the cosmic bean stalk into the kingdom of the universe's giants.
You might think our sun is pretty big,
and it is. With a radius of 700,000 km
across, if the sun were a football or a
soccer ball for you Americans out there,
the Earth would be 109 times smaller,
the size of a 2 mm wide peppercorn.
Even if you added up all the mass of all
the solar systems planets, the sun would
still be 743
times more massive than all of them
combined. But when it comes to other
stars, our sun is nothing special. Some
of our closest neighbors, the Alpha
Centauri binary stars, are a similar
size, and Sirius A is twice as big. But
how big can a star actually grow? To
understand that, we need to look at
stellar evolution.
So our journey begins here in the heart
of something known as the Herzbrung
Russell diagram. Independently invented
in the early 1910s by both Danish
astronomer Ana Herzbrung and American
Henry Norris Russell. It plots stars
temperatures against how bright or
luminous they are. And running down the
center of this diagram lie what's called
main sequence stars. This spine is where
most stars spend the majority of their
lives. If they're here, it means they're
in a stable phase of existence.
Having gone through the chaotic
molecular cloud collapse of birth, a
main sequence star is now continuously
fusing hydrogen in its core, generating
energy that pushes outward against the
brutal inward force of its own gravity.
It's a balancing act, and once it's
reached, a star is said to be in
hydrostatic equilibrium.
Almost all of the stars on the main
sequence are in this state of so-called
rest, but that doesn't mean they are all
the same. Our sun sits comfortably in
the middle of the Herzbrung Russell
diagram. It's a fairly average G-type
main sequence star. The G is what's
called the spectral class which
essentially classify stars by their
temperature and therefore color into a
seemingly arbitrary naming system of
OBFG
KM where O is the hottest and M is the
coldest. I remember it using the
pneummonic, "Oh, be a fine girl. Kiss
me. Do with that what you will." The sun
and other G-type stars have surface
temperatures around 5,778
Kelvin, giving them the yellow white hue
that we're all familiar with. Their
cores steadily fuse hydrogen into
helium, converting about 600 million
tons of hydrogen per second and emitting
energy that, in the case of our sun, has
powered life on Earth for billions of
years. But if you move up or down the
main sequence, the stars change and a
pattern begins to emerge. It's probably
not surprising that dim stars are
usually cool and that the hotter a star
gets, the brighter it gets, too, at
least on the main sequence. But what
might be less intuitive is how a star's
mass relates to this. And there is a
clear correlation. The brightest star
also tends to be the most massive ones.
Take for example Bellatrix, the 26th
brightest star in the sky and the left
shoulder of the Orion constellation.
It's a Btype star, one of the brightest
classifications, and has a surface
temperature around 22,000 Kelvin, almost
four times hotter than our sun. It's
also 8.6 times more massive. The effect
of this greater mass is to crush the
hydrogen in Bellatrix's core far more
than in other stars. Greater pressure
increases the rate of fusion reactions,
and therefore far more energy is
released. This fusion is so powerful
that it forces the star to swell. It's
as if gravity almost can't contain it.
So, Bellatrix's volume is a whopping 200
times greater than our son's. This comes
at a price. Massive main sequence stars
burn hot and fast. While the sun will
likely have a total lifespan of 10
billion years, Bellatrix has been
burning for 25 million and is only 7
million years left.
This actually makes large stars quite
rare. Their instability and rapid
existences mean we're simply less likely
to see them than their longer lived,
less massive cousins. So, is that the
answer? Then, if we want to find the
largest star, should we simply seek out
the heaviest? It's thought that stars
can't grow much bigger than 150 times
the mass of the sun without becoming so
unstable that they blow themselves
apart. However, the universe has found
ways to cheat when it comes to this
limitation.
When two massive stars collide and
merge, the resulting star is a true
beer, much larger than anything possible
through the slow devouring of an
accretion disc. Perhaps this is the
explanation for the truly staggering and
excitingly named R136A1.
potentially the most massive and most
luminous star in the universe.
At the furthest top left point on our
Herzbrung Russell diagram, R136A1
is a monster. Forget the 150 times mass
limit. This beast has been estimated to
be 265 times the mass of our sun and has
a radius 40 times larger. It's part of a
particularly rare group known as Wolf
Ray stars. We found just 220 in our
galaxy, although scientists expect there
could be as many as 2,000. They are
massive and in an advanced but short
phase of life, one that comes just
before they collapse into supernova
explosions.
Because they are so unstable, they throw
off vast amounts of plasma in great
winds, ejecting as much as 10 solar
masses every million years at speeds of
up to 3,000 km per second. But they are
also incredibly luminous with R136A1
releasing as much light in just 4
seconds as the sun produces in a year.
Although to our eyes, it's actually only
164,000 times brighter than our star
because most of its radiation is UV
light.
To be honest, we're lucky to have seen
this star at all. It will likely only
exist for 3 million years, the blink of
a cosmic eye. But this still isn't the
universe's largest star. Not by radius,
at least. Because although it is
certainly one of the most massive, there
are far less massive stars that grow
much larger.
How? It turns out the very largest stars
have a trick up their sleeves or rather
in their shells.
So far we have focused on main sequence
stars, those burning hydrogen in their
core. However, as a star dies,
eventually that hydrogen will run out.
