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00:01What is the largest star in the universe, and why is it that large?
00:05And what are stars anyway?
00:08Things that would like to be stars.
00:11We begin our journey with Earth, not to learn anything, just to get a vague sense of scale.
00:17The smallest things that have some star-like properties are large gas giants or sub-brown dwarfs,
00:24like Jupiter, the most massive planet in the solar system.
00:2811 times larger and 317 times more massive than Earth, and more or less made of the same stuff as our Sun.
00:35Just much, much less of it.
00:37The transition towards stars begins with brown dwarfs, failed stars that are a huge disappointment to their mums.
00:45They have between 13 and 90 times the mass of Jupiter.
00:49So even if we took 90 Jupiters and threw them at each other, although fun to watch, it wouldn't be enough to create a star.
00:58Interestingly, adding lots of mass to a brown dwarf doesn't make it much bigger, just its insides denser.
01:05This increases the pressure in the core enough to make certain nuclear fusion reactions happen slowly, and the object glow a little.
01:13So brown dwarfs are a sort of glowy gas giant that don't fit into any category very well.
01:20But we want to talk about stars, not failed wannabe stars, so let's move on.
01:25Main sequence stars
01:27Once large gas balls pass a certain mass threshold, their cores become hot and dense enough to ignite.
01:36Hydrogen is fused to helium in their cores, releasing tremendous amounts of energy.
01:42Stars that do that are called main sequence stars.
01:45The more massive a main sequence star is, the hotter and brighter it burns, and the shorter its life is.
01:52Once the hydrogen-burning phase is over, stars grow, up to hundreds of thousands of times their original size.
02:00But these giant phases only last for a fraction of their lifespan.
02:06So we'll be comparing stars at drastically different stages in their lives.
02:10This doesn't make them less impressive, but maybe it's good to keep in mind that we'll be comparing babies to adults.
02:18Now back to the beginning.
02:20The smallest real stars are red dwarfs, about 100 times the mass of Jupiter, barely massive enough to fuse hydrogen to helium.
02:29Because they are not very massive, they are small, not very hot, and shine pretty dimly.
02:34They are the only stars in the main sequence that don't grow once they die, but sort of fizzle out.
02:42Red dwarfs are by far the most abundant type in the universe.
02:46Because they burn their fuel very slowly, it lasts them up to 10 trillion years,
02:52a thousand times the current age of the universe.
02:55For example, one of the closest stars to Earth is a red dwarf star, Barnard's star,
03:01but it shines too dimly to be seen without a telescope.
03:03We made a whole video on red dwarfs if you want to learn more.
03:08The next stage are stars like our sun.
03:11To say the sun dominates the solar system is not doing it justice,
03:15since it makes up 99.86% of all its mass.
03:20It burns far hotter and brighter than red dwarfs,
03:23which reduces its lifetime to about 10 billion years.
03:26The sun is 7 times more massive than Barnard's star,
03:30but that makes it nearly 300 times brighter, with twice its surface temperature.
03:36Let's go bigger.
03:38Small changes in mass produce enormous changes in a main sequence star's brightness.
03:43The brightest star in the night sky, Sirius, is two solar masses,
03:48with a radius 1.7 times that of the sun,
03:51but its surface is nearly 10,000 degrees Celsius,
03:55making it shine 25 times brighter.
03:58Burning that hot reduces its total lifespan by four times to 2.5 billion years.
04:04Stars close to 10 times the mass of our sun have surface temperatures near 25,000 degrees Celsius.
04:12Beta Centauri contains two of these massive stars,
04:16each shining with about 20,000 times the power of the sun.
04:20That's a lot of power coming from something only 13 times larger,
04:24but they'll only burn for about 20 million years.
04:26Entire generations of these blue stars die
04:30in the time it takes the sun to orbit the galaxy once.
04:34So is this the formula?
04:36The more massive, the larger the star.
04:40The most massive star that we know is R136A1.
04:44It has 315 solar masses
04:47and is nearly 9 million times brighter than the sun.
04:50And yet, despite its tremendous mass and power,
04:54it's barely 30 times the size of the sun.
04:56The star is so extreme and barely held together by gravity
05:01that it loses 321,000 billion tons of material through its stellar wind.
05:07Every single second.
05:09Stars of this sort are extremely rare
05:12because they break the rules of star formation a tiny bit.
05:16When supermassive stars are born,
05:18they burn extremely hot and bright,
05:20and this blows away any extra gas that could make them more massive.
05:23So the mass limit for such a star is around 150 times the sun.
05:29Stars like R136A1 are probably formed
05:33through the merger of several high-mass stars in dense star-forming regions
05:36and burn their core hydrogen in only a few million years.
05:40So this means they are rare and short-lived.
05:45From here, the trick to going bigger isn't adding more mass.
05:48To make the biggest stars, we have to kill them.
05:53Red giants.
05:54When main-sequence stars begin to exhaust the hydrogen in their core,
05:59it contracts, making it hotter and denser.
