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Life Cycle Of A Star - From Start To Finish


The numbers given within this article are based on generally accepted averages and ranges which have been produced throughout many experiments performed by people who have dedicated their entire lives to understanding the universe that we live in.

But people make mistakes... Suffice to say that much still remains in flux!

Anyway, with that important disclaimer out of the way, let’s discuss the life cycle of a star, right from their birth to their eventual death.

All stars are born within nebulae which are vast clouds of gas and dust that can vary dramatically in size - the largest known nebula stretches over 1,000 light years across and the smallest a still astronomically large 0.4 light years.

These clouds are the nurseries of our universe and harbour the necessary environment for stars to eventually form (albeit over hundreds of millions of years).

Here's an example of a nebula - The Horse Head Nebula... Credit: NASA

The Horsehead Nebula

Nebulae are mostly comprised of hydrogen, but also contain helium and other heavier elements in trace amounts.

Over millions of years, the denser parts of the nebulae start to clump together under a combined effort of their acceleration due to gravity towards one another and the presence of dark matter, which is a topic for a different day!

Briefly though, dark matter permeates space and exhibits gravity-like behaviours as it applies an attractive force on normal matter, pulling everything together.

I'll be releasing articles on the "dark universe" very soon, and you can sign up here to be notified when they drop!

As they clump together, their increased total gravitational "force" results in more gas being pulled to these areas.

As this happens over millions of years, the clumps start to become sufficiently large enough where the gases at the centre are being compressed enough to start heating up, and friction begins to accelerate this process.

As both the heat and pressure increase due to the mass pulling in more and more of the surrounding gases, hydrogen nuclei at the centre have sufficient kinetic energy to start travelling incredibly quickly - quick enough to inevitably collide with each other.

Nuclei feel an incredible Coulombic force of repulsion between each other which is due to nuclei having the same overall charge (+), however when atoms have enough momentum, they can overcome this force of repulsion on collide with each other.

To properly understand this, an understanding of the 4 fundamental "forces" in the universe would be necessary, which are:

  • the strong nuclear force

  • the weak nuclear force

  • electromagnetism (the Coulombic force) and;

  • gravity

Einstein's general relativity tells us that gravity isn't a force per say, rather a curvature of space time due to mass, but how this curvature interacts with matter can be explained using classical physics like newtonian mechanics, which describes gravity as a force.

The two forces that we need to consider when we're trying to understand how atomic nuclei can fuse together is the strong nuclear force and the Coulombic force.

The strong nuclear force is the binding attraction that nucleons (protons and neutrons) feel when they're extremely close to each other.

In other words, the strong nuclear force is incredibly strong, but acts over a very small distance.

In contrast, the Coulombic force, which is the force of repulsion or attraction that charges feel to each other (depending on if they're oppositely or similarly charged), acts over an infinite distance but is comparatively very weak.

As an interesting side note, notice how the Coulombic force acts over an invite distance, just like gravity - everything in the universe feels an attraction to everything else gravitationally, and either an attraction or a repulsion electrically.

When nuclei have enough kinetic energy to resist the Coulombic force and get close to enough to feel the strong nuclear force, they fuse together under the extreme binding attraction of the strong nuclear force.

This therefore explains another principal which we'll come on to later - why nuclear fusion stops at iron.

Having enough momentum for this is incredibly difficult though, as temperature and pressure needs to exceed what is mostly not possible on planet Earth.

When two nuclei combine, not all of their mass is preserved, and since Einstein's famous equation E=mc² tells us that mass and energy are essentially one and the same, some mass is converted into highly energetic electromagnetic waves known as gamma rays.

This outward force prevents the force of gravity from crushing the now well-established core of superheated, pressurised gases.

You can check out this article if you're interested in knowing how pressure can affect temperature.

At this point, a protostar (think - baby star) has been born and it’s massive enough to have a very strong gravitational field that allows it to keep pulling in more of the surrounding gases, most of which is adding to the mass of the star.

Nuclear fusion (the process of the nuclei slamming into each other) only becomes more frequent as the core becomes hotter and the pressure continues to increase with the addition of more gases.

Heavier elements like helium start to fuse together to form beryllium, which fuse together to form oxygen and so on...

... until Iron is produced (we'll elaborate on why in a minute...)

Once these adolescent protostars hit an almost equilibrium, where the force of gravity trying to crush the star is almost equal to the force produced from nuclear fusion trying to expand the star, then the star reaches its main sequence stage (think - adult star).

I say "almost" here, since the force from nuclear fusion will slightly outweigh the "force" of gravity, meaning that all protostars will continue to grow to and through the main sequence stage.

