In this episode of Space Sparks, ESA / Hubble summarizes their findings.
Sometimes you find something completely different in science when you look for something that you predicted. This was the case recently when astronomers using the Hubble Space Telescope examined the core of a globular cluster, a “ball” of tightly packed ancient stars called NGC 6397, in search of a central black hole of average mass. Instead, they unexpectedly found a whole swarm of small black holes, as reported by NASA and ESA on Feb. 11, 2021.
There are two established types of black holes: the stellar-mass black hole formed when a large star runs out of fuel and collapses and weighs only a few times the mass of our Sun, and the supermassive black hole believed to exist at the center of every major galaxy and contains the mass of many millions of stars. Aside from these two types, astronomers believe there should also exist an average black hole – the medium black hole, with a mass of 100 to 100,000 times the mass of our Sun – and there are multiple candidates for this one. , but only a few confirmed cases.
These researchers’ analysis of data from the Hubble Space Telescope and Gaia Space Observatory on this spherical cluster did not provide evidence of a medium-sized black hole in the core that the astronomers were looking for. Instead, they made the very first detection of a collection of black holes in the center of a spherical cluster.
Globular cluster NGC 6397, containing hundreds of thousands of stars, was imaged by the Hubble Space Telescope. The cluster’s blue stars are hotter and towards the end of their lives, while the orange are sprawling red giant stars. The white stars include stars that resemble our sun. Image via NASA / ESA / T. Brown and S. Casertano (STScI).
Astronomers focused on this particular globular cluster, NGC 6397, because it is one of the closest globular clusters to Earth at 7,800 light-years away. More importantly, scientists believe that a globular cluster is an ideal place to find medium-sized black holes because of the dense collection of stars at their centers.
Globular clusters are large globular collections of stars that orbit the peripheries of galaxies. The clusters are old, sometimes almost as old as the universe itself. A globular cluster collapsed into the core, such as NGC 6397, is one old enough to pull the heavier stars towards the center of the cluster and the younger stars towards the outer edge of the cluster. This gives the spherical cluster a very dense core.
Because these black holes are not directly observable, the astronomers measured how the stars in the cluster move – their speeds – to determine the distribution of mass in the cluster. The locations where stars were found to move faster are areas where more mass is concentrated. Yet the distribution of these stars was not limited to a single point-like central location in the core, as would be expected in the presence of a medium-sized black hole. Instead, the mass appears to be more randomly distributed, to a few percent of the size of the cluster.
Therefore, based on the evolution of the stars, the astronomers concluded that the remnants of stars in the form of black holes of stellar mass populate the innermost regions of the globular cluster. NGC 6397 could house more than 20 of these, the “lightest” kind of black holes.
This artist’s concept shows the concentration of black holes in the center of spherical cluster NGC 6397. In reality, the black holes are far too small for direct observation through existing telescopes. This globular cluster can contain more than 20 black holes. Image via ESA / Hubble / N. Bartmann.
Like many scientific discoveries, the discovery of a concentration of black holes in a spherical cluster raises more questions, the most important of which is: Could the merging of these black holes squeezed into tight spaces result in gravitational waves? Maybe scientists will find out if black holes colliding with each other are causing the gravitational ripples recently seen by some instruments, and maybe they’ll discover something they didn’t even intend to do.
In short, astronomers searched for a medium-sized black hole in the center of a globular cluster and found a swarm of smaller black holes instead.
Source: Does NGC 6397 contain a medium-sized black hole or a more diffuse inner subcluster?
Via NASA
Via ESA
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