If you have ever observed the night sky, you surely know that it swarms with countless stars of various sizes. It is remarkable when we realise that the light of these stars has travelled many years through space-time before it reached our minuscule blue planet and hit the retina of our eyes, where we are able to detect it and interpret its originator as a peculiar tiny dot in the sky. But what is more, the light from the stars we can observe today is often even several centuries old – this is how long it takes for light to travel the enormous distances that separate us from these stars. Observing the night sky is therefore in its essence like traveling into the past. Who knows, some of the stars in today’s night sky may not even exist anymore.

But even more remarkable is the immense number of stars. If you happen to be very lucky and observe the night sky far away from the cities’ light pollution, you may behold up to 2000 diminutive dots. That may seem like an impressive number, but it is only one fifty-millionth of all the stars hiding in the heart of our galaxy. The Milky Way contains an estimated number of staggering 100 billion stars, our Sun of course being one of them. If we add all the other stars from billions and billions various galaxies of the universe, we get a truly incredible number. The entire observable universe might hold up to 100 billion billion billion stars!

And each star is a little unique. Some finish their lives in a massive explosion, others leave this world in a considerably more peaceful manner. Stars are mesmerizing and omnipresent inhabitants of the cosmos, without which life could not possibly arise. Therefore it is appropriate to understand them at least a little bit.

The life of stars begins in huge clouds (nebulae) made predominantly from the lightest elements. These cosmic clouds perpetually come into contract due to gravity, which gradually raises their temperature. Once the temperature of the nebula reaches a sufficient value, the electrons inside of it decide that they no longer wish to form atoms and a peculiar state of matter called plasma is created. At this moment, the interstellar cloud consists of negatively charged electrons separated from positively charged atomic nuclei.

These hydrogen nuclei then move fiercely throughout the nebula and often come across different nuclei. But once two nuclei get too close to one another, electromagnetic interaction starts showing and swiftly splits them apart again. However, we should not forget that the temperature of the cloud still rises thanks to gravity. Eventually, it raises to such an extent that the nuclei manage to trick the electromagnetic force. The velocity of individual nuclei grows with the temperature of the cloud, so in the end they are able to overcome the immense repulsion of electromagnetism by getting so close that the enormous power of the strong interaction shows itself, and the nuclei are united into a single helium nucleus. At this moment, nuclear fusion has just began in the nebula, which can only mean one thing – a star has been born.

Our newly created star then continues with nuclear fusion, which becomes the source of tremendous energy. Due to this energy, the star is able to stop its own gravitational collapse – up to this moment, the original cloud (star) kept shrinking. Thanks to the energy from fusion, the star is able to create photons – the particles of light, which give stars their distinctive glow. Each star sends off billions of photons into the surrounding cosmos every second. These photons then travel freely through space-time until they reach an impediment that would absorb them and steal their energy.

Sometimes we do not even realize how dependent we actually are on our parental star’s photons. If the Sun suddenly stopped supplying us with its precious light, the Earth would change dramatically in no time. Eight minutes and twenty seconds after the Sun’s extinction, the Earth would submerge into an eternal darkness.

The temperature would fall beyond the freezing point in just a week, which would cause the freezing of all world’s oceans – water in its liquid form would exist just near the ocean floor, due to the heat from the Earth’s heart. Plants would immediately stop producing atmospheric oxygen by photosynthesis, and they would die shortly thereafter. This would cause starvation and early death of all herbivores. Carnivores and omnivores would follow in just a moment – including humans, understandably.

The differences in the Earth’s atmosphere would even out before long, any kind of wind would therefore cease to exist. The same goes for all the rivers of the world, since it would never rain again. All of these huge changes would significantly limit our last chance of survival – the production of electric energy. It is reasonable to assume that only a handful of lucky individuals would be able to survive, though not for long. All the remaining life on Earth would be concentrated at the bottom of the oceans. The Earth would become a dim and eternally frozen wasteland.

However, we do not have to worry about anything like that – for now. The Sun is about to stay here with us for at least a few billion years. But not all stars are this lucky. Some only live a fraction of our closest star’s life.

The strong interaction is, as suggested by its name, the strongest of all interactions. It is about a hundred times stronger than electromagnetism (and about a hundred trillion trillion trillion times stronger than gravity). How is it possible that we almost never hear about this interaction if it has such an immense strength? The problem is that its range is just billionth of a millionth of a meter. It may seem that a force with such a short range would never be able to influence our universe in a significant way, but the truth is that without the strong interaction, humans would never be able to exist.

As already mentioned, the universe was flooded with elementary particles right after the Big Bang. Those then started clumping together to create composite particles – quarks started forming protons and neutrons. But what caused them to attract? Why were quarks so keen on creating more complex particles? As you may already suspect, the attraction between individual quarks was provided by the strong interaction.

It does not end here, though. The strong interaction is to blame for another crucial phenomenon of our reality – the existence of atoms. We already know from the previous chapter that the same electric charges repulse each other. However, this means that protons in the nuclei of complex atoms should repulse and escape into all conceivable directions. But the strong interaction ensures that protons remain together. If electromagnetism were just a tiny bit stronger than the strong interaction, the existence of atoms would simply be impossible.

The final force, the weak interaction, may be the least known and the least interesting of all interactions. However, this is not to say it is not important. The weak interaction has the power to turn a neutron into a proton. Why would it do that? Within some atoms, there is an unstable ratio between the number of electrons and the number of protons in the nucleus. And the weak interaction is here to make sure that this instability is eliminated. When there are too many neutrons in a nucleus, the weak force simply turns one of them into a proton. This phenomenon is called the beta decay.

As we know from the first chapter, the number of protons in the nucleus determines what atom we are dealing with. But this means that any time an atom undergoes beta decay, which adds one proton into its nucleus, the whole atom changes into a different element. The weak interaction can therefore turn carbon into nitrogen or hydrogen into helium just by turning neutrons into protons.

And that is all. We have reached the end of the story of interactions. They make sure that our universe functions the way it does and without them, we would have never existed – perhaps with the exception of the weak interaction, which is the only one that does not affect the course of the cosmos in a significant way. Now, we can focus our attention on another fascinating element of our universe – huge fusion factories which are making billions of photons each moment.