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At the core of the dense globular star cluster 
NGC 6397 lies an unexpected stellar graveyard.

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Globular clusters are spherical groups 
of stars tightly bound by gravity.

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For years, astronomers have debated whether 
mid-sized black holes might exist in globular clusters.

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These “intermediate-mass” black holes are the 
long-sought missing link in black hole evolution. 

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They are smaller than the supermassive black holes 
that lie at the centers of large galaxies...

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...but larger than stellar-mass black holes 
formed when massive stars collapse.

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Scientists hoped to find an intermediate-mass
black hole at the heart of this globular cluster...

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...but instead, they found stellar-mass black holes 
lurking there, the remains of collapsed stars.

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Astronomers found them using observations 
from the Hubble Space Telescope…

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...combined with data from the Gaia space observatory.

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Spanning several years, these observations 
measured tiny apparent motions of stars in the cluster.

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From these motions, this research calculated
how mass is distributed throughout the cluster.

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The results suggested the stars were orbiting 
not one intermediate-mass black hole... 

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...but a tight concentration of stellar remnants
with small black holes dominating its mass.

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This is the first time a collection of stellar-mass 
black holes has been found in the core of 
such a dense globular cluster like NGC 6397.

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[ SILENCE ]