And without the explosive energy of
fusion to keep it stable, a star's
intense gravity causes its core to start
collapsing. With this comes even greater
pressure, which once again turns up the
heat in the core. Eventually, the core
becomes hot enough to kickstart helium
fusion, causing the star to enter an
entirely new phase of life. But this
heat is also enough to warm the outer
shells of a star. They can reach
temperatures that used to only exist at
the center. And suddenly, hydrogen atoms
in the outer layers are also able to
start fusing. This causes the star to
expand dramatically as it becomes a red
giant.
Our sun's radius is currently 700,000
km. But when this process begins, the
sun will expand to a diameter of 300
million km, which will make it big
enough to consume Mercury, Venus, and
possibly even the Earth. This process
can take place in stars 0.8 8 to 8 times
the mass of the sun. A current example
of a red giant is the fascinating mirror
A, which is part of the Sus
constellation.
It's only between 1 and 1.2 times as
massive as our star and yet its radius
is at least 332 times bigger. And this
is just the baseline. Mirror A pulses
over the course of various 80 to 1,000
day cycles. When those cycles align,
mirror physically puffs up. So its
maximum radius is actually much larger.
It can reach around 42 times that of our
sun. But even a red giant isn't the
biggest type of star. When you get to
eight solar masses, another
classification appears. One that's even
bigger. The red super giant.
Towering, mighty, vastly larger than
their smaller cousins, but also doomed
to a tragic end. These monsters of the
universe will end their lives in an
explosive supernova.
But we're interested in the moments
before that when they swell to become
the largest stars we see in the
universe. Beetlejuice is one of them.
With a diameter of 1.2 billion km,
making it more than 700 times the size
of the sun. If it was at the center of
our solar system, all the rocky inner
planets would be engulfed. Even Jupiter
wouldn't escape. Now, these super giants
are violent beasts. In the last throws
of their life, they pulse and throw out
huge amounts of material, which makes it
difficult to determine where the edge of
the star ends, and space begins. But
despite its enormous size, we know even
Beetlejuice isn't the largest star out
there. There is one other candidate, and
as I hinted at at the beginning of this
video, it lurks in the constellation
Scutum.
This is Uy Scooty and it is colossal.
So vast that five billion suns could fit
within it. Surprisingly, it's not very
hot. In fact, it's 40% cooler than the
sun and glows a somber red. Another red
super giant.
Because of its vast distance and low
temperature, UI scooty is not actually
visible to the naked eye. You need
powerful binoculars or a small telescope
to spot it.
At a distance of 5 to 10,000 lighty
years, there is some uncertainty about
UY Scooty's true size with many other
candidates vying for the title of
largest known star.
But UI Scooty won't be around for much
longer. It's already 10 to 20 million
years old and may now only have a few
million years left on the clock. Within
that time, it may even get smaller,
transforming into a yellow hyper giant.
This class of star is incredibly bright.
But in order to achieve this, it would
first have to shed its outer layers,
becoming even hotter to sustain the last
possible fusion reactions.
Hyper giants are capable of blowing off
the mass of Jupiter in just one
explosive burp. And they have lots of
them. Filled with heavy elements like
oxygen, carbon, and nitrogen, these
ejected materials can form vast clouds
10,000 astronomical units in length.
That's 300 times the distance from the
sun to Neptune.
And they are vital to the universe's
development. These swirling expulsions
mix with dust clouds and material from
other stars, combining to create stellar
nurseries filled with the ingredients
for life. This is still several
hundred,000, if not a few million years
away for UI Scooty,
but there is a giant star that's even
closer to this final destruction.
We've only ever properly imaged one star
outside the Milky Way, and it was WHG64,
a red super giant, a bit like UI Scooty,
160,000 lighty years away in the large
melanic cloud. Recent studies suggest it
may have already turned into a yellow
hyper giant. In the last 10 years, it
has become dimmer as it's thrown off
material and become shrouded in dust.
The problem is once you get to these
distances, it's hard to measure things
precisely. Maybe WHG64
is actually bigger than UI Scooty.
We don't know if stars can get bigger
than this. It's unlikely as giants like
UI Scooti are scraping the edge of
what's called the Hayashi limit. This is
the maximum size a star can reach given
its mass and you can see it as a line on
the Herz Russell diagram. If a star
crosses this line, then convection
inside takes over and gravity starts to
win, making it smaller. So, one day we
might find a star slightly bigger than
UI, Scooty, but it won't be by much
unless we're missing something. Perhaps
there's a type of giant star we simply
don't have in our galaxy. We don't know.
And unfortunately, we don't have the
technology to find out yet.
What I do know is that humans can't
really wrap our heads around anything
bigger than a few thousand km. So the
scale of the biggest stars is far beyond
our comprehension. They are cosmic
monsters.
But in the grand scheme of things, even
these stars are tiny. Galaxies are tens
of thousands of light years across,
millions of light years apart, and
individual filaments of cosmic web
stretch billions of light years through
space. The universe is so vast, we can't
even begin to pretend to understand it
all. That doesn't mean we can't try,
though. And bigger isn't always better.
I think living on a tiny planet around
an average star is working out pretty
well for us so far.
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