Let's take a quick step back now though and look at why stars do not fuse nuclei that would result in a nucleus heavier than iron.

Why stars call it a day at producing Iron

The reason for why the process does not go beyond iron is fairly straight-forward.

Immense amounts of energy are required to fuse nuclei together, and this only becomes greater as the mass of the nuclei that you’re trying to collide gets bigger (remember Isaac Newton’s 2nd law of motion where force = mass*acceleration, or rearranging to give acceleration = force/mass - the higher the mass, the more force is required to achieve the same acceleration).

However, the process of them fusing releases vast amounts of energy as well, and in the case of every element below iron, the amount of energy to fuse the nuclei is less than the amount of energy that’s released by fusing them.

However, beyond iron, so for example gold, the amount of energy that would be required to fuse two nuclei together to form the nucleus of a gold atom would simply be greater than the amount of energy that would be released by doing so.

Hence, the process becomes inefficient and would result in a net loss of energy, and therefore a quick death for the star...

So we now have the fledgling equivalent of a star, formally known as a protostar.

It’s burning brightly and only getting bigger in terms of both mass and volume as time goes on - things are looking good so far!

Just as a quick side note here, this can be happening multiple times throughout a nebula, and in fact very often does.

Interestingly, something that's become apparent to astronomers is that most the star systems in fact contain more than 1 star - our solar system is kind of an odd-one-out.

This is the reason for star clusters, such as the Seven Sisters that you can see with your own eyes if you're in a dark enough location - you can use this map if you’re interested in knowing if there's a dark site near you.

So now we can skip about 500,000 years or so, as the protostar is just going to keep burning and keep getting bigger until it consumes all of the surrounding gases that don't evolve into planets and hits a (sort of) equilibrium where it’s outward force provided by nuclear fusion is almost equal to the force of gravity that’s trying to crush the star down to the core.

At this point, the star is considered to be stable.

Henceforth, these next millions if not billions of years will form the main sequence stage of the star.

As a quick side note, this is the stage that the sun is currently in.

What happens after that is very much determined by the mass of the object that would be left behind from the explosion ie how much mass the star was able to accumulate (or if it combined with another star).

Just to recap from earlier, a star will always continue to grow from nuclear fusion, right until the point that it runs out of atoms to fuse.

Hence, a star is never in a perfect equilibrium, but the rate at which it increases in size is drastically decreased for the portion of the main sequence stage, compared to when it's a protostar, just like how a child grows more than an adult in x amount of time.

After the main sequence phase, we have either the red-giant phase, or the super red-giant phase, depending on how massive the star is.

This is a phase in a star's lifetime where nuclear fusion starts to happen more regularly, due to the fact that as nuclear fusion happens in a star, the temperature rises which causes a loop effect of increasing the frequency of nuclear fusion (as the particles have more energy at higher temperatures).

Technically, the sun is heating up with every passing second, albeit extremely slowly (by about 1% every 110 million years).

But it's not just getting hotter - the increasing frequency of nuclear fusion means that the star is also expanding and becoming more luminous, both of which would be bad news for the Earth once billions of years have passed.

At this point in time, the star is a red giant, but this phase doesn’t last for very long as it’s consuming its source of fuel very readily - ie the frequency of nuclear fusion is much higher now than during the main sequence phase.

This process can last about 1 billion years or less, and accounts for approximately 10% of a star's lifetime.

If the star at question is more than 5 solar masses, then the same process happens only a super red giant is formed, which lasts for even less time than a regular red giant.

This is quite simply because more massive stars burn through their fuel much quicker than less massive stars since they require more fusion to counter-act their stronger gravitational "force" (which is because they have more mass).

Just to mention something very quickly here, you might be thinking that when this happens, we're imminently destined for a fiery death.... but it's generally believed that we could push the orbit of the Earth back as the the sun begins to swell to avoid being either consumed completely or photo-disintegrated by radiation.

As I mentioned before, it's really null-and-void at the time of writing this since the growth in size of the sun is minimal, but if we're still around in a few billion years or so, then the rate at which the sun will grow would be problematic for sure.

There's a few ways that this could theoretically be done: we could use light energy from the sun to power an extremely powerful laser that would constantly bombard the Earth with highly energetic electromagnetic radiation, producing a force large enough to gradually push it back over a long enough time frame, or we could use asteroids to nudge the Earth back over time with the interaction between gravitational fields.

The latter of which uses the principal of conservation of momentum, which is something that we experience every single day only on much smaller scales!

