Sun 101: How the brilliant yellow star formed from just a cloud of gas

Updated: Jan 20, 2021

It took thirty million to 50 million years to form the star we call the sun. Which may sound like a long time, but in the universe's scale, it is not so long. With lots of help from science and its advanced tools, we know a good deal of how stars form in the universe.

One star’s birth starts at another stars' death.

Stars die in one of four ways, either they become brown dwarfs, blow up as supernova or become neutron stars or black holes. Whichever way a star finally meets its doom as a supernova, it throws its material back into the galaxy. Over billions of years, this stellar debris gathers themselves into the colossal clouds that astronomers call interstellar matter.

The universe is the transcendent recycling machine. Starting around 4660 million

years ago, from the remnants of exploded stars, a new one finally rose: a star known as the sun.

Recipe for a star: Take 104 solar masses of molecular gas. Sprinkle liberally with carbon and silicate dust spiced with metals. Freeze to 10 K and stir well until mixture is frothy. Hammer until lumpy. No need for oven; stars will form and bake themselves. Watch out for hot bubbles flying from the pot.
—James Kaler, “Cosmic Clouds”

Further reading: How stars are formed? , The Ultimate guide to understanding black holes


Giant Molecular Cloud

Time zero

4.6 billion years is the age of our sun. But the process for making this star started way before that. About four thousand million years ago, our solar system was nothing but a cloud of raw materials. Each and every object we see around us from the small meteors that fall in from the sky to planets like Jupiter and even the sun itself came from this gas cloud. The layers of the sun formed one after the other from this parent cloud.

These accumulations of interstellar gas were as common billions of years ago throughout the universe. We call them giant molecular clouds. The name is because hydrogen exists in a molecular form in these clouds. Molecular clouds are 73% molecular hydrogen; rest is helium, with traces of heavier stuff like carbon, nitrogen and oxygen, and silicate particles.

This ancient cloud from which the solar system born was about 50–100 light-years across, orbiting the core of the Milky Way, about two-thirds of the way out from the centre. These enormous molecular clouds hold enough matter to outweigh millions of stars like the sun.

A molecular cloud's life is like thin ice, forever on the brink of destruction. A push, a pull, a tug, anything could trigger enough drama to create an entire star system in these things.

Two or more Molecular clouds may smash into each other triggering cloud collapse each other, or a supernova explosion close by can send shock waves from the dying star burrowing into the cosmic mist.

Something like this triggered star formation about 4660 million years ago in one of the spiral arms of the milky way galaxy, and it led to the formation of our local star, the sun.




Formation of the Solar Globule:

Time- 2 000 000 years:


Once the collapse molecular cloud starts, it does not stop until an abundance of nuclei forms in the regions where the density is higher. About two million years have passed by now, and denser areas pull in more gas from their surroundings. The parent cloud now fragments into thousands of small concentrated regions. Most of these regions would later form stars, and one of them would become the sun.

As the gas closest to the cloud's centre fell inward, the core became hotter. The final inevitable result was a gas and dust cocoon: a shell of dark material surrounding a denser, warmer core. We identify such an object as a Bok globule. Solar globule is the one from which the sun formed. This cocoon-like formation of gas and dust served as an incubator for the young sun.

The name might sound ironic as of now because the solar globule was dark. It emitted no light but infrared radiation.




Protosun formation:

Time: 2 030 000 years

Over thousands of years, the gases got pulled relentlessly towards the dense core at the centre. The centre is now about a 10 000 degree celsius, and it is hot enough to emit radiation. The generated radiation makes the globule opaque the heat radiation generated in the core gets trapped inside.

As energy could no longer escape, the nucleus heats much faster, and the core shrinks in size.

At this point, the nuclear reactions started at the stellar core, and it had reached a form that astronomers call the protostar. Or in our case, we can call it the proto-sun.

The gases the proto-sun accumulates over the years don't just stand there, but they start to swirl because of the angular momentum.

As the protosun grows smaller and hotter, it spins faster and faster.



The Solar Nebula formation

Time: 2 130 000 years

Within 100 000 years after the protosun formation, the entire gas cloud has become a swollen semi-spherical mass, flattened at the edges by rotation.

Over the years, the accelerated rotation has flattened out the gases. Now, an immense pancake-looking disk of swirling gas and dust encircles the protosun. The whole thing now almost resembles a sunny-side-up egg. We call this structure the Solar Nebula.

The central region, closer to the protosun, the temperature topped 2000 Celsius. But at about the current location of Jupiter, the temperature dropped below 70 degree Celsius. And further beyond that, it was even more frozen.

This chilling disk of dust and debris surrounding the protostar would act as the raw material for planet formation in the distant future. We call this cold, vast accumulation of matter as the proto-planetary disc or proplyd for short.

