What happens when a black hole crashes directly into a massive star?
You might expect the black hole to simply destroy the star and continue moving through space. But new research suggests something far more surprising can happen.
Under the right conditions, a stellar-mass black hole can become trapped inside a star, surrounded by a huge amount of hot gas. The result is a strange object called a “black hole star,” or BH*.
Scientists led by Shi and colleagues studied this unusual process using computer simulations, mathematical models and stellar-evolution calculations. Their work could help scientists understand how massive black holes grew in the early Universe and may even provide clues about mysterious objects discovered by the James Webb Space Telescope.
What Happens When a Black Hole Hits a Star?
Stars are enormous balls of extremely hot gas. A stellar-mass black hole is much smaller, but its gravity is incredibly strong.
When a black hole moves through a star, it has to push through the star's gas. This creates a powerful drag force.
Think about moving your hand through water. The faster you move, the more resistance you feel. A black hole moving through stellar gas experiences a similar effect, although the physics is much more extreme.
This drag removes energy and slows the black hole down.
If the black hole enters the star slowly enough, the drag can become strong enough to stop it from escaping. The black hole then remains inside the star's envelope.
If it enters too quickly, however, it can pass through the star and escape back into space.
The researchers found that the speed of the collision is therefore extremely important.
A Critical Speed
The study suggests that black holes can usually be captured when their incoming speed is below roughly twice the characteristic escape-speed scale of the star-black-hole system.
The exact result depends on several factors, including the black hole's mass, the star's mass, the angle of the collision and the speed at which the black hole approaches.
Interestingly, very small black holes can be harder to capture.
A black hole weighing around 5 times the mass of the Sun or less experiences weaker drag because of its lower mass. As a result, it may pass through the star and escape even when heavier black holes would become trapped.
When a black hole is successfully captured, the process can happen surprisingly quickly—often within only several hundred hours.
The Black Hole's Mass Makes a Huge Difference
One of the most important findings is that the final outcome depends strongly on the black hole's mass compared with the star.
The researchers studied collisions involving a 100-times-the-Sun-mass star and black holes ranging from about 5 to 100 solar masses.
They found an important limit at around 20% of the star's mass.
For a 100-solar-mass star, this corresponds to roughly 20 solar masses.
When the black hole is below this limit, the collision can create a black hole star.
When the black hole is much more massive, the collision becomes far more destructive.
The black hole deposits a huge amount of energy into the star through shocks and drag. This can disrupt the stellar material and cause large amounts of gas to escape.
What Is a Black Hole Star?
A black hole star is not an ordinary star.
It is a massive envelope of gas with a black hole sitting deep inside it.
After a suitable collision, much of the original star's material can remain around the black hole. Instead of being completely destroyed, the star's gas forms an extended structure around the black hole.
The researchers found that low-mass black holes can produce an envelope that is dense, extended and approximately spherical.
The object can reach a state called quasi-hydrostatic equilibrium. In simple terms, this means the pressure inside the envelope can balance its gravity well enough to keep the structure together.
The black hole remains at the center while the surrounding gas forms a giant envelope.
This is why scientists call it a black hole star.
This Idea Has Been Around for Decades
The idea of putting a compact object inside a star is not completely new.
Scientists Kip Thorne and Anna Żytkow proposed a strange object known as a Thorne–Żytkow object.
In this theoretical object, a neutron star becomes trapped inside a giant star.
The neutron star sits at the center while the surrounding stellar material forms a huge envelope.
A black hole star is somewhat similar.
The major difference is that the central object is a black hole rather than a neutron star.
The black hole can release energy as gas moves toward it. This energy may help provide pressure and support to the surrounding envelope.
Why Doesn't the Black Hole Immediately Destroy the Star?
A major question is whether the enormous heat produced during the collision would cause the entire envelope to explode or rapidly expand.
The researchers investigated this using stellar-evolution calculations.
Their results suggest that, for sufficiently low-mass black holes, the heated envelope can gradually lose its extra heat and settle down instead of undergoing a runaway expansion.
This is important because it suggests that a black hole star could exist for a significant period after its formation.
However, scientists still need to study what happens over much longer periods.
The black hole will continue to interact with its surroundings, and it may gradually consume material from the envelope.
The Collision Also Mixes the Star
The collision does more than simply trap the black hole.
