KATHMANDU, FEBRUARY 22
Science works in a peculiar way. It puts us in awe when we least expect it. A peculiar discovery put scientists in a similar position when they found evidence of a swarm of black holes lurking at the heart of the globular cluster NGC 6397, when in fact they were searching for an intermediate-mass black hole.
Black holes are mostly either stellar or supermassive. According to NASA, the stellar black holes form when a massive star undergoes an explosive death and leaves behind the small, heavy core of a star, which, if massive enough, will collapse to form a black hole. Supermassive black holes, on the other hand, exist in the center of most galaxies, including the Milky Way Galaxy, and are astonishingly heavy with masses ranging from millions to billions of solar masses.
Other than these two, scientists believe in the existence of another "intermediate-mass" black hole (IMBH)" which astronomers think could have formed as a result of when a single black hole devours lots and lots of material, or if individual black holes merge together.
In the release issued on February 11, ESA stated that data from the Hubble Space Telescope -- a project of international cooperation between ESA and NASA -- for the first time measured the extent of the collection of these black holes.
NGC 6397 (also called core-collapsed cluster), is an old globular cluster (extremely dense stellar systems where stars are packed closely together), as old as the Universe itself, it is said, and is 7800 light-years far from the Earth, making it the closest one.
Eduardo Vitral and Gary A. Mamon of the Institut d'Astrophysique de Paris are the ones that are studying the positions and velocities of the cluster's stars.
They conducted their study on the core of NGC 6397, expecting to find evidence for an IMBH. These are smaller than the supermassive black holes that lie at the cores of large galaxies but larger than stellar-mass black holes formed by the collapse of massive stars.
"Our analysis indicated that the orbits of the stars are close to random throughout the globular cluster, rather than systematically circular or very elongated," explained Mamon.
"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 percent of the size of the cluster," added Vitral.
It is explained that this invisible component could only be made up of the remnants (white dwarfs, neutron stars, and black holes) of massive stars whose inner regions collapsed under their own gravity once their nuclear fuel was exhausted. The stars progressively sank to the cluster's centre after gravitational interactions with nearby less massive stars, leading to the small extent of the invisible mass concentration. Using the theory of stellar evolution, the scientists concluded that the bulk of the unseen concentration is made of stellar-mass black holes rather than white dwarfs or neutron stars that are too faint to observe.
Recently, researchers have also found another interesting fact about Cygnus X-1 black hole, which is the first black hole ever detected, that it is in fact bigger than previously thought.
Compiled by Kriti Joshi for The Himalayan Times.