06:02This leads to hotter and faster fusion,
06:05which pushes back against gravity
06:06and makes the outer layers swell in a giant phase.
06:10And these stars become truly giant indeed.
06:15For example, Gacrox.
06:17Only 30% more massive than the sun,
06:20it has swollen to about 84 times its radius.
06:23Still, when the sun enters the last stage of its life,
06:26it will swell and become even bigger.
06:29200 times its current radius.
06:32In this final phase of its life,
06:34it will swallow the inner planets.
06:36And if you think that's impressive,
06:38let's finally introduce the largest stars in the universe,
06:42hypergiants.
06:44Hypergiants are the giant phase
06:46of the most massive stars in the universe.
06:49They have an enormous surface area
06:51that can radiate an insane amount of light.
06:55Being so large,
06:56they're basically blowing themselves apart,
06:59as gravity at the surface
07:00is too weak to hold on to the hot mass
07:03which is lifted away in powerful stellar winds.
07:07Pistol Star is 25 solar masses,
07:10but 300 times the radius of the sun,
07:12a blue hypergiant aptly named
07:14for its energetic blue starlight.
07:17It's hard to say exactly how long Pistol Star will live,
07:20but probably just a few million years.
07:23Even larger than the blue hypergiants
07:25are the yellow hypergiants.
07:27The most well-studied is Roe Cassiopeia,
07:30a star so bright
07:31it can be seen with the naked eye,
07:33although it's thousands of light-years from Earth.
07:36At 40 solar masses,
07:38this star is around 500 times the radius of the sun
07:41and 500,000 times brighter.
07:45If the Earth were as close to Roe Cassiopeia
07:47as it is to the sun,
07:49it would be inside it
07:50and you would be very dead.
07:53Yellow hypergiants are very rare, though.
07:56Only 15 are known.
07:57This means they're likely just a short-lived intermediate state
08:01as a star grows or shrinks
08:03between other phases of hypergiant-ness.
08:07With red hypergiants,
08:08we reach the largest stars known to us,
08:11probably the largest stars even possible.
08:14So, who's the winner of this insane contest?
08:18Well, the truth is,
08:20we don't know.
08:20Red hypergiants are extremely bright and far away,
08:26which means that even tiny uncertainties in our measurements
08:28can give us a huge margin of error for their sights.
08:32Worse still,
08:33red hypergiants are solar system-sized behemoths
08:36that are blowing themselves apart,
08:38which makes them harder to measure.
08:40As we do more science
08:42and our instruments improve,
08:44whatever the largest star is will change.
08:47The star that is currently thought to be among the largest we've found
08:51is Stevenson 218.
08:53It was probably born as a main sequence star
08:55a few tens of times the mass of the sun
08:57and has likely lost about half its mass by now.
09:01While typical red hypergiants
09:03are 1,500 times the size of the sun,
09:06the largest rough estimate places Stevenson 218
09:10at 2,150 solar radii
09:13and shining with almost half a million times the power of the sun.
09:18By comparison,
09:20the sun seems like a grain of dust.
09:22Our brains don't really have a way of grasping this kind of scale.
09:27Even at light speed,
09:28it would take you 8.7 hours to travel around at once.
09:32The fastest plane on Earth would take around 500 years.
09:36Dropped on the sun,
09:38it would fill Saturn's orbit.
09:41As it evolves,
09:42it will probably shed even more mass
09:44and shrink down into another hotter hypergiant phase,
09:48accumulate heavy elements in its core
09:50before finally exploding in a core-collapsed supernova,
09:54giving its gas back to the galaxy.
09:57This gas will then go on
09:59to form another generation of stars of all sizes,
10:03starting the cycle of birth and death again
10:05to light up our universe.
10:08Let's make this journey again,
10:10but this time without the talking.
10:13The universe is big.
10:15There are many large things in it.
10:17If you want to play a bit more with size stuff,
10:39we have good news.
10:40We have created our first app,
10:43Universe in a Nutshell,
10:44together with Tim Urban,
10:46the brain behind Wait But Why.
10:48You can seamlessly travel from the smallest things in existence,
10:51past the coronavirus,
10:52human cells and dinosaurs,
10:54all the way to the largest stars and galaxies,
10:57and marvel at the whole observable universe.
11:00You can learn more about each object,
11:03or simply enjoy the sheer scale of it all.
11:06The app is inspired by the Scale of the Universe website by the Huwang Twins,
11:10that we spent a lot of time with when it came out years ago,
11:13and felt that it was finally time to create a Wait But Why and Kurzgesagt version.
11:17You can get it in your app store.
11:19There are no in-app purchases and no ads.
11:22All future updates are included.
11:25And since this is our first app,
11:26we'd love to hear your feedback,
11:28so we can improve it over time.
11:30If this sounds good to you,
11:32download the Universe in a Nutshell app now,
11:34and leave us a five-star review if you want to support it.
11:37Kurzgesagt and all the projects we do
11:39are mostly funded by viewers like you.
11:43So if you like the app,
11:44we'll make more digital things in future.