What Made Black Hole Stars So Special (And Weird)?#

Besides their incredible size, the weirdest thing about these stars was what was hidden deep inside them: a constantly growing black hole, like a cosmic parasite.

It sounds impossible, right? But Black Hole Stars challenge everything we thought we knew about how stars are born and grow. They could only exist for a short time in the early universe, but if they did exist, they might solve one of the biggest puzzles in understanding the cosmos.

Scale That’s Hard to Imagine#

Let’s get a sense of their size.

• The biggest stars we know today might have around 300 times the mass of our sun (300 solar masses).
• A Black Hole Star could have up to 10 million solar masses, made of almost pure hydrogen.

To help picture this:

• Compare our Sun
• Then a star like Weson LL pegasi (the largest star known today)
• Finally, the Black Hole Star.

Its scale is unbelievable.

• Over 800,000 times wider than our sun.
• About 380 times larger than the largest star we know today.

And hidden far below its surface? A black hole, gobbling up billions upon billions of tons of matter every second, growing incredibly fast.

How Normal Stars Are Born (And Why They Have Limits)#

Normally, stars start from giant clouds of gas, mostly hydrogen, containing thousands or even millions of solar masses.

1. Matter starts clumping together in the densest spots.
2. These clumps get denser, their gravity pulls in more matter, and they grow faster.
3. Eventually, they get so hot and dense in the center that nuclear fusion starts – a new star is born!

But there’s a catch, and this limits how big normal stars can get:

• Nuclear fusion creates lots of radiation energy.
• This radiation is so powerful it blows away the surrounding gas cloud.
• The new star can’t collect any more mass.

From that point on, a normal star lives in a delicate balance:

• Gravity: Pulling inwards, trying to crush the star.
• Radiation: Pushing outwards from fusion, trying to blow the star apart.

After millions or billions of years, the star’s core runs out of fuel. The balance breaks, and the star is destroyed.

The Early Universe: A Unique Nursery#

Black Hole Stars were totally different. They were “beasts” of the early universe.

• A few hundred million years after the Big Bang.
• The universe was much smaller, so all the matter was much more concentrated.
• It was denser and hotter than today.

In this unique environment:

• Dark Matter was everywhere, forming giant structures called Dark Matter Halos.
• These halos had immense gravity, pulling in and concentrating vast amounts of hydrogen gas.
• This created the birthplaces for the first stars and galaxies.
• Giant clouds of hydrogen formed, some as massive as 100 Million Suns – more than the mass of small galaxies today.

This environment was unique and will never exist again.

How Black Hole Stars Formed and Grew#

The enormous gravitational pull of those Dark Matter Halos drew gas right into the center of these immense clouds, setting the stage for extremely massive stars.

Remember how a normal newborn star blows away its gas cloud? That didn’t happen here.

• These early universe gas clouds were so unbelievably large and massive.
• Even after a star was born in the center, more and more gas kept piling onto the new star.
• The young star was forced to keep growing, reaching those unbelievable sizes, up to 10 million times the mass of our sun.

Crushed by all that gravity, the core got hotter and hotter, desperately trying to push back outwards with fusion. But it was useless. There was just too much mass and too much pressure. The normal balance was impossible.

It was like a supernova happening way too fast – the core collapsed and was crushed into a black hole.

Surviving Their Own Death#

Normally, that’s the end of the story. A star goes supernova, a black hole forms, and things settle down (relatively speaking!).

But in the case of these giants, the star survived its own death.

• A huge explosion ripped through the star from the inside.
• But the star was so incredibly large and massive that even this supernova wasn’t enough to tear it apart completely.

So now, it had a black hole for a heart! At first, this black hole would be tiny – maybe a few tens of kilometers across – sitting right in the center of a star as big as our entire solar system. And this monster would grow.

The Black Hole Inside: A Force-Fed Parasite#

Stars form from spinning gas, so they spin too. When a black hole forms from a star’s core, it keeps that spin (angular momentum).

• Matter falling towards the spinning black hole doesn’t go straight in.
• It orbits faster and faster, forming a flattened disc called an accretion disc.
• Gas in this disc orbits at almost the speed of light.
• In normal situations, only a little gas actually falls into the black hole at any moment. It’s like a black hole has tons of food but only “nibbles”.

The matter in the accretion disc has a rough time, though.

• Friction and collisions heat it up to millions of degrees.
• Actively feeding black holes have incredibly hot, powerful accretion discs.
• This heat and the radiation it creates actually limit how fast the black hole can eat! Just like a star’s core pushes gas away, the radiation from the super-hot disc blows away most of the food nearby. So, even with lots of gas around, a normal black hole grows slowly.

But a black hole inside a Black Hole Star is different!

• The immense pressure from the surrounding star pushes matter directly down towards the black hole.
• This overcomes the normal restrictions on how fast it can consume material.
• This “force-feeding” is so violent and releases so much energy that the accretion disc gets hotter and releases more radiation pressure than any star core ever could!
• This incredible outward radiation pressure is enough to push back against the crushing weight of 10 million Suns.

An unbelievably dangerous balance was created: millions of solar masses pushing in, met by the fierce radiation of a force-fed black hole pushing out.

Life, Death, and the Aftermath#

This impossible balance lasted for a few million years.

• During this time, the Black Hole Star was consumed from the inside by its parasite.
• The black hole grew significantly, reaching thousands of solar masses.
• And the bigger the black hole got, the faster it ate, which heated the star up even more, causing it to expand.

In its final stage:

• The Black Hole Star expanded to be over 30 times wider than our entire solar system. Truly the largest star that could ever exist.
• Intense magnetic fields around the core shot out powerful jets of plasma from the black hole’s poles. These jets pierced through the star and shot out into space, making it a bright cosmic beacon. It must have been an awe-inspiring sight!

But this was also the end.

• The star got too stretched out.
• The accretion disc inside became too powerful.
• The parasite finally destroyed its host, blowing the star apart.

What was left? A black hole with a mass of perhaps 100,000 solar masses, ripping its way out to find new things to consume, leaving behind only the remnants of the star it devoured.

The Supermassive Question#

If Black Hole Stars existed, they could help answer one of the biggest mysteries in the universe: the existence of Supermassive Black Holes (SMBHs) at the centers of galaxies.

• Many SMBHs are just too big, especially the ones we see early in the universe. They don’t seem possible based on normal growth rates.
• Black holes from regular supernovae are only a few tens of solar masses at most.
• They grow slowly afterward because of the reasons we explained (accretion disc limits).
• Black holes can merge to form slightly larger ones (over 100 solar masses), but even with mergers and slow accretion, it should take billions of years to make black holes with hundreds of thousands or millions of solar masses.

Yet, we’ve found evidence that some Supermassive Black Holes had masses of 800 million solar masses only 690 million years after the Big Bang! That’s just not enough time for slow growth from small seeds.

Black Hole Stars offer a sort of cosmic “cheat code”.

• If they formed very early on, the black holes they left behind were already thousands or even 100,000 solar masses.
• These massive black holes could act as “seeds” for SMBHs.
• These large seeds could settle in the centers of the earliest galaxies, merging with others and pulling in lots of matter, allowing them to grow into SMBHs quickly and reliably.

Looking for Evidence#

We might get proof that these incredible stars existed very soon.

• The James Webb Space Telescope (JWST) is looking out into the universe farther than ever before.
• Looking farther means looking back in time, specifically to the early universe that was previously hidden from us.

So, with a bit of luck, the JWST might catch glimpses of these “tragic titans” during their short, violent lives between forming and being destroyed.

Until then, let’s just appreciate the visual – stars are big, but Black Hole Stars were on a whole other level!