Within the Milky Way galaxy, it is estimated by NASA that between 10 million
and a billion black holes exist, most of which are thought to lead fairly isolated lives.
But astronomers using the Hubble space telescope have recently announced that they have discovered
something unexpected, not one, or even two, but a swarm of black holes concentrated at the heart
of an ancient, enormous cosmic structure that is almost as old as the universe it's self.
For the first time, astronomers from the Paris Institute of astrophysics have detected a swarm
of black holes crowding the core of a dense globular cluster named NGC 6397.
A Globular cluster is a densely packed collection of ancient stars
and NGC 6397 is just one of more than 150 known to exist within the Milky Way Galaxy.
The enormous cosmic structure contains around 400,000 stars, most of which are tightly bound
by gravity and populate the centre. It is located only
7,800 light-years away and is one of the closest to Earth, making it one of the easiest to study.
Astronomers studying NGC 6397 originally thought that at the centre of this massive star cluster
lurked the elusive and long sought after "intermediate-mass black hole".
This type of black hole is thought of as the "missing link" between the already known,
supermassive black hole, which can be found at the heart of most large galaxies
and the smaller, stellar black hole, which are the remains of collapsed massive stars.
Although the existence of intermediate-mass black holes is widely debated,
a few candidates have been identified and the centre of NGC 6397 was thought of as an ideal
target. But when astronomers looked for this elusive object,
they were surprised to find not one, but more than 20 stellar black holes instead.
Because black holes cannot be observed directly, astronomers used the Hubble space telescope and
the Gaia space observatory to peer inside of the globular cluster and measure how the
stars were moving. Because the more mass at one location, the faster the stars travel around it.
Over several years of observing the tiny movements made by the tightly packed stars, astronomers were
able to calculate the distribution of mass within the centre. But instead of finding a
point-like mass affecting the stars around it, which would be expected for a single,
solitary intermediate-mass black hole, they found a mass that appeared to be spread more randomly.
Suggesting that the stars near the centre of NGC 6397 were being affected
by a concentration of stellar corpses, including white dwarfs and neutron stars,
but mainly multiple stellar black holes. The researchers concluded that this
mysterious family of at least 20 black holes must have progressively sank to the centre
of the globular cluster, after gravitational interactions with nearby less massive stars.
The discovery of this black hole family, also offers an exciting possibility according to
astronomers, the possibility that the tightly packed horde of black holes may one day
merge, causing gravitational waves to ripple
through space-time. An event that scientists would be able to detect
using The laser interferometer gravitational-wave observatory or LIGO as its more commonly known.
Like most scientific discoveries, more questions are raised than answered
and although the elusive intermediate-mass black hole wasn't identified, a swarm of stellar black
holes at the centre of a dense globular cluster was for the first time, and I'm sure that we will
hear much more about this mysterious stellar graveyard as further studies are completed.