It can also change the star's internal chemistry.
As the black hole moves through the star, it creates a powerful shock wave behind it. This shock can move hydrogen-rich material from the outer layers toward the core.
In one of the researchers' tests, the amount of hydrogen in the core increased from around 20% to about 30%.
This mixing could affect how the black hole star evolves later.
The newly formed object is also much more inflated than the original star, while the central region around the black hole can remain relatively dense.
Where Could These Collisions Happen?
These collisions are unlikely to happen frequently in normal regions of space.
However, some parts of the Universe are extremely crowded.
One example is a globular cluster, where huge numbers of stars are packed into a relatively small volume.
Another is a nuclear star cluster, located around the center of a galaxy. These regions can contain enormous numbers of stars and black holes.
There is another important environment: active galactic nucleus (AGN) disks.
An AGN contains a supermassive black hole surrounded by a bright, gas-rich disk. Stars and smaller black holes can become trapped within this disk.
Because many objects are moving through the same region, collisions may happen more often.
A Possible Way to Build Bigger Black Holes
The most exciting possibility is that black hole stars could help create much larger black holes.
Imagine a dense stellar environment containing many stars and stellar-mass black holes.
One black hole collides with a massive star and becomes trapped.
Later, another black hole could enter the same stellar envelope.
The black holes could move toward the center because of interactions with the surrounding gas. Eventually, they might form a binary system and merge.
If this process happens repeatedly, multiple smaller black holes could combine into a much larger one.
This could provide a possible pathway for creating intermediate-mass black holes, which are heavier than ordinary stellar black holes but lighter than the supermassive black holes found at the centers of galaxies.
Black Hole Mergers Inside a Star
There is another fascinating possibility.
Two black holes trapped inside the same stellar envelope could eventually form a binary.
The surrounding gas would create friction and remove energy from their motion. This could cause the black holes to move closer together.
Eventually, they could merge and produce gravitational waves.
The researchers suggest that some such mergers could occur on timescales of only years.
Because the merger would happen inside a large amount of gas, it could be different from an ordinary black-hole merger happening in nearly empty space.
The gas could potentially affect the gravitational-wave signal and might even produce an electromagnetic event.
Could Black Hole Stars Explain JWST's “Little Red Dots”?
Perhaps the most interesting connection is with a mysterious population of objects discovered by the James Webb Space Telescope (JWST).
Astronomers have found many compact, unusually red objects at very high redshifts, meaning they existed when the Universe was much younger.
Scientists have nicknamed them “little red dots.”
These objects have unusual features, including red optical colors, strong hydrogen emission lines and signs of very fast-moving gas.
One possible explanation is that some of these objects contain massive black holes surrounded by large amounts of gas.
A black hole star could potentially create similar conditions.
Its enormous envelope could produce relatively cool-looking radiation with a temperature of around 5,000 kelvin, while the extended atmosphere could produce some of the spectral features seen in these distant objects.
This does not mean scientists have proven that little red dots are black hole stars. Other explanations are still being studied.
But the new research provides an interesting physical mechanism for creating these unusual objects.
A New Cosmic Possibility
The study shows that a black hole crashing into a star does not always mean the star is completely destroyed.
If the black hole is small enough and the collision is slow enough, gas drag can trap the black hole inside the star.
The result can be a strange, enormous object with a black hole at its center and a massive stellar envelope surrounding it.
For black holes below roughly 20% of the star's mass, this process can create a relatively stable black hole star. More massive black holes are more likely to heat and disrupt the star, causing large amounts of material to escape.
If these collisions happen repeatedly in dense stellar environments, they could help black holes grow rapidly.
And in the early Universe, this process might have contributed to the formation of massive black holes and possibly some of the mysterious little red dots observed by JWST.
Scientists still need to understand how long these objects survive, how quickly their black holes grow and what exact signals they would produce.
But the idea is remarkable: a black hole may not always destroy a star. Under the right conditions, it can turn the star into a completely new cosmic object—with the black hole itself hiding at the center.
Reference: Yanlong Shi, Qingru Hu, Zhenghao Xu, Douglas N. C. Lin, Norman Murray, "Formation of black hole stars via star--black hole collisions", Arxiv, 2026. https://arxiv.org/abs/2608.27596
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