Our sun is pretty big it's so big that  about a million Earths could fit inside  

but how does our life-sustaining ball of  plasma compare to some of the giants out there  

when you start to think about it the scale  of it all becomes almost incomprehensible  

some stars are a thousand times larger than  our sun others are millions of times larger  

and the biggest are nearly 10 billion times  larger than a sun that's already a million  

times the size of our pale blue dot welcome  back to fact nominal today we're exploring  

some of the largest stars in the universe  and whether or not they should even exist

astronomers measure a star from its photosphere  or the outer surface of the star from which light  

is emitted they compare the size of the star to  our own sun using a term called solar radii so  

if you have a star that measures 10 solar radii  that means 10 suns would line up neatly across  

its diameter according to theoretical models a  star should max out at 1500 solar radii this is  

mind-numbingly large stars that have a big radius  of around 648 million miles if you compare by  

volume 800 million suns could fit into a star that  big but we've found a couple stars that are even  

bigger up until recently astronomers thought  that the largest observed star was UY Scuti.  

UY Scuti is a part of the constellation Scutum in  our Milky Way galaxy it's not visible to the naked  

eye because it sits in a part of the galaxy whose  stars are blocked by light from the milky way's  

core but astronomers found it back in 1860 and it  has turned out to be very very big nearly 10 000  

light years away from earth UY Scuti was thought  to have a solar radius of seventeen hundred with a  

margin of error of about two hundred that means  about five billion suns could fit inside it  

UY Scuti has a circumference larger than the  orbit of Jupiter around the sun it's so big that  

some hypothetical object traveling at the speed of  light would take seven hours to travel around it  

if there was a planet orbiting Scuti it would have  to be over 600 astronomical units away from the  

massive sun one astronomical unit is the distance  from earth to our sun about 93 million miles to  

put into perspective just how far 600 astronomical  units is Pluto is a mere 39 astronomical units  

away from the sun however recent measurements  from the Gaia space telescope have thrown a wet  

blanket on the hype surrounding UY Scuti revealing  that it might only be some 5,000 light years away  

making its solar radius around 755 still  huge but nothing compared to Stephenson 2-18

in 2021 Stephenson 2-18 took the crown as the  largest known star like ui Scuti stevenson 2-18  

is also in the constellation scutum but it's  a lot farther away some 19 000 light years  

away from earth it's part of a cluster of  26 super giant stars in the region called  

Stephenson ii but is brighter larger and more of  an outlier than any of its brothers and sisters  

if the distance of Stephenson 2-18 is accurate  it means it measures in at a whopping 2150 solar  

radii that's 10 billion times the volume of  our sun its circumference would reach farther  

than saturn's orbit around the sun an object  traveling at the speed of light would take nine  

hours to make a complete loop around Stephenson  2-18 what's more Stephenson 2-18 is relatively  

young and it's still growing if this is true  then it could be on its way to transitioning  

into a luminous blue variable star an incredibly  rare type of hyper giant star that occurs when a  

super giant ejects its outer layers and becomes  incredibly unstable and variable in luminosity  

but doesn't quite supernova instead luminous  blue variable stars undergo giant outbursts  

that resemble mini supernovas and are  sometimes called supernova imposters  

Stephenson 2-18 could also turn into a wolf ray  at star these stars have completely lost their  

hydrogen and emit about 10 suns worth of material  every million years as their cores collapse they  

can either burst into supernovas or condense  further into black holes depending on their masses  

one of the most puzzling things about Stephenson  2-18 is its intense luminosity it's 440,000 times  

brighter than the sun this is a number that was  thought to be impossible based on the Eddington  

limit of luminosity which is around 320,000 times  that of our sun the Eddington limit is basically  

the maximum luminosity or brightness a star can  achieve without bursting apart by balancing itself  

in a state of hydrostatic equilibrium between  the outer force of its radiation and the inward  

force of its gravity how can Stephenson 2-18 be so  big and so bright somehow define the mathematical  

models of the universe before we answer that let's  see how it could get so big in the first place

stars are born from interstellar dust clouds  that can be hundreds of light years in diameter  

at first these clouds are cool and diffuse and  consist mostly of hydrogen and helium over time  

gravity pulls more and more atoms together  within the cloud adding to its mass if a  

cloud becomes massive enough it becomes  unstable and undergoes gravitational collapse  

as the cloud collapses it gets hotter and hotter  as hydrogen and helium atoms pack tighter together  

within the core of the cloud more mass means  more gravity which sucks in more and more  

cosmic material in a process called accretion a  protostar begins forming cloud collapses usually  

result in hundreds to tens of thousands of stars  forming almost simultaneously creating embedded  

stellar clusters basically a bunch of stars still  surrounded by the remnants of their mother cloud  

there are a few situations that can trigger the  gravitational collapse that leads to the birth  

of stars helium and hydrogen aren't enough to  collapse a cloud there needs to be traces of  

other heavier elements these can come from the  remnants of older stars that have gone supernova  

and send shocked matter into the clouds at high  speeds the collision of entire galaxies produce  

enormous amounts of energy that trigger starburst  formations as the gas clouds from each galaxy are  

squashed into each other jets from supermassive  black holes have also been proposed as stimulating  

star formation injecting energy into the stellar  cloud and catalyzing its collapse whatever the  

cause at a certain point the atoms within the  condensing cloud become so tightly compressed  

that they're stripped of their electrons  leaving highly charged ions with exposed nuclei  

