When you gaze up at a clear night sky, you see myriad glimmering stars scattered all over—ever wondered how these massive objects begin their life journey spanning millions to billions of years. It is estimated that there are 100 billion stars in our Milky Way galaxy alone. Do you know a star is born from a vast cloud of gas and dust, or a star might conclude its life cycle as a black hole is chiefly dependent on its mass? The life cycle of a star has many phases. This article covers the various stages of life cycle of a star from birth to death in a comprehensive way.
Nuclear Fusion Inside a star
Stars are massive, burning balls of hot gases. The life cycle of any star is hugely dependent on the amount of mass it contains. Stars are chiefly made of Hydrogen and Helium. At a star’s core, hydrogen atoms undergo a fusion process to release massive energy. When a star’s core runs out of material, it collapses, leading to a star’s death. The outcome depends entirely on the mass of the star. From its birth to its death, the entire life cycle of a star spans millions to billions of years. The energy released by nuclear fusion balances out the immense gravity exerting inward. Therefore, a star can maintain a spherical shape. The fuel inside the core affects its size. The bigger the size, the faster a star consumes its fuel. Ironically, the more massive a star is the shorter its lifespan.
Blue stars, which are considered to be massive ones, end their lives much more quickly than red or orange medium sized stars.
Types of stars:
There are many types of stars. We can categorize them based on their colours and sizes. Various types of main-sequence stars are given below.
1) O type stars: The most massive, rarest, and brightest blue supergiant stars. They are 15-90 times massive than the sun.
2) B type stars: They are also massive but non-supergiant blue stars. Not very common too. They all are heavier than our sun.
3) A type stars: Blueish white, more commonly found stars. They are 1.5 to 2 times massive than our sun.
4) F type stars: They are yellow-white—1 to 1.4 times massive than our sun.
5) G type stars: Our sun is a class G star. they are orange yellowish. Fun fact, do you know the sun fuses more than 600 million tons of Hydrogen into Helium each second.
6) K type stars: They are cooler and lighter(0.5-0.8 solar masses) than our sun. They are orange in colour.
7) M type stars: They’re the most found, lightest, and smallest of all stars. They are red in colour.
Simply put, a star’s colour varies from blue to red, blue being the heavier, bigger, and less common ones and red being the lighter, smaller and more common ones. The bigger a star is, the more quickly it consumes its fuel and nears its end quickly. Our sun is predicted to live for almost 10 billion years. At the same time, there are much bigger stars that live only a few million years.
LIFE CYCLE OF A STAR STAGES: BIRTH OF A STAR
STAGE 1: NEBULA & PROTO STAR
Every star begins its life as giant clouds of Hydrogen and Helium. This massive cloud which can be several light-years across is called a nebula. The most famous of them is Eagle Nebula. These nebulae consist of hydrogen, helium, molecular clouds, cosmic dust etc. These molecules collide, and they form a cluster of molecules called Proto star. This phenomenon sets the course for the subsequent chain of events spanning millions of years.
STAGE 2: THE MAIN SEQUENCE PHASE
When enough matter constricts upon itself due to gravity, the nuclear fusion of hydrogen nuclei kicks in, and enough energy is released to balance out the immense inward pressure due to gravity, where a star is at its most stable phase, called the Main sequence phase.
When hydrogen atoms fuse to form deuterium atoms releasing beta decay and eventually forming helium atoms, the star consumes this fuel until it exhausts. Sun is at this stage and maintains its size, colour and brightness.
A large and massive star consumes more fuel to counteract the immense pressure of gravity and live only tens of millions of years. Higher temperature means helium atom is fused to form nuclei of higher masses such as Silicon, Oxygen, Neon and Iron.
STAGE 3: RED GIANT PHASE
When a star’s hydrogen fuel slowly creeps to a low level, the core starts to shrink, consumes its fuel at an even faster pace, finally exerts more outward pressure than gravity, and its outer layers drift away to become much bigger, cooler than they were before. This phase is called the Red Giant star.
DEATH OF A STAR:
The core becomes hotter and consumes fuel at an even higher pace, resulting in the fusion of three helium nuclei to form a carbon nucleus and release even more energy. This phase is called the triple-alpha phase. After this phase, the star consumes more and more of its helium fuel to form larger nuclei, and the star starts to shrink due to gravity, becomes cooler and duller. This phase is called the horizontal branch phase.
STAGE 4: WHITE DWARF
As this phenomenon continues for millions of years, helium nearly exhausts, and the core collapses. The star enters the asymptotic giant phase. The outer layer starts expanding rapidly to become a massive red star until the material in the outer layer gets ejected, leaving a small dense core behind. Finally, this core becomes dense and cools down completely. This phase is called the White dwarf phase. This core simply does not have enough fuel to overcome electron degeneracy pressure. Eventually, they cool down to become brown dwarfs. This eventuality occurs for low mass stars with a core of less than 1.4 solar masses(Chandrasekhar limit).
STAGE 5: SUPERNOVA
For a large and massive star, the fuel starts to get consumed at an alarming pace until only the iron core is left behind. If the core is 1.4-3 solar masses, the core collapses as it is unable to support its gravity and electrons are squeezed together with protons to form neutrons. The outer layers are ejected after triggering an explosion called a supernova explosion. This explosion is one of the most spectacular and violent events in the universe. Such high energy is released quickly, creating elements of a much higher mass than iron. In fact, all the elements having an atomic number greater than 26 are formed during these high energy phenomena. They are even brighter than an entire galaxy and release more energy than our sun emits in its lifetime. The remnants of supernova are not white dwarfs.
STAGE 6: NEUTRON STAR & BLACK HOLE
Neutrons are left behind in a tiny, dense, compact ball called a neutron star. The entire mass of the core is constricted into a small space no bigger than a large city; the density of this star is exceptionally high. Even a teaspoon of neutron star weighs millions of tons. Suppose the core weighs more than three solar masses. In that case, gravity will overcome this neutron degeneracy pressure; neutrons are collapsed together to form a point of infinite density called a black hole. The entire core mass is restricted to a point called the singularity, and gravity becomes so immense that even light can not escape. In 2019, first image of the black hole was generated. You know, gravity affects space and time. Thus, these fascinating objects are indeed remnants of a massive star collapsing under its gravitational force. Every star goes through this cycle, and our sun is no exemption.