The evolution of galaxies: life of stars

Introduction

I’m sure many of us have taken moments to look up and just admire the stars dazzling in the clear night’s sky. But did you know these shining stars have actually been rapidly burning for the past 4.6 billion years? That’s right, throughout the scientific existence of mankind, these stars have been there, helping to support life on earth. But think of stars like living creatures- they go through a living cycle, and ultimately, they “die”. Scary thing is that our Sun is a star and it shall “die” too. The earth, by that stage, won’t exist to see what happened!

What is a star?

A star is a massive object in space, composed together due to gravitational forces. The distinguishing feature between stars and planets is that stars are luminous – they produce their very own light, whereas planets are non-luminous. In stars, the combination of the forces of gravity alongside molecular forces causes atoms to be somewhat smashed together. New elements form as a result, causing a release of energy. This is also known as nuclear fusion. 

The Life of a Star

  • Stage one: The nebula 

    The first stage of a star is known as a nebula- a giant cloud of dust and gas, namely hydrogen. Over millions of years of gravity causes the hydrogen gas to collect in a cloud and as more and more hydrogen collects in this cloud; it starts to spin. As the cloud spins, atoms of hydrogen gas bump into each other and therefore the faster it spins the more collisions there are and as a result the temperature rises. At temperatures greater than 10 million degrees Celsius, nuclear fusion begins to take place. This results in the formation of helium Gas from 2 hydrogen atoms colliding together. Stars form inside relatively dense concentrations of interstellar dust and gas known as molecular clouds.

  • Stage 2: the protostar

    After the initial phase of the nebula, the life cycle of a star begins. The first stage is the protostar, where the star is still essentially growing. This is when it is still gathering dust and material from the cloud that formed it. The star in the protostar stage is actually only 1% of its final mass as a fully developed star. Yet due to the interaction of gravity, this mass builds up relatively quickly. When the thermonuclear fusion plays in, however, it transforms from a protostar into a main sequence star.

  • Stage 3: The T-Tauri Phase

    As the star develops, they push away all the unattached molecules and left out gases. This leaves the new star to rapidly spin.This young star is 10,00 years old, and has an average rotational speed of 10-12 days (whereas for the Sun it is about a month). 

    The nuclear fusion doesn’t produce enough heat for hydrogen fusion to occur, however, so it relies on the gravitational pull so it can contract itself.

    After about 100 million years, it concludes it’s T-Tauri phase and moves on to it’s main sequence phase.

  • Stage 4: Main Sequence Star

    At this point the star has gained stability – the internal thermal pressure balances with the contraction of the gravitational forces on the outer layers. This reaches hydrostatic equilibrium, which gives the star their unique shape. 

    A star is in its main sequence for 90% of its life, where it forms hydrogen and helium. Sometimes, stars in their main sequence can be named as dwarf stars due to their low luminosity and the relatively small size of the star.

  • Stage 5: Red Giant Phase

    By the time a star becomes a red giant, it has a relatively large radius and a cool temperature.The outer layers of gravity can no longer support hydrostatic equilibrium as the inner core’s expansion becomes too large. These stars are typically extremely large and bright, due to continuous cooling and expansion as the star loses balance of opposing forces.

    A star is a red giant until the star stops making hydrogen and helium. Once it stops, it can no longer feed the core and continue the fusion process, causing it to lose hydrostatic balance.

    As the fusion process continues in the core, the outer layers become cooler. This is where a convective process occurs, and the star stops expanding- it just becomes more and more bright. 

    What happens next to the fate of the star depends on its size and mass. The more the mass, the quicker they “burn” hence they won’t live for as long as smaller stars.

  • Stage 6: Ending Stage

    • White Dwarfs:

    Stars like our sun, which are considered relatively small stars, run out of hydrogen after about 10 billion years to form white dwarfs. A good way to imagine a white dwarf is the mass of the sun confined in a size like the Earth. 

    Did you know: the denser the dwarf, the smaller they are? So this means that stars with a greater mass than our sun would form smaller white dwarfs. 

    White dwarfs don’t produce energy: they cool. This is how eventually they fade away, as they can no longer hold on together, and it forms a black dwarf.

    • Red Dwarfs: 

    These stars are hardest to see as they aren’t bright. They are the smallest and the coolest, and the most common in our galaxy. One example of a red dwarf is the Proxima Centauri, which is really close to our sun.

    • Supernova: 

    Giant stars and supergiant stars blow up in a huge explosion, also known as the supernova. A supernova sometimes leaves behind a tiny, dense, fast-spinning star called a neutron star. Such a star may give out radio waves in pulses as it rotates. These bursts of radiation are called pulsars.

    • Black hole: 

    Sometimes, a neutron star becomes so dense that the star disappears into itself. This is also known as a black hole, where the gravitational pull is so strong that the star disappears within itself.The gravitational pull is so strong that everything nearby is pulled inside: even light can’t escape from it.

Imagine we were from another world, far off from the sun but witnessing the Sun’s life cycle. It would have just been another one of many mesmerizing stars and the stages that they go through. Stars have long life cycles, and the sun (according to the predictions in current astronomy) have countless years till it “dies”. One thing is for sure, is that all life will come to an end and there will be a time it is no longer a main sequence star. What do you reckon would have happened to our solar system by then?

References

https://www.superprof.co.uk/blog/what-is-a-star-how-is-it-formed/

A huge thanks to all the people below for our images:

Alexander Andrews on Unsplash

Denis Degioanni on Unsplash

Joel Filipe on Unsplash

NASA on Unsplash

Drew Beamer on Unsplash

Sven Brandsma on Unsplash

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