Cover Image Credit: Kurzgesagt
There are over 100 billion stars in the Milky Way alone, and quadrillions more in the universe. They make up almost all the matter in the universe, and they can be huge! Let’s look at some of the largest stars in the universe.
Things That Would Like to be Stars
Let’s start with Earth. Not to see or learn anything, but just to get a sense of scale. The most “star-like” things that aren’t stars are gas planets and brown dwarfs. Sorta like Jupiter. Jupiter is 317 times more massive than the Earth (a good approximation for pi×100), and is made up of hydrogen and helium, which is the same as the sun and most stars, just a lot less.
The transition from planets to stars begins with brown dwarves, which are failed stars. They have 13-90 times the mass of Jupiter. So even if we threw 90 Jupiters at each other, although fun to watch, wouldn’t be enough to create a star. So brown dwarves are a sort of glowy gas giants that aren’t planets or stars. But we want to see real STARS, not failed stars, so let’s move on.
Main-Sequence Stars
Once gas balls reach a certain mass, their cores become hot and dense enough to ignite, fusing hydrogen into helium. Stars that do that are called main-sequence stars. The more massive a main-sequence star is, the hotter it burns, and the shorter its life span. After the main-sequence phase of their life, stars reach a giant phase, where they grow up to hundreds of thousands of times their original size. However, this stage only lasts for a fraction of their lifespan. So just remember that we’ll be comparing babies to adults.
The smallest real stars are red dwarves. Barely able to burn hydrogen into helium (100x the mass of Jupiter), these slowly exhaust their fuel. And when I say slowly, I mean SLOWLY, burning out after about 10 trillion years! These are the only stars that don’t grow once they die! Instead, they fizzle out, like a dying campfire. Red dwarves are the most common type of star in the universe. One of the closest stars to Earth is a red dwarf. Barnard’s star is a tiny, quickly moving star that has 0.14 solar masses (masses of the sun).
The next stage is stars like our sun. The sun makes up 99.86% of the solar system’s mass, meaning that it dominates the solar system. The sun is hotter and seven times more massive than Barnard’s star, meaning that its lifespan is shortened to 10 billion years. However, that extra mass makes the sun 300 times brighter than Barnard’s star, and twice its surface temperature.
Let’s get bigger!
Tiny changes in mass cause enormous changes in brightness. For example, the brightest star in the sky. Sirius. It has twice the mass of the sun and is 1.7 times bigger than the sun. However, its surface temperature is nearly 10,000ºC, meaning it’s 25 times brighter. Burning that hot reduces its lifespan to 2.5 billion years. Stars close to 10 times the mass of the sun have surface temperatures close to 25,000ºC (baking cookies in about half a second, and burning them in the next half-second). Beta Centauri has two of these stars, both huge, shining with about 20,000 times the sun. That’s a lot of power, for an object only 13 times larger. But these stars will only burn for about 20 million years. Entire generations of these stars die, while the sun happily cruises along in its galactic orbit. So is that how to get big stars? More mass equals the largest star?
R136a1 is the most massive star we know, at about 215 solar masses. This star is over 6 million times brighter than the sun, and almost ten times hotter, at 42,000ºC. And yet, despite its crazy temperature and mass, it’s still barely 30 times the mass of the sun! Well, I say “barely,” but that is TECHNICALLY a lot of mass, and that is HUGE! But bigger things are on their way, don’t worry.
So, no, it’s not the mass that determines the size. If you add more mass, the mass is so large that the outer layers of the star will get pulled in, due to the gravity of the star. To get the biggest stars, we need to kill them.
Red Giants
When Main-Sequence Stars run out of fuel, their core contracts, meaning hotter fusion. That extra energy is used in expanding the outer layers of the star, making them MASSIVE. This phase is called the giant phase, and these stars get giant indeed.
For example, Gamma Crucis, also known as Gacrux. Only about 30% more massive than the sun, it’s swollen to over 80 times the radius! Still, when the sun enters its giant phase, it’s going to get EVEN LARGER, 200 times its current radius!! And if you think THAT’S big, let’s look at the largest stars in the universe, Hypergiants.
Hypergiants
Hypergiants have an enormous surface area that can radiate an insane amount of light. Being so large and so bright, these stars are practically blowing themselves apart, because gravity isn’t strong enough to hold onto the outer layers. Pistol star is a blue hypergiant at about 28 solar masses, and will probably only last a few million years.
Then we reach the yellow hypergiants, such as Rho Cassiopeiae! A star SO bright that we can see it without a telescope, even though it’s thousands of light-years away. At 40 solar masses, this star is 500 times larger than the sun, and 500,000 times brighter. If Rho Cassiopeiae were to replace the sun, the Earth would be inside it! Yellow hypergiants are rare, though. This means that yellow hypergiants are probably short, intermediate states as a star grows and shrinks between other phases of hugeness.
With red hypergiants, we reach the king of kings, the largest star in the universe. So, who’s the biggest?
Well… We don’t really know. Small differences in measurements can lead to huge differences in size. Worse still, red hypergiants are huge solar system-sized behemoths that are blowing themselves apart. As our accuracy of instruments gets better, the largest star will change. The star that we think could be the largest is Stephenson 2-18. It was probably born as a main-sequence star a few tens of times the mass of the sun and probably has lost about half its mass by now. While typical red hypergiants are 1,500 times the radius of the sun, the largest estimate places Stephenson 2-18 at 2150 times the radius of the sun, and shining with almost half a million times as bright! It’s so huge that even at light speed, it would take you 8.7 hours to travel around it. If we placed it on top of the sun, it would eat up Saturn. Eventually, this huge star would become a hotter, smaller hypergiant, release mass, and finally explode in a core-collapse supernova, giving its gas back to the galaxy.
In conclusion, stars are HUGE. They have super mind-boggling temperatures, brightness, and radii, and some are big enough to have just under 10 million suns in them! They are the giants of giants, in star sizes. These stars make the sun look like a grain of sand, and the Earth even smaller! Do you feel small yet?
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