1
00:00:00,000 --> 00:00:06,152
Graveyard of stars black hole mystery.

2
00:00:06,195 --> 00:00:17,677
At the core of the dense globular star cluster 
NGC 6397 lies an unexpected stellar graveyard.

3
00:00:17,677 --> 00:00:27,870
Globular clusters are spherical groups 
of stars tightly bound by gravity.

4
00:00:27,870 --> 00:00:40,273
For years, astronomers have debated whether 
mid-sized black holes might exist in globular clusters.

5
00:00:40,273 --> 00:00:51,663
These “intermediate-mass” black holes are the 
long-sought missing link in black hole evolution. 

6
00:00:51,663 --> 00:01:03,053
They are smaller than the supermassive black holes 
that lie at the centers of large galaxies...

7
00:01:03,053 --> 00:01:13,706
...but larger than stellar-mass black holes 
formed when massive stars collapse.

8
00:01:13,706 --> 00:01:25,096
Scientists hoped to find an intermediate-mass
black hole at the heart of this globular cluster...

9
00:01:25,096 --> 00:01:35,842
...but instead, they found stellar-mass black holes 
lurking there, the remains of collapsed stars.

10
00:01:35,842 --> 00:01:42,077
Astronomers found them using observations 
from the Hubble Space Telescope…

11
00:01:42,077 --> 00:01:48,220
...combined with data from the Gaia space observatory.

12
00:01:48,220 --> 00:01:56,848
Spanning several years, these observations 
measured tiny apparent motions of stars in the cluster.

13
00:01:56,848 --> 00:02:08,882
From these motions, this research calculated
how mass is distributed throughout the cluster.

14
00:02:08,882 --> 00:02:17,003
The results suggested the stars were orbiting 
not one intermediate-mass black hole... 

15
00:02:17,003 --> 00:02:30,000
...but a tight concentration of stellar remnants
with small black holes dominating its mass.

16
00:02:30,001 --> 00:02:41,575
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.

17
00:02:41,575 --> 00:02:46,916
[ SILENCE ]