Astronomers are stunned to discover a so-called "middleweight" black hole, the first of its kind known to science. It is located about 13,000 light years from Earth, and it doesn't fit in any of the established categories. The team of international researchers has discovered that it is 2200 times the mass of Earth's sun, and it sits at the center of a globular star cluster.

The research, published in the journal Nature on Thursday, reveals a black hole surrounded by spherical globular star clusters. The stars of the 47 Tucanae star cluster are tightly bound by gravity as they orbit a galactic core en masse. Previously, attempts were made to search for a central black hole in the star region, but it had proven unsuccessful.

Holger Baumgardt, a researcher from the University of Queensland explains that all previously known black holes fall into only two categories. The first one is the small, stellar-mass black holes that weighed a few times the mass Earth's sun, while the other includes the supermassive black holes that could weight billions of times more. The newly discovered one is considered as a middleweight black hole that weighs 100 to 10,000 times the mass of our sun, the ABC reports.

Project leader Bulent Kiziltan, from the Harvard-Smithsonian Center for Astrophysics, says that intermediate-mass black holes were "missing links" between stellar-mass and supermassive black holes. He thinks that they may be the primordial seeds that grew into a super massive in the centers of galaxies today, The Sydney Morning Herald reports.

He also explains that the crowded center of 47 Tucanae makes it difficult to observe the motions of individual stars. This means that there needs to be a modification on what to focus on to prove the existence of a black hole in 47 Tucanae. The middleweight black hole had eluded detection for so long, leading researchers to suggest that similarly sized black holes could exist in other globular clusters.