Two other recent studies also proposed that stellar remnants, particularly stellar-mass black holes, could populate the inner regions of globular clusters. “We used the theory of stellar evolution to conclude that most of the extra mass we found was in the form of black holes,” said Mamon. Through a process called dynamical friction, more massive objects gradually sink to the GCs center while less massive stars migrate to the periphery. They think the CUO includes the remnants of massive stars: neutron stars, white dwarfs, and black holes. The paper’s authors say that stellar evolution explains what’s likely there. The CUO has from 1000 to 2000 solar masses whatever they are, they’re dense.

They call it a CUO: a cluster of unresolved objects. The researchers haven’t wholly concluded that the mass in the center of the GC is all black holes. Astronomers thought that the cores of GCs were a likely place to find the elusive IMBHs. “We found very strong evidence for an invisible mass in the dense core of the globular cluster, but we were surprised to find that this extra mass is not ‘point-like’ (that would be expected for a solitary massive black hole) but extended to a few percent of the size of the cluster,” said Eduardo Vitral, one of the paper’s authors.įinding a clump of stellar-mass black holes was surprising and went against expectations. The 2021 paper found something extraordinary: there wasn’t one black hole at the center of NGC 6397 there was a group of them. The black hole’s massive gravitational force affects those stars’ trajectories and velocities, and by measuring them, astronomers can estimate the black hole’s mass. Brown (STScI), Stefano Casertano (STScI), Jay Anderson (STScI)īlack holes can’t be seen, so to infer their existence, astronomers study the motion of the stars in the GCs core. The small white objects include stars like our Sun. The blue stars are massive stars near the end of their lives, and the red stars are stars that have burned most of their hydrogen and are now red giants. The globular cluster NGC 6397 glitters with the light from hundreds of thousands of stars in this Hubble Space Telescope image. In a 2021 study, astronomers looked at Hubble images and data from other facilities to examine NGC 6397 for evidence of an intermediate-mass black hole. It’s about 7,800 light-years from Earth and is one of the two closes GCs. NGC 6397 is a globular cluster in the constellation Ara. That central region is where astronomers thought IMBHs might be hiding. The stars in a GCs center are more tightly packed than the stars toward the edges. When the Hubble came along, its ability to discern individual stars in GC helped astronomers detect IMBHs. Then there are intermediate-mass black holes (IMBHs.) They can contain billions of times more mass than the Sun. Supermassive Black Holes (SMBHs) are gargantuan and reside at the center