16 Facts about stars for world building

World-building can be overwhelming if you don’t know where to start. Whether you are writing fantasy or science fiction, world-building is the foundation of your story. You have to create the world in which your characters live.

If you want to know where to start, you can start at the very beginning. Stars are the basic foundation of the universe. Everything we see around us is made out of stars, and we couldn’t have the universe we do without them.

So, stars must be at the beginning of your world-building process. Think of our sun, its composition, how it behaves, and how it has shaped our solar system, our world, and how we live.

Here are some astronomical and cultural facts and ideas about stars to help you with world-building.

Why are stars important?

Stars are the substance of the universe—there are trillions of them. The way stars behave, and their characteristics can have a profound impact on our world.

Stars give light, heat, and gravity. Without our own star, our sun, Earth could not have life. And life on Earth is dependent on the sun. 

Any other world you create for your fantasy or science fiction stories should have a sun. Or two or three. 

So, let’s learn about stars, the types of stars in the universe, and their formation and life cycle. This will help provide a scientific context for your world.

The life cycle of stars

Nothing lasts forever. Just like all life, stars are born, live, and eventually die. 

Stars may seem to stay the same forever, but that’s because their lives stretch over millions or billions of years. But in truth, from the time they are born, they are slowly dying. 

So how is a star born?

Nebulas: The stellar nurseries

If you have ever looked through a telescope and marveled at the beauty of the horsehead nebula or the spectacular pillars of creation, you are actually looking at a giant gas cloud where stars are being born. Stars will form in the very densest part of the nebula when the gas begins to be drawn in together by the force of gravity. 

Over the course of millions of years, these dense gas clumps will grow in mass, pressure, and heat until they ignite nuclear fusion, the process in which the hydrogen molecules at the center of the mass get squeezed together, releasing a massive amount of energy—the birth of a star. 

The amount of gas left over after the star has formed will determine what planets may be able to form around it.

Main sequence stars

Once nuclear fusion has occurred, the star will remain stable (somewhat) for the rest of its life. How long that is will depend on a few characteristics. Massive stars tend only to live millions of years, whereas medium-sized stars like the sun will live for billions of years. 

Sometimes, more than one-star forms in a solar system, and you can get a binary or trinary star system. 

If you could imagine a planet forming in a star system where there is more than one star, you could imagine the sort of stories that could arise. Will there be two stars in the sky? Will one be smaller than the other? How will that shape your world’s culture?

There are many possibilities. During this period, as long as the star continues to burn hydrogen in its core, it is called a main sequence star.

Red Giant

What is born must someday die. That includes our own sun, which is about 4.5 billion years old. It is estimated that it will exist as it is now, gradually growing more luminous and powerful throughout its life cycle, which is estimated to be about 10 billion years. 

10 billion years may seem like an astronomical amount of time, but it’s not forever. The sun will begin to die when it exhausts the hydrogen available for nuclear fusion in its core. At this time, it will swell to become a red giant.

The red giant sun will swell to engulf everything out to Earth, which is obviously unfortunate for us. But don’t worry—we’ll be long gone by then; as the sun ages, it will get hotter and more powerful and eventually turn Earth into Venus. 

This is why Venus is the way it is. It was too close to the sun, and the sun’s power became too much for it. 

You could play around with this phase of life. If you are into dystopian or science fiction stories, you can imagine life on a planet slowly becoming Venus. Or maybe a planet that survives to see its sun become a red giant. Maybe the civilization must develop technology to combat this situation or develop the ability to leave. 

We’ll have to figure this out ourselves someday, after all. 

Supernova

The very end of a star depends on its size. The end will come in one terrible day for a massive or supermassive star, which is many times the size of our sun. An explosive supernova. 

Supernovas are among the most destructive explosions in the known universe. Few will survive in the vicinity of one. 

Sometimes binary star systems where one star is sucking energy from the other can end in a supernova as well. 

You can well imagine what that might look like for a civilization. Most stars massive enough to explode have lifespans of only millions of years, not billions. 

The star will likely give plenty of warning that the end is near, so you could develop a civilization that is struggling to cope with the reality of its dying sun. 

White Dwarf

After our sun goes through its red giant phase, where it has thrown off its outer shell, it will shrink in size to a tiny white cinder about the size of the Earth. In this situation, it will no longer be producing heat or light. It won’t be able to give us the life energy it had been in its lifetime. 

All the planets around it will freeze and drift away as the sun’s gravity will no longer be strong enough to support its family of planets. 

What would life be like around our sun then?

The color and classification of stars

We’ve already talked about how the size of stars matters with regard to how long they will live. But stars are classified according to their size and color, or rather, the color they appear to us.

