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Black hole stars may have been  the largest stars to ever exist.  

They burned brighter than galaxies and were Larger  than any star today or that could ever exist in  

the future. But besides their scale, what makes  them special and weird is that deep inside, they  

were occupied by a cosmic parasite, an endlessly  hungry black hole. How is that even possible?

Black hole stars take the weirdness of black  holes and go beyond to break everything we know  

about how stars form and grow. They were only  possible during a short window of time in the  

early Universe, but if they existed, they would  solve one of the largest mysteries of cosmology. 

Black Hole Stars were excessive any way  you look at them. The most massive stars  

today may have about 300 solar masses – a  black hole star had up to 10 million solar  

masses of nearly pure hydrogen. Let us take  a moment to look at what this means visually.  

The sun. Wezen. LL Pegasi. The largest star. And  finally the black hole star. Its scale is beyond  

words: over 800,000 times wider than our Sun, 380  times larger than the largest star we know today.

And far below its surface is a black hole,  

growing rapidly as it devours billions  upon billions of tons of matter per second.

Normally, stars are born from gigantic clouds,  collections of thousands to millions of solar  

masses of mostly hydrogen. In these clouds,  matter starts to accumulate around the densest  

spots inside. As these spots get denser, their  gravitational pull intensifies and they grow  

faster. Eventually, they generate so much heat and  pressure that they ignite fusion reactions, and a  

new star is born. But this puts a limit on their  size: Nuclear fusion releases enough radiation  

energy that the surrounding gas cloud is blown  away. The new baby star can’t gather more mass.

From now on the star is living  on the edge between two forces:  

Gravity pulling in, trying to squash the  star, and radiation created by fusion,  

pushing outwards, trying to blow the star  apart. After millions to billions of years,  

the core runs out of fuel and the  balance breaks, destroying the star.

But Black hole stars were very, very different.

The Beasts of the Early Universe

A few hundred million years after the Big Bang,  when the universe was much smaller, all the matter  

in existence was much more concentrated.  The universe was much denser and hotter.  

Dark matter was a dominant player, forming  giant structures called dark matter halos. 

These dark matter halos were so massive that  they pulled in and concentrated unthinkably  

gigantic amounts of hydrogen gas, becoming the  birthplaces of the first stars and galaxies. 

Epic clouds of hydrogen formed, some as massive  as 100 million Suns, more than the mass of small  

galaxies. In this unique environment, that will  never exist again, the enormous gravitational  

pull of the dark matter halos drew gas into  its center and created extremely massive stars.

As we said before, when a star is born it blows  away the gas cloud that created it – but these  

titanic gas clouds in the early universe were so  large and massive that even after their birth,  

more and more gas piled on the newborn star,  making it grow to unbelievable proportions.

The young star is forced to grow and grow and  grow, getting more and more massive, until in  

some cases, it reaches up to ten million times the  mass of our sun. Crushed by gravity, its core gets  

hotter and hotter, desperately pushing outward,  trying to blow itself apart – but to no avail.  

There is too much mass and too much pressure.  The balance is impossible to uphold.

Like a supernova on fast forward, the  core gets crushed into a black hole.  

Normally that would be the end  – today’s stars go supernova,  

a black hole forms and things calm down. But  in this case, the star survives its own death.

A tremendous explosion rocks  the star from the inside,  

but it is not enough – the star  is so large and massive that not  

even a supernova can destroy it – but  now it has a black hole for a heart.  

It is tiny, a few tens of kilometers, in the  center of a thing the size of the solar system.

The Monster Grows

Stars are born from ever faster spinning and  collapsing gas, and so they also spin. When  

a black hole is born from the core of a star, it  keeps its angular momentum. This means that matter  

that gets drawn in doesn’t just fall in a straight  line, but instead begins orbiting the black hole,  

in smaller and smaller circles going faster  and faster. The result is an accretion disk  

where gas orbits at nearly the speed of light.  Only a small amount of gas actually falls in at  

any given moment. Basically, black holes put a  lot of food on the table and only nibble at it.

But the matter trapped in the accretion  disk doesn’t have a good time: Friction  

and collisions between particles heat it  up to temperatures of millions of degrees.  

Actively feeding black holes have accretion  disks that are incredibly hot and powerful.  

This heat from the disk further restricts how  much a black hole can devour, just like the core  

of stars, the superhot material creates radiation  that blows away most of the food within its reach.  

So even if a black hole had access to as much  food as it desired, it can only grow slowly.

A black hole embedded inside a black hole star is  different. The enormous pressure surrounding it  

pushes down matter directly into the black hole,  overcoming all restrictions on how fast it can  

consume. This process is so violent and releases  so much energy that the accretion disk becomes  

hotter and releases more radiation pressure  than any star core ever could – enough to  

push back against the weight of 10 million Suns.  An impossibly dangerous balance has been created  

– millions of solar masses pushing in, the angry  radiation of a force fed black hole pushing out.

For the next few million years, the black hole  star is consumed from within. The black hole  

grows to thousands of solar masses and the bigger  it gets, the faster it eats, which heats up the  

star even more and causes it to expand. In its  final phase, the black hole star has become over  

30 times wider than our solar system – truly,  the largest star to ever exist in the universe.  

The intense magnetic fields at its core spew  out jets of plasma from the black hole’s poles,  

which pierce through the star and shoot out  into space, turning it into a cosmic beacon.  

It must have been one of the most awe  inducing sights to ever exist in the universe.

But this also marks the end. It becomes too  stretched and the accretion disk within too  

powerful: the parasite destroys its host, blowing  it apart. A black hole with the mass of 100,000  

Suns rips its way out to hunt for new prey,  while leaving behind nothing but a star carcass.

The Supermassive Question

If Black Hole Stars existed, they could explain  one of the greatest mysteries of the Universe.

The supermassive black holes we  see at the center of galaxies  

are just … too big! They should not be possible.

Black holes born from regular supernovas  can be a few tens of solar masses at most.  

And because of the process we explained before,  they grow slowly after that. If black holes  

merge together, they can form slightly larger  black holes of over a hundred solar masses.  

It should take billions and billions of  years to make black holes with hundreds  

of thousands or even millions of solar masses.

And yet, we know that some super  massive black holes already had  

800 million solar masses only 690  million years after the Big Bang.

Black Hole Stars are a sort of black hole cheat  code. If they formed very early in our Universe  

and the black holes that emerged from them were  thousands of solar masses, then they could be the  

seeds for supermassive black holes. These seeds  could take root in the center of the earliest  

galaxies, merging with others and drawing in  enough matter to grow quickly and reliably.

Very soon, we may be able to verify their past  existence. The James Webb Space Telescope is  

turning its sensors to explore the farthest  reaches of the Universe, looking back in time,  

back to the early universe that we could not see  before. So, with luck, we might be able to witness  

glimpses of these tragic titans in the brief  moment between their formation and destruction.  

Until then, let us do the visual  journey again, just for fun.  

Stars are big – Black hole stars bigger.

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And as the year 12,022 is slowly  coming to an end (just kidding,  

time flies by way too fast) we will  take a short break to recharge our  

batteries. Thanks to all of you out  there. For all the love and support!  

We wish you a peaceful and cozy turn of the  year. See you back at full throttle in january!