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Learn about hypergiant stars -- stars that make the sun look ridiculously tiny.

Hosted by: Hank Green
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Okay I wanna make this clear from the start. No one here is putting down the Sun.

The Sun is by far my favorite star, and it's mind-boggling how much bigger it is than Earth.  Like, if you shrunk the whole solar system down so that the Sun was the size of like a playground kickball-thing, the Earth would be a tiny speck smaller than a peppercorn.

The diameter of the Sun is about 1.2 million kilometers, more than 1000 times the diameter of Earth, so when we compare the two the Sun naturally seems huge.

But the thing is, there are stars out there that make our Sun look ridiculously tiny.  In general, the biggest stars in the galaxy are also among the most luminous.  Some of them are more than a million times as luminous as the Sun. 

And most of these super-fried stars are a class of stars that astronomers call "supergiants" or "hyper-giants".  They're actually stars with a lot more mass than the Sun that are in the final stages of their lives.

These stars have about 15 times the mass of the Sun, sometimes more, but for most of their lives they're only a few times bigger than the Sun in diameter.  Nothing to write home about.

But when they start running out of fuel and the end is rapidly approaching, they can grow to be more than 1000 times as big across as the Sun. 

That's because, in order to keep their fusion reactions going, their cores have to reach further and further out to find fresh fuel.  That outward movement of the fusion reaction pushes their outer layers away, causing these aging, high-mass stars to swell to hundreds of times larger than their original size. 

Take VY Canis Majoris, a yellow orange star nearly 5000 light-years away that has the reputation of being the largest star know. And don't get me wrong, VY Canis Majoris is a huge star; it's diameter is about 1400 times as big as the Sun's. 

If we dropped it down in the center of our solar system it would eat all of the inner planets. That's Mercury, Venus, Earth, and Mars, and keep going on unimpeded and swallow up Jupiter as well.

But despite it's reputation VY Canis Majoris isn't actually the biggest star in our galaxy. There are at least 2 other hyper-giant stars currently known that are probably even bigger and there could be even bigger stars out there that we don't know about yet.

One of the top contenders for biggest known star in the Milky Way is NML CYGNI, a star that's more than 1600 times as big across as the Sun.

Another, UY SCUTI is a hyper giant that's nearly 10,00 light-years from us. Our best measurements so far show us UY SCUTI is about 1700 times as big as the Sun.

If either of these stars were at the center of our solar system they would gobble up all of the inner planets, keep on going past Jupiter almost to the orbit of Saturn. But of course, they won't be that big forever.

These types of hyper-giant stars are really unstable and constantly pulsing, growing bigger or smaller or hotter or cooler in as little as a few years.

That's because inside a dying hyper-giant a huge number of different fusion reactions are possible; like turning Helium into Carbon or Carbon into Oxygen. And those different reactions can take place in different layers of the star at any given time. Some of them might run out of fuel and stop, while at other places new ones start.

These fluctuations in energy can sometimes push the outer surface of the star farther out and it looks bigger and cooler. Other times the star becomes bunched up more tightly making it smaller and warmer at the surface.

So what we think is the biggest star today might not be the biggest star literally tomorrow.

But regardless of whether these hyper-giants are only 1400 times as big across as the Sun or 1600 times or even 2000 times as big, these stars are all absolutely enormous.

So listen, no one loves the Sun more than I do, but the fact is there are stellar giants out there lighting up the galaxy that make our home star look practically insignificant.

I'm glad you heard it from me first, but lets just not tell the Sun about it, OK?

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