The Universe houses billions of planetary systems. Our planetary system is established around the star ‘Sun’ formed about 4.5 billion years ago. The solar system, situated in the outer region of the Milky Way’s spiral disk went through various stages of the Sun to form what it is today.
The Birth of the Sun:
The Sun’s birth, marks one of the first stages of the Sun. The Nebular Theory suggests that the Solar System comprising the Sun and all the planets started as an enormous cloud of molecular dust and gas (in the solar nebula). Approximately 4.57 billion years ago, this giant cloud collapsed. The reason behind this may be shock waves from a supernova or a passing star resulting in gravitational collapse.
This collapse caused a collection of pouches of gas and dust into denser regions. With this, more and more dust, gases, and matter got pulled in the denser regions and started to rotate to satisfy the conservation of momentum. The rotation increased pressure and catalyzed heat generation. Majority of the mass got cumulated together, forming a massive ball at the center while the residual matter circled around it like a flattened disk.
The gigantic ball of matter at the center of the solar nebula ultimately led to the formation of the Sun, while the circling flat disk of matter formed the planets, moons, asteroid belt, etc. For about 100,000 years, the Sun was a collapsing Protostar; then the pressure and temperature in the ball’s interior increased to ignite fusion at its core.
Initially, the Sun was a T Tauri star, i.e. a violently dynamic star that blasted out a powerful solar wind. It took the Sun a few million years to settle down into its current form. Here, the life cycle and various stages of the Sun began.
The Main Sequence:
Similar to most stars, the Sun is currently experiencing the main sequences of stage of its life span and in the course of the main sequence, nuclear fusion reactions (fusing hydrogen into helium) occur vigorously in the star’s core. Approximately 600 million tons of matter is transformed into solar radiations, neutrinos, and 4 x 1027 W of energy per second. The Sun has been generating energy by this process for 4.57 billion years now.
Like every other process, this also has an expiration date. The amount of hydrogen gas in the Sun’s core is finite and therefore, cannot fuel the process forever. To date, the Sun has converted roughly 100 times the Earth’s mass into helium and solar radiations. As this process continues, more hydrogen is transformed into helium resulting in the continual shrinking of the Sun’s core. This allows the Sun’s outer layers to increase their proximity towards the center and face an intense gravitational pull.
As the closeness of the outer layers increases, more pressure is applied to the core, which is repelled by a subsequent rise in the rate of fusion. Essentially, this refers to the fact that the rate of fusion speeds up and the heat-light output of the Sun increases as the Sun consumes hydrogens and this process is resulting in a one percent increase in luminosity and heat-producing capacity of the Sun every 100 million-year and a 30 percent increase since the last 4.57 billion-year.
Around 1.1 billion years from today, the Sun is likely to be 10% brighter and hotter than the present day. This is similar to Venus’ runaway warming that transformed the planet into a hellish environment.
After 3.5 billion years, the Sun would become 40% brighter and hotter than it is currently. This intensification of heat and light would boil the oceans, melt the ice caps permanently, and release all water vapor in the atmosphere to space. Under these environmental states, life on Earth as we know would cease to survive. This would turn our Earth into a hot and dry body like Venus. The main sequence is one of the most vital stages of the Sun.
Core Hydrogen Exhaustion:
The Universal rule ‘A thing that starts needs to end’; is true for everything, even the Solar system. However, eliminating something as huge as a planetary system involves hundreds of billions of years. The end of the Sun is not likely to take place anytime in near future. But in the distant future, the Sun would burn out all its hydrogen fuel and gradually crawl towards death. As the Sun exists the main sequence about 5.4 billion years later, one of the last stages of the Sun begins.
Once the hydrogen present in the Sun’s core is used up, the inert helium ash formed there will be unstable and deteriorate under the influence of its weight. Due to this, the core would heat up and become denser, resulting in the growing size of the Sun leading towards the Red Giant phase of its evolution. It is estimated that as the Sun expands, it will grow huge enough to embrace Mercury’s, Venus’, and maybe even Earth’s orbit. If by chance the Earth survives the embrace, the extreme heat from the red sun would scorch the planet.
Final Phase and Death:
The last stages of the Sun involve the Red-Giant-Branch (RGB) phase and once the Sun touches the RGB phase, it will have an active lifespan of roughly 120 million years left. But this phase would witness a series of activities. First, the helium-ash-filled core will ignite viciously in a helium flash in which around 40% of the mass of the Sun and 6% of the core will be transformed into carbon within minutes!
In its RGB phase, the Sun is likely to shrink to around ten times its present size and 50 times its luminosity, with a considerably lower temperature than today. The helium present in the Sun’s core will continue to burn for the next 100 million years until it is exhausted completely. After exhaustion, the Sun would enter its Asymptotic-Giant-Branch (AGB) phase, where it would rapidly expand again and become more luminous.
Eventually, in the upcoming 20 million-year, the Sun would begin to demonstrate instability and will undergo a set of thermal pulses of mass-loss and these phenomena are predicted to take place every 100000 years, increasing the Sun’s size to over 1AU radius and luminosity to be 5,000 times brighter.
This stage of the Sun’s expansion will either embrace the Earth or leave it totally incompatible for life. Even the planets present in the Outer Solar System (beyond the asteroid belt) will transform drastically. With an increase in the absorption of energy from the Sun, the water ices will start to sublimate, forming a thick atmosphere and dense surface oceans. In 500,000 years from this stage, the Sun’s current mass will be reduced by half, and its outer envelope of gases will evolve into a planetary nebula.
The post-AGB stage evolution of the Sun will be comparatively faster. This happens as the expelled mass ionizes to form a planetary nebula and the bare core reaches a temperature of 30,000 K. The final temperature of the exposed core will be over 100,000 K, following which the residue will cool to form a white dwarf. The planetary nebula formed will gradually diffuse in around 10,000 years, but the white dwarf will subsist for trillions of years before diminishing to black.
Ultimate Fate of our Sun:
The stages of the Sun or any other star end with its death. Generally, the death of stars is associated with massive supernovas and the formation of black holes. However, in the case of the Sun, such formations may not take place because the Sun is not massive enough for undergoing such processes. When compared to the Earth, the Sun appears to be massive, but it is a comparatively low-mass star. There are some colossal high mass stars in the Universe many times larger than the Sun. If the Sun were ten times more massive, then the final stage of its lifespan would be a lot more explosive.
In such a case, iron would start to form in the core of the star. When iron undergoes nuclear fusion, it doesn’t give off any considerable amount of energy. Due to this, the star no longer experiences an outward pressure in its core, and hence it avoids collapsing inward.
The Sun is likely to catastrophically implode with a tremendous amount of energy when iron of around 1.38 times its mass is collected at the core. This unfathomable amount of energy would reach the Earth in just eight minutes and totally destroy it along with the entire solar system. A new nebula (similar to the Crab nebula) might form and be visible from the nearby star systems. The last remains of the Sun might be a stellar black hole or a swiftly spinning neutron star.
But our Sun doesn’t have this fate due to its mass. The Sun would just burn itself out into a white dwarf star. And by that time, life would long be extinct. This marks the end of the various stages of the Sun.
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