hydrogen atoms combine to form a Helium atom, which is heavier and denser than Hydrogen. That gives out immense amounts of energy, making the star shine. This energy also keeps the core hot enough for nuclear fusion to take place continuously. The nuclear fusion exerts a force in the outward direction which balances the gravitational force that is directed inward of the star. Hence, stars are stable and spend most of their lives in this state. Moreover, stars come in various sizes and colours; the hotter the star, the more massive it is.

As stars age and use up their hydrogen for fusion, they undergo significant changes in their size, temperature, and brightness. When a star runs out of hydrogen, it is unable to perform nuclear fusion, which leads to gravity overpowering the outward force, and the star contracts into a small ball. As the star shrinks, its temperature rises, and when it is so hot, nuclear fusion takes place again and the star expands. This time, however, nuclear fusion results in the formation of heavier elements (the heaviest being iron) and not just Helium.

Red Giants and White Dwarfs

Exactly how much the star expands depends on the size of the initial star. If the star was small or medium (like our Sun), it would become a Red Giant. A Red Giant’s surface is typically 100 to 1000 times wider than our Sun and, because of its large volume, relatively colder than it, which results in its red-orange appearance. Its core, however, keeps contracting and after a certain period, sheds the outer red layer in huge clouds of dust and gas. What is left is a dense core that packs all its mass into a sphere about as big as the Earth. That is called a White Dwarf.

Red Supergiants

Stars that are about 10 times as heavy as our Sun take a different path as they age. They become Red Supergiants, entering the final act of their stellar evolution. In this phase, these massive stars undergo a series of intense transformations. The core, under the pressure of gravity, continues to fuse heavier elements until iron is formed.

When iron is produced in the star’s core, nuclear fusion can no longer generate energy. At this critical juncture, gravity triumphs over the outward pressure, causing the star to collapse rapidly. This dramatic collapse results in an explosive event known as a supernova, a dazzling display that can outshine entire galaxies for a brief period. The supernova scatters heavy elements formed in the star’s core into space, contributing to the cosmic recycling of materials necessary for the formation of new stars and planets.

Neutron Stars and Black Holes

Following a supernova, the remnants of a massive star can take two distinct paths. If the core is less than about three times the mass of the Sun, it collapses into an incredibly dense object known as a neutron star. Neutron stars are so compact that a teaspoon of their material would weigh billions of tons on Earth. They also exhibit rapid rotations, emitting beams of radiation detectable as pulsars.’ On the other hand, if the collapsing core is more massive, nothing can withstand the force of gravity, resulting in the formation of a black hole. Black holes are regions in space where gravity is so intense that not even light can escape, creating an invisible cosmic abyss.

Conclusion

In conclusion, the journey of stars is a fascinating exploration of cosmic processes marked by diverse stages of evolution. From the humble beginnings of protostars in nebulous regions to transformative events like supernovae, stars play a crucial role in shaping the vast expanse of the universe. The remnants left behind, whether neutron stars or black holes, contribute to the ongoing cosmic dynamics, influencing the birth of new celestial bodies. As we continue to explore the mysteries of the cosmos, the story of stars serves as a reminder of the intricate and ever-changing nature of the universe.

(Written by Shrinidhi (UG’27), Ashoka University)