Most of the stuff from the parent cloud fell into the protosun and the rest into the proplyd. The globule surrounding the star had disappeared for the first time, and it revealed the newly forming star to the outer cosmos. It was still a toddler among other giants, but preparing itself for the next–and most violent–stage in its early life cycle: the T-Tauri phase.


Further reading: The story of the solar system and how it formed


The T-Tauri Phase

Time: 3 million years

Three million years have passed after the initial collapse of the cloud. The protosun has shrunk itself into a smaller radius. The core temperature is now a whopping 5 million degrees Celsius, while the exterior boiled and churned at around 4500 Celsius. It is no more a protostar, but it is what we call a T-Tauri star.

The name may sound like a fancy sushi restaurant, but the T-Tauri phase is one of intense ferocity. A powerful magnetic field drives this phase.

Let me show you how the magnetic field of the sun forms. The gases around the young star get ionised and become a mix of positively and negatively charged particles. As the star rotates, these particles create a series of enormous electric currents. Thus the spinning star develops a magnetic field.

But what is so violent about it? During the T-Tauri phase, the star would spin at a much faster rate than it is spinning today. It made the T-Tauri sun's magnetic field much intense than what we see today.

And the proto-planetary disc sun was wrapping still around it. So, as the sun swirled around, it dragged its magnetic field throughout the disc. Where ever the magnetic field touched the proplyd, enormous clumps of gas got sucked right into the young sun. And where ever these hordes of the gas hit, the agitated star gave out violent flares that are the trademark events of the T-Tauri phase of star formation.

Thus the adolescent sun was very much more violent than the star we know it today. The sunspots that on the solar surface were very much larger than what we see today.

Possibly the most striking aspect of the T-Tauri phase is the molecular outflow which would come next.





The Bipolar Outflow

Time: 3 million years

At the T-Tauri phase, the sun developed what we call a stellar wind. The sun still has recurring stellar winds.

What are stellar winds anyway?

It is an ocean of charged particles that stream out from the solar surface. But T-Tauri winds are much more violent and hold more mass, travelling at speeds of up to 200 kilometres per second.

As the wind crashed away from the young sun’s surface, the disc carried the flowing gas further away into space. Because of this violent activity, charged, accelerated particles began strutting away from the young sun at both the poles. Astronomers call this a bipolar molecular outflow. The sun lost much of its original mass through this process. However, the sun continued to shrink under gravity because the pressure at its heart was not yet enough to halt the shrinkage.

By the time the wind had stopped, 10 000 years had passed, and the sun’s mass started to stabilise. But the sun still has tens of millions of years before it is wholly mature.


Further reading: Check out this insightful post by Jillian Scudder The sun won't die for 5 billion years, so why do humans have only 1 billion years left on Earth?


The Main Sequence phase:

Time: 30–50 million years

After 30 to 50 million years, the sun’s contraction finally stopped. The sun’s inherent temperature had struck a whopping 15 000 000 Celsius.

And at 15 000 000 Celsius, the positively charged hydrogen nuclei at the sun’s core started speeding up. This marks the beginning of the nuclear fusion reaction in the sun's core.

Let me explain the reaction occurring in the sun, making it shine. Being at the centre of the sun, an enormous amount of mass encases you, squeezing down on you from all sides. Similarly, the hydrogen gas in the sun's core gets crammed in so rigorously that four of them fuse to form one helium atom. We call this nuclear fusion, which converts some mass of the hydrogen atoms into energy as light.

The sun's core produces the same amount of energy as 15 billion hydrogen bombs each second.

It doesn't blow to pieces because the gravity just precisely balances tremendous energy produced. The fusion reaction slowly converts the hydrogen into helium in the sun’s nucleus through a series of nuclear reactions. The output of these reactions is nothing but pure energy.

The core is now a self-sustaining mass that has now produced a considerable amount of energy in its nucleus. The pressure created by this reaction was so intense the gravitational contraction finally met its equivalent. As the power precisely matched against any further gravitational contraction, the sun had reached hydrostatic equilibrium. It had become a stable star, what astronomers call a main-sequence star.

It took tens of millions of years for the sun to get to this point. About 4600 million years later, it is not quite halfway through its main-sequence journey. It still has a long life ahead.




Thank you for taking this journey with us, as we take a deep dive into the secrets of the universe. And if you want to read more from us, there’s always a subscribe button somewhere nearby!


Want to know what happens when the sun dies? Check out this amazing post by Michelle Star in this link: https://www.sciencealert.com/what-will-happen-after-the-sun-dies-planetary-nebula-solar-system



References:

  1. The story of the solar system, book by Mark A. Garlick

  2. Catching Stardust_ Comets, Asteroids and the Birth of the Solar System, book by Natalie Starkey


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