NGC397 – Cluster of Black Holes Instead Of One?

by Drishti Ranjan

NGC397 – Cluster of Black Holes Instead Of One?

March 8, 2021

Astronomers went looking for one medium-sized black hole in the centre of a globular cluster and instead found a whole cluster of black holes, smaller in size. What does this mean? How will it change the way we look at Black Holes? Read more to find out!

 History is a testament to the fact that most life-altering discoveries were made by accident.  In yet another accidental discovery, scientists have stumbled upon a cluster of black holes, instead of one singular intermediary blackhole in the heart of the dense globular cluster NGC397.

NGC397, a cluster of Black Holes

NGC 397, a newly found cluster of Black hole.

As per NASA released statement, globular clusters are dense stellar systems that hold stars which are closely packed together. While most of the globular clusters are considered old from an astronomical point of view, but the NGC 6397 is different. Scientists have reasons to believe that it may be as old as the universe itself.

NGC397, also known as the ‘core collapsed cluster, has a very dense nucleus and is placed in the Ara cluster. It is situated 78,000 light-years away from Earth, which makes it the closest globular cluster to Earth. The cluster consists of approximately 4,00,000 stars and can be visible to the naked eye if observing under good conditions. 

Black Holes - The phenomenon

A Blackhole is an astronomical object with an extremely strong gravitational pull. The gravitational pull is so strong that not even light can escape it. Owing to this reason, radiation, matter, or any other substance may fall into it but cannot get out of it.

All Black Holes can exist at any mass. There is no precise definition of how big or small a black hole can be. Their masses are measured in terms of the mass of the sun (solar mass).

Black hole is an astronomical object with heavy gravitational pull.

However, on a comparative scale, black holes are mostly either stellar or supermassive. The stellar black holes are formed when a massive star undergoes an explosive death and leaves behind the small, heavy core of a star, which will collapse to form a black hole. This happens only in case it is massive enough and has only a few-times the mass of the sun. On the contrary, supermassive black holes exist in the centre of most galaxies and have masses ranging up to billions of solar masses!

However, there is a huge gap of almost 100- 100,000 solar masses between both types of black holes. This is where scientists believe intermediate-mass black holes exist. Intermediate mass black holes are believed to have been formed by a single black hole devouring multitude of material or by separate black holes merging together.

Intermediate-mass black holes (IMBH) are smaller than supermassive black holes that lie at the core of large galaxies. At the same time, they are larger than stellar-mass black holes formed by the collapse of massive stars. The IMBH constitute a ‘missing link’ in the evolution of Black Holes and have a lot of controversy around their mere existence, although a few candidates have been found.

NGC 6397 and the Cluster of Black Holes

This artist’s concept shows the concentration of black holes at the center of globular cluster NGC 6397. In reality, the black holes are far too small for direct observation by any existing telescopes. This globular cluster may contain more than 20 black holes.

The Hubble telescope at the Gaia observatory of the European Space Agency detected imprints of an intermediate-mass Black Hole in NGC 6397.

Since the black holes are not easy to observe or analyse, the scientists measured velocities to study the distribution of mass in the cluster. The locations on the black holes where stars move faster is exactly where more mass is concentrated. On looking closely, the distribution of these stars were not confined to one-point or a central location. Instead, it appeared the mass was more randomly distributed, unlike an intermediate black hole.

Research by Eduardo Vitral and Gary A. Mamon of the Institut d’Astrophysique de Paris(IAP) shows that a concentration in the orbit of stars was close to random throughout the globular cluster, instead of being systematically circular or elongated.
Upon the discovery, Vitral said, “We found very strong evidence for invisible mass in the dense central regions of the cluster, but we were surprised to find that this extra mass is not point-like but extended to a few per cent of the size of the cluster.

There is a possibility that it could only be made up of the remnants of massive stars like white dwarfs, neutron stars and black holes. Their inner regions must have collapsed under their own gravity once their nuclear fuel was consumed. They added that, “The stars then progressively sank to the cluster’s centre after gravitational interactions with nearby less massive stars.” The whole process must have led to the invisible concentration of the mass.

Scientists further conclude that rather than white dwarfs or neutron stars that are too faint to observe, the bulk of the unseen concentration is made of stellar-mass black holes. This has also been proven by the theory of stellar evolution. This further points to the issue that there may be a lot of other IMBHs which remain unknown, solely because they are invisible to us.

These mergers of tightly packed Cluster of Black Holes appear to exist in the form of globular clusters. Researchers note that this discovery raises the possibility that mergers of these tightly packed holes may be an important source of gravitational waves. 


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