Stars are classified using the MK system, which grades them according to their temperatures and luminosity. Stars are labeled M, K, G, F, A, B, and O, with M being the coolest and O being the hottest. 

O, B, A-type stars

These are the hottest and most massive stars, ranging in color from white to blue.

These stars would provide some problems for world-building purposes because they are short-lived, so they don’t allow much time for a planet to form and for life to start and evolve. 

But that isn’t to say you can’t use your imagination. Just because life evolved on Earth in the manner that it did doesn’t mean it will be exactly the same elsewhere. 

But if you want to stick with a realistic time frame, these O-A-type stars will be a challenge. 

Type O stars live for around 10 million years, which is a mere fraction of the lifespan of our sun. B-type stars live for around 40 million years, and A-type stars live for about a billion years. So, if an O-type star had been formed simultaneously with our sun, it would have died before Earth formed. 

An example of an O-type main sequence star is 9 Sagittarius, a massive pair of stars in a binary system. They were formed about a million years ago and are very young. If you could imagine what life was like on Earth one million years ago.  

F, G type stars

Our sun is a G-type star. These are your medium-sized stars. Type F stars are blueish white, and type G is yellow or white. 

These stars are the most conducive to life. They have a large enough lifespan to account for life’s evolution in the way it has here on Earth. When scientists search the skies for stars with possibilities of life, F—and G-type main sequence stars are the best bets because they most mimic our own sun.

K, M type stars

K and M-type stars are smaller than the sun and orange to red in color. They are the most numerous and oldest stars; M-type stars can have lifetimes of trillions of years. These are truly ancient stars!

The trade-off, though, is that because they are small, not as hot, and not as powerful. A civilization on a planet around an M-type star may live in a generally cold world, or maybe it needs to be a little closer to the star than Earth is to our sun.

And maybe a Mars-sized planet would be your best bet as well. 

The good news is that these stars are rarely alone. Many of them exist in binary or trinary star systems, so you could create a world that revolves around two small red stars. Or maybe it orbits in between them. 

How might magic affect the physics of stars?

So far, I’ve discussed stars from a purely scientific perspective. But when you are writing fiction, science becomes optional. 

Magic is a common component of fantasy fiction. How magic works in your world is completely up to you; there are no rules.

But magic can be a way to circumvent the rules of science in your world. If you want your world to revolve around an O-type star, you don’t think the 10 million-year lifespan will be enough time for life to evolve properly, and you think that the supernova finale might pose a dilemma, magic can be your solution. 

Much of how the universe works depends on the force of gravity. Gravity pulls matter into stars and planets, breaks matter apart, and attracts objects. A lot of this is up to chance. A billion things had to happen for our sun to form the way it did and for Earth to form the way it did. Anything could be different. We could have had two stars. 

But maybe another force is at work besides gravity, and maybe that other force can be magic. I like to think of magic as an invisible force, much like gravity. Maybe your world works this way, and magical forces have come together to allow your planet to exist inside a nebula, for instance, around no sun. Maybe magic takes the place of a sun. 

What would a world without a sun look like?

Culture and religion surrounding stars

The next aspect to consider with your world is how your civilization will regard this star or multiple stars in their sky. How will their culture revolve around it?

Sun worshiping 

On Earth, the sun has held an important position in the hearts and minds of people. It’s a constant in our lives. The earliest civilizations saw the sun as something to revere and worship. There are countless instances of sun worship, from the Aztecs to the Incas and many European and Asian cultures. 

The sun is seen by many as a symbol of strength and a giver of life itself. 

Would your civilization’s star be seen in a similar light? If there are two stars in the sky, would one be worshiped and not the other?

Stars and navigation

Before technology, many early mariners used the stars for navigation. Because even though we’ve established that the universe is constantly changing, it is changing slowly. 

The stars have remained more or less the same for the entirety of human existence. The night sky gave travelers a frame or reference. It changed depending on the location of the Earth and the time of year. People used them to guide them across the face of the Earth. 

Would your civilization have constellations and use them to move about their world?

Nature and agriculture and the role of stars

The final thing to consider about stars is how your star will affect nature.

Plants need sunlight for photosynthesis. Many animals eat plants, and their behavior is dictated by the sun. Do they hunt at night? Is there a night at all in your world? Does night last for years?

And, of course, the sun will dictate the seasons. Civilization on Earth evolved around farming and agriculture. We were able to move beyond our hunter-gatherer ancestors and live more sedentary lives because of agriculture and the seasons certainly played a role in that. 

Conclusion

Stars. They are the substance of the universe. And now, I hope I have helped you understand them a little better. You can use this information as a base to start your world-building process. 

In the next lesson, we will move on to planets, how they form, the types of planets you might see in a solar system, and how what your world will look like.