As the star burns through all of its fuel, it begins to collapse under an overwhelming gravitational "force", and its density starts to drastically increase as all of its mass is existing in a smaller volume of space (density = mass/volume).

Consequently, the pressure also starts to increase and what happens next is what depends on the mass of the star.

This is is because the overall amount of mass will determine the strength of the gravitational force that’s crushing the star.

Also, I mentioned earlier on in this article that elements are fused together to form heavier ones, up until Iron.

Well, now that the pressure is absurdly high and only getting higher, atoms have no choice but to slam into each other regardless of energy efficiency, and heavier elements like gold begin to form.

At some point, the pressure becomes too high and the star explodes like a balloon full of too much air, except the explosion of a star is one of the most violent events known in the universe... A supernova.

However, this isn't true for all stars.

For star like the sun, who just don't cut the mustard in terms of mass, then as they expand outward during the red giant phase, the gases end up having velocities that exceed the star's escape velocity at that distance.

Consequently, they run off into space forming what we call a planetary nebula, and what's left behind is a super-hot, dense called a white dwarf that eventually call over millions of years into a black dwarf.

White dwarfs are incredibly dense as the pressure from the core collapse of the star has forced atoms to bunch up next to each other, reducing the overall volume while retaining mass.

More specifically, if the remaining object that's left behind after the gases escape into space is around or less than 1 solar mass (ie its mass is less than or around equal to the sun's mass), then the gravitational force due to its mass is not strong enough to fuse subatomic particles together, namely protons and electrons.

If you’re not very familiar with atomic structure, then I'd highly suggest giving it a quick search on YouTube or the like, although I do plan on making a separate article here at Expansive (so make sure to subscribe so you can be alerted to new posts!)

See, white dwarfs aren't massive enough to collapse any further, despite undergoing no nuclear fusion and hence having no source of energy to resist the gravitational collapse.

This then begs the question of why they don't collapse if they've not got nothing to contest their own gravitational attraction?

Well, they do have something up their sleeve - electron degeneracy pressure.

Now, if you'd like much deeper understanding of what electron degeneracy pressure is and how it arise, then I'd highly suggest that you give this article here a read, since it takes much more holistic approach than what I'll describe in this particular article.

In essence, electron degeneracy pressure arise because electrons are not classical particles - they're quantum particles (or waves...), which is significant because it means that electrons essentially follow different rules.

They're assigned 4 quantum numbers which I go into detail of in the other article, but suffice to say that electrons are bound to very specific places around a particular nucleus, and can't exist in between these places, or "shells" as they're referred to as.

Given this, they exert an outward pressure that's strong enough (in the case of a low mass star) to contest the force of gravity, and halt the collapse the star.

Remember, white dwarfs DO NOT undergo nuclear fusion and are stable only because of the degeneracy pressure of their electrons.

So to recap, the outer layers of gas will expand outward into the cosmos, almost as if the star is shedding its own skin.

This leaves behind a superheated and pressurised core that we call a white dwarf, that’s surrounded by gases and dust that are be spread over the cosmos, which we call a planetary nebula.

White dwarfs are relatively small, highly-pressurised, super-heated balls of particles that will continue to thrive for a long time, despite the fact that they no longer undergo nuclear fusion.

Electron and protons do not collapse together under the gravitational attraction of the white dwarf and the degeneracy pressure of electrons halts the celestial body from collapsing any further.

However, given their extreme temperature, they will continue to radiate energy from the collapse for a long time, slowly fading away until they become a black dwarf which is a cool, dormant star that has nothing left to give off in the form of radiation (but remember that gravity can’t collapse it further, as it’s not strong enough to fuse electrons and protons together).

Within an atom, there's a lot of "room" between electrons and protons that's simply empty space.

There's so much room in fact that atoms can be said to be 99.999999996% empty space!

I know that sounds absolutely insane, and if you're after some serious evidence to back that up (which I wouldn't blame you for) then consider checking out this article here.

Under extreme pressures, this gap can be filled by fusing protons with electrons, resulting in a much smaller, denser object.

So what happens when the remaining star is greater than 1 solar mass?

A neutron star is born.

If the remaining star’s mass is between 1.5 - 3 solar masses, then its gravitational "force" is strong enough to overwhelm the degeneracy pressure of electrons, forcing them to neutralise with protons to form a body that's almost exclusively made from neutrons... hence the name.

Much like electrons, neutrons obey an “exclusion principle” which means that they can only occupy a single elementary cell.

When all the protons and electrons have combined to form neutrons, what’s left behind is a bunch of degenerate neutrons that, under this gravitational force, won’t fuse together.

