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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