Black holes are considered as the apex of astronomical curiosities that we humans have garnered for the last few decades. Last year we even succeeded in capturing an image of a black hole for the very first time since its conception. Why are we so obsessed with the black holes?? The answer lies in the obscurity of these gravitational giants. The secrets about the beginning of space and gravity are believed to lie beyond the horizons of these black holes. Humans have been delayering these secrets in order to find out what truth lies inside the core of a black hole.
A number of attempts have been made towards discovering the existence of black holes in our Milky Way galaxy and other distant galaxies. So far, we have been discovering a number of black holes in the far away galaxies but to our surprise, a black hole has been recently found right in our cosmic backyard. It’s a surprise because despite its existence being so close to our solar system, we haven’t been able to notice its existence yet.
A team of astronomers from the European Southern Observatory (ESO) in collaboration with other institutes examined a triple star system (a star system comprising three stars in a formation that the two stars orbit it other in smaller orbits and the third star orbits around the inner pair in an outer bigger orbit). Initially, upon analysing the observations of the system called HR 6819, as a part of their study of double star systems, they revealed a previously undiscovered body in the HR6819: a black hole. This path-breaking discovery was made at the ESO observatory at La Silla, Chilli through its MPG/ESO 2.2-metre telescope. The observations with the FEROS spectrograph on the telescope showed that one of the two visible stars orbits an unseen object every 40 days, while the second star is at a large distance from the inner pair.
The unseen black hole in the HR6819 belongs to the family of the very first stellar-mass black holes found that do not interact violently with their surroundings and thus accretion (disk formation) is seen around them, therefore, are called nonaccreting black holes as they appear truly black. However, its presence could be spotted and its mass could be calculated by studying the orbit of its companion star in the inner orbit. According to the theory of stellar mass limits, this invisible body fits the definition of a black hole as it has a mass four times that of the sun. According to Thomas Rivinius, Emeritus Astronomer based in Chilli, “an invisible object with a mass at least 4 times that of the sun can only be a black hole.”
The black hole found in the HR6819 is special in a way that it doesn’t react to its environment. Out of a couple of dozen black holes found in our galaxy strongly interact with their environment and make their presence known by releasing powerful X-rays. But, after the discovery of an invisible black hole, it is estimated that, over the Milky Way’s lifetime, a significant number of stars ended up as black holes as they died. Their locations in the Milky Way can now be traced by the clues that this discovery of an invisible black in the HR 6819 has provided. As Rivinius shares, “There must be hundreds of millions of black holes out there, but we know about only a few. Knowing what to look for should put us in a better position to find them.” This can be well understood as the fact that this discovery is just the ‘tip of an iceberg’.
As science lovers, we must find this discovery as a ticket to a roller coaster that runs across the deep drown secrets of our Milky Way galaxy and way into an unbridled ride of the cosmic ocean. We must understand and observe the might of gravity and the phenomena it is capable to cause. The discoveries of these triple star systems (HR 6819) provide clues about the violent cosmic mergers that release gravitational waves strong enough to reach our planet Earth. Astronomers believe that studying these systems could help them understand how stellar collisions can happen in these systems. According to Marianne Heida, a postdoctoral fellow at ESO, “by finding and studying the triple star systems like HR 6819 or LB-1, we can learn a lot about the formation and evolution of those rare stars that begin their lives with more than about 8 times the mass of the Sun and end with a supernova explosion that leaves behind a black hole.” The existence of this invisible black hole so close to our home has paved way for the possibilities for many more surprises to come in the near future. Who knows, maybe we can find a black hole right in our backyard someday.