The Largest Star in the Universe – Size Comparison - video Dailymotion

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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.

The Largest Star in the Universe – Size Comparison

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