the rapid compression and super high temperatures  of these exposed hydrogen and helium nuclei  

sets off nuclear fusion which creates a star's  fusion plasma generator once the star has stopped  

collapsing in on itself and nuclear fusion  is humming within its core the plasma of the  

star starts to expand however the expansion is  counteracted by the stars own gravity and the  

plasma is pulled back in hydrostatic equilibrium  is achieved which we mentioned earlier where the  

star's gravity and its radiation pressure equalize  this results in a more or less stable star  

stars are then maintained by a continuous series  of explosions as hydrogen is fused into helium  

our sun fuses a remarkable 620 million  tons of hydrogen into helium each second

but how does a star become a giant one of the  factors is the composition and size of the gas  

cloud that formed the star and the initial size of  the protostar once a protostar has settled down it  

becomes what's known as a main sequence star these  stars can range between 1 3 to 8 times the size of  

our sun throughout a star's lifetime it burns  through hydrogen as it fuses into helium within  

its core in some of the larger main sequence  stars this process is faster because they burn  

through their hydrogen faster in smaller stars  it happens more slowly it can take anywhere from  

a few million years to a few billion for a star  to exhaust its hydrogen supply within its core  

once a star burns through its hydrogen supply  it's left with a bunch of helium helium takes  

a lot more energy and higher temperatures diffuse  which the star doesn't have so the star begins to  

actually contract as its hydrostatic equilibrium  is thrown off and gravity starts compressing in  

when it does the core is heated up  again as more matter is pulled in  

additional hydrogen is sucked in towards the  core and creates a shell around the helium  

center where fusion resumes then something called  the mirror principle occurs as the inner core  

within the shell contracts the layers outside  the shell expand and the star becomes hundreds  

of times larger meaning the core becomes hot  enough to start fusing helium to carbon when  

this happens a massive amount of energy is  released ballooning the star into a red giant  

this is what will eventually happen to our sun  however because the sun is only a medium-sized  

star it would balloon into a red giant eject most  of its gas and then compress into a white dwarf  

stars that are at least eight times the size of  our sun can undergo an additional stage where they  

become super giants if they have enough mass these  large stars can begin fusing their carbon cores  

into even heavier elements like oxygen sulfur and  silicon creating yet even more energy that powers  

a further expansion creating some of the largest  stars in the universe stars like Stephenson 2-18.

So how can a star like Stephenson 2-18 exist  if its luminosity and size seem to exceed  

the limits proposed through the mathematical  equations of the Eddington luminosity limit  

and Stellar Evolutionary Theory the first  answer is the simplest we might have gotten  

the measurements wrong calculating the distance  and luminosity of a star is pretty complicated  

determining a star's radius requires two initial  observations its angular diameter which is  

how large it appears from earth and its actual  distance from earth the angular diameter is easy  

enough to calculate but determining precisely  how far away a star is from earth is actually  

very difficult interstellar radiation plays with  the light from a star as it comes towards us  

also many giant stars like UY Scuti and Stephenson  2-18 have variable fluctuating luminosities and  

expel huge amounts of gas making them tough  to see the light from our own milky way galaxy  

also obscures many stars again making them  tough to see for nearby stars astronomers use  

what's called parallax to determine a star's  distance from the earth parallax works like  

this say you're looking out at some trees and  distant mountains from the side window of a car  

as it drives down the road you'll notice that as  you drive along the trees seem to be moving faster  

than the mountains in the background the same  thing happens in the sky nearby stars seem to move  

faster across the sky over the course of a year  by taking one measurement of a star's position  

and then another measurement six months later to  calculate the star's distance based on its angle  

but parallax measurements are only accurate  up to a distance of about 400 light years  

for more distant stars astronomers have used the  spectral data of the stars we can measure through  

parallax to figure out two elements that can help  estimate a distant star solar radius luminosity  

and temperature nearby supergiants tend  to have a lower temperature and a red hue  

and astronomers take that data and project it  onto more distant stars like Stephenson 2-18  

in short it's not the most perfectly accurate  system but it's still pretty effective

so if the measurements are correct our model of  stellar evolution seems incomplete the answers  

to why these supergiant stars are so luminous  might have to do with a phenomenon called going  

Super Eddington the Eddington limit is the maximum  luminosity a star can achieve while in a state of  

hydrostatic equilibrium when there's a balance  between its internal pressure radiating outward  

and the gravitational force pressing against it  from the outside if you remember any star larger  

than 1,500 solar radii should simply burst into a  supernova but Stephenson 2-18 with its 2,150 solar  

radii hasn't however there are several ways a star  can potentially go Super Eddington and become even  

more luminous while still retaining its shape the  first is a theory called atmospheric porosity it  

imagines an atmosphere around the star where there  are dense regions surrounded by less dense regions  

this would basically contain the mass  loss of the star to the outer layers  

producing more intense luminosity while the  denser inner core of the star remains intact  

another explanation has to do with what are called  photon bubbles photon bubbles are radiation driven  

instabilities in the stellar atmosphere they are  proposed to develop within a highly radiated star  

where the radiation pressure is higher than its  internal gas pressure basically these bubbles are  

capable of transporting photons lights at faster  more efficient rates and allow massive stars to  

glow brighter there's a whole lot of uncertainty  regarding the largest stars in the universe  

for now Stephenson 2-18 takes the cake but as  we develop better satellites and are able to  

look farther into the cosmos with more precision  there seems to be no limit to what we may find  

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