This resistance halts the collapse of the star, and what’s left behind is a neutron star, named simply because it’s a bunch of neutrons!

I have a separate post that goes into the details of neutron stars here, so at risk of repeating myself, I’ll leave it at saying that they’re essentially the extreme version of a white dwarf.

They’re hotter, denser and have tons of other features that make them extremely interesting to study.

Neutron stars, like white dwarfs, will cool over the next billions of years, and their rotational speed will also decline.

What’s interesting here, although theoretical at the time of writing this, is that because neutron stars are said to be 95% neutrons and have a very thin crust of protons, it’s said that the crust will undergo proton decay and will eventually disappear over an unimaginably long time frame (we’re talking trillions of trillions of trillions of years… but interesting anyway!)

A neutron star.... well, strictly speaking it's meant to be an illustration of a pulsar. You can read up on the differences between the two here (but they're pretty much the same).

Image of a pulsar

Now we’re into the less well known category where the stellar remnant is something that simply doesn’t comply with our current models of physics, so we really don’t know all that much about them despite the hundreds of theories out there.

Yes, I’m talking about the infamous black hole.

If the remaining star has a mass greater than 3 solar masses, it’s said that the gravitational force will be strong enough to overwhelm the degeneracy pressure of neutrons, causing them to collapse down to a point of supposedly zero volume - a singularity.

Now, my article on black holes is honestly probably my most holistic, detailed article, so if you want a really deep understanding of basically anything and everything about black holes, then I'd definitely recommend give it a read.

I would just like to mention though, simply because I think the concept of a singularity throws a lot of people out there, that a singularity is NOT something that actually exists.

Just like Schrödinger's cat, the idea of a singularity is just a way of saying that our current models are incorrect, and cannot accurately describe the environment of a black hole beyond the event horizon.

Nobody knows what exists in that spherical region where light can't escape, but if a black hole was made from neutrons then its density wouldn't be any greater than that of a single neutron, unless they were somehow arranged in a way that is completely unbeknownst to us.

Unlike white dwarfs and neutron stars, it’s not totally clear whether black holes survive indefinitely or not.

If you’re familiar with Hawking Radiation, then you might be inclined to think that they slowly disintegrate over time (you can click here for more info on that), but their gravitational field strength is so strong that, particularly in the case of black holes found at the centre of galaxies (supermassive or ultramassive blackholes), they consume matter in the form of stars and even other black holes, so they generally have a huge amount of “food” available to them.

This is a picture that was taken by the Event Horizons Telescope of the supermassive black hole at the centre of our galaxy, called Sagittarius A*. Credit: Event Horizon Telescope collaboration et al.

Image of a black hole

Seeing as this is about the life cycle of a star, which we’ve now described from birth to death of every known scenario, I think it’s best to conclude here:


Stars a born from clouds of gases called nebulae, and live millions to billions of years depending on their size until they're eventually crushed by gravity when they run out of nuclei to fuse together as fuel.

If the remaining star is less than or around the same mass as the sun, then the outer layers of gases are ejected into space leaving behind a planetary nebula and a hot, pressurised core called a white dwarf that cools over time into a black dwarf.

White dwarfs are solely supported by the degeneracy pressure of electrons essentially refusing to move any closer to their nuclei.

If the remaining star is 1.4 - 3.2 solar masses, then electron degeneracy pressure is overwhelmed, meaning that electrons and protons are fused together at the core to form a neutron star.

The rest of the star explodes outwards under the extreme pressure of the core-collapse of the star, leaving behind a nebula.

Neutron stars also cool over time and the crusts of which perhaps even decay.

Finally, if the remaining star is more than 3.2 solar masses, then matter is compressed down to a point that our current models of physics cannot explain, whether that be classical or quantum ... every conclusion drawn is non-sensical.

Black holes might decay incredibly slowly due to a process that has arisen from Stephen Hawking's work on attempting to reconcile relativity with quantum theory.

This process involves the emission of so-called Hawking Radiation, which results in the mass of a black hole slowly shrinking over time, but the gravitational pull of black holes generally mean that a lot of them end up consuming other matter in the form of stars and black holes anyway!

To summarise everything in a simple picture... Credit: NASA

lifecycle of a star by NASA

And that’s about it...

I hope that this article was an interesting read and, most importantly, I hope that you could learn something from it, however theoretical it might have been!

Maybe stick around and consider signing up to receive daily facts about our universe - totally for free!

Maybe death isn’t just a natural part of life, but a natural part of this universe, Ryu.

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