Things That Would Like To Be Stars#

We’ll start our journey with Earth. Not to learn anything specific about it, but just to get a rough idea of how small we are compared to everything else we’ll talk about.

The smallest things that have some properties similar to stars are large gas giants or sub-brown dwarfs. Think of Jupiter, the most massive planet in our own solar system. Jupiter is eleven times larger and 317 times more massive than Earth. It’s pretty much made of the same stuff as our Sun, just way, way less of it.

Moving up in size, we encounter brown dwarfs. These are sometimes called “failed stars” – maybe a bit of a disappointment to their cosmic parents. They have a mass between 13 and 90 times the mass of Jupiter. So, even if you somehow mashed 90 Jupiters together (which would be fun to watch, I’ll admit), it still wouldn’t be enough mass to actually create a star.

Here’s something interesting about brown dwarfs: adding a lot more mass to one doesn’t make it much bigger. Instead, its insides just get denser. This increased pressure in the core can cause certain nuclear fusion reactions to happen slowly, which makes the object glow a little. So, you can think of brown dwarfs as sort of glowy gas giants that don’t neatly fit into either planet or star categories.

But we’re here to talk about real stars, not failed wannabes, so let’s move on!

Main Sequence Stars#

Once a large ball of gas gets massive enough, it passes a certain mass threshold. At this point, the pressure and temperature in its core become high and dense enough to ignite. Inside, hydrogen atoms are fused together to create helium, releasing a huge amount of energy. Stars that are doing this hydrogen fusion in their cores are called main sequence stars.

Here’s a key point about main sequence stars:

• The more massive a main sequence star is, the hotter and brighter it burns.
• But, burning so intensely means it uses up its fuel faster, leading to a shorter lifespan.

It’s also important to remember that after a star runs out of hydrogen fuel in its core, it goes through phases where it can grow to hundreds of thousands of times its original size. These giant phases, however, only last for a tiny fraction of the star’s total life. So, when we compare star sizes, we might be comparing stars at very different points in their lives. It doesn’t make them less impressive, but it’s a bit like comparing babies to grown-ups.

Okay, back to the beginning of the main sequence:

• The smallest real stars are called red dwarfs. They have about 100 times the mass of Jupiter – just barely enough mass to actually fuse hydrogen into helium. Because they aren’t very massive, they are small, not super hot, and shine quite dimly. They are unique among main sequence stars because they don’t expand into giants when they die; they just sort of fizzle out. Red dwarfs are the most common type of star in the universe by far. This is because they burn their fuel incredibly slowly, which allows them to live for up to ten trillion years – that’s a thousand times longer than the current age of the universe! For example, Barnard’s Star, one of the closest stars to Earth, is a red dwarf, but it’s too dim to see without a telescope. (By the way, there’s a whole separate video about red dwarfs if you’re curious to learn more).

• The next stage up includes stars like our Sun. Saying the Sun “dominates” the solar system is a huge understatement – it makes up a whopping 99.86% of all the solar system’s mass! The Sun burns much hotter and brighter than red dwarfs, which shortens its lifetime to about 10 billion years. Our Sun is 7 times more massive than Barnard’s Star, which makes it nearly 300 times brighter and gives it twice the surface temperature.

• Let’s go bigger! Small increases in mass lead to enormous increases in a main sequence star’s brightness. Sirius, the brightest star you see in the night sky, has 2 times the mass of the Sun and a radius 1.7 times larger. Its surface is nearly 10,000°C, making it shine 25 times brighter than the Sun. Burning that hot means its total lifespan is reduced by 4 times, down to 2.5 billion years. Stars closer to 10 times the mass of our Sun have surface temperatures near 25,000°C. The system Beta Centauri contains two stars like this, each shining with about 20,000 times the power of the Sun. That’s an incredible amount of power coming from something only roughly 13 times larger than the Sun! But they will only burn for about 20 million years. Entire generations of these bright blue stars live and die in the time it takes for the Sun to orbit the galaxy just once.

Mass vs. Size in Main Sequence Stars#

So, is the simple rule that the more massive a star is, the larger it is?

Let’s look at the most massive star we know: R136a1. This star is enormous with 315 solar masses and is nearly 9 million times brighter than the Sun. And yet, despite its tremendous mass and power, it’s barely 30 times the size of the Sun!

Why isn’t it bigger? This star is so extreme that gravity barely holds it together. It’s constantly losing material through powerful stellar winds – we’re talking about 321 thousand billion tons every single second! Stars like R136a1 are extremely rare because they bend the usual rules of star formation a little. When supermassive stars are born, they burn so hot and bright that the intense radiation and wind can actually blow away any surrounding gas that might otherwise be added to them. This naturally limits the mass of such a star to around 150 times the Sun. Stars like R136a1 were probably formed when several high-mass stars merged together in crowded star-forming regions. They burn through their core hydrogen in only a few million years, meaning they are both rare and very short-lived.

So, for main sequence stars, being super massive doesn’t necessarily mean being super sized in comparison to other stars.

Getting Bigger: The Giant Phases#

From here, the key to making the biggest stars isn’t just adding more mass during their main life. To make the biggest stars, you actually have to wait for them to start “dying”.

When main sequence stars begin to run out of the hydrogen fuel in their cores, the core starts to contract under gravity, becoming hotter and denser. This causes hydrogen fusion to start happening in a shell around the core, and this fusion is hotter and faster. This increased energy production pushes outwards against gravity, causing the star’s outer layers to swell dramatically into a giant phase. And these stars become truly, truly giant.

• Take Gacrux, for example. It’s only about 30% more massive than the Sun, but it has already swollen to about 84 times the Sun’s radius.
• Even our own Sun, when it enters the final stages of its life, will swell up and become even bigger – about 200 times its current radius! In this final, giant phase, it will actually engulf and swallow the inner planets of our solar system.

The Largest Stars: Hypergiants#

If you thought Red Giants were impressive, prepare to meet the undisputed champions of size: Hypergiants.

Hypergiants are the giant phase of the most massive stars in the universe. They develop an absolutely enormous surface area, allowing them to radiate an insane amount of light. Because they are so huge and luminous, gravity at the surface is very weak compared to the outward pressure from the hot interior. This means they are basically blowing themselves apart, losing mass constantly through powerful stellar winds.

• Blue Hypergiants: An example is the Pistol Star. It has about 25 solar masses but is 300 times the radius of the Sun. It’s called a blue hypergiant because of its energetic blue light. It’s hard to pin down its exact lifespan, but it’s probably only a few million years.

• Yellow Hypergiants: Even larger than blue hypergiants are the yellow hypergiants. The most studied one is Rho Cassiopeiae. This star is so incredibly bright that you can actually see it with your naked eye, even though it’s thousands of lightyears away from Earth! Rho Cassiopeiae has 40 solar masses but is around 500 times the radius of the Sun and 500,000 times brighter. If Earth were as close to Rho Cassiopeiae as it is to the Sun, Earth would be inside the star, and you would be very, very dead. Yellow hypergiants are very rare – only about 15 are known. This rarity suggests they might be a relatively short-lived, temporary state as a star expands or contracts between other hypergiant phases.

• Red Hypergiants: With red hypergiants, we reach the largest stars known to us. They are probably the largest stars that are even physically possible! So, who is the absolute winner of this incredible size contest? Well, honestly, we don’t actually know for sure. Red hypergiants are extremely bright and very far away. This means that even tiny uncertainties in our measurements can lead to a huge range of possible sizes. To make things trickier, these stellar behemoths are often expanding so much they are actively shedding material, which makes them harder to measure accurately. As our scientific instruments get better and we do more observations, the star currently thought to be the “largest” might change.

The star that is currently considered to be among the largest we’ve discovered is Stephenson 2-18 (St2-18). It likely started its life as a main sequence star only a few tens of times the mass of the Sun and has probably lost about half its mass by now as it evolved. While typical red hypergiants might be around 1500 times the size of the Sun, the largest rough estimate places Stephenson 2-18 at an incredible 2150 solar radii! It shines with almost half a million times the power of the Sun.

Compared to this star, our Sun seems like nothing more than a tiny grain of dust. Our brains can’t really grasp this kind of scale easily.

• If you could travel at the speed of light, it would take you about 8.7 hours to travel just once around Stephenson 2-18.
• The fastest plane on Earth would take around 500 years to do the same journey!
• If you dropped Stephenson 2-18 where our Sun is, its outer edge would reach out and fill the orbit of Saturn!

As Stephenson 2-18 continues to evolve, it would probably shed even more mass and shrink back down into another, hotter hypergiant phase. Eventually, it will build up heavy elements in its core before finally exploding in a core-collapse supernova. This explosion blasts its gas and elements back out into the galaxy, providing the raw materials for a new generation of stars of all different sizes. And so, the cycle of cosmic birth and death continues, lighting up our universe.

The Universe is BIG!#

Let’s take a step back and just appreciate the scale. The universe is incredibly big, and there are so many enormous things in it.

If you’d like to explore size and scale yourself, we have good news! We’ve created our first app, called Universe In A Nutshell, together with Tim Urban, the brilliant mind behind the website Wait But Why. With this app, you can seamlessly travel from the smallest things we know of in existence (past things like the coronavirus, human cells, and even dinosaurs) all the way up to the largest stars and galaxies, and marvel at the entire observable universe! You can learn more details about each object you encounter or simply enjoy the sheer, mind-boggling scale of it all.

The idea for the app was inspired by the fantastic Scale Of The Universe website created by the Huang twins, which we spent a lot of time with years ago. We felt it was finally time to make a version from us and Wait But Why. You can find the app in your app store. There are no in-app purchases and no ads. All future updates are included automatically. Since this is our very first app, we would absolutely love to hear your feedback so we can keep improving it over time.

If this sounds like something you’d enjoy, please download the Universe In A Nutshell app now! And if you like it and want to support us, leaving a 5-star review is a great way to help.

Just so you know, Kurzgesagt and all the projects we work on are mostly funded by viewers like you! So, if you enjoy the app, it helps us keep going and encourages us to create more digital things in the future.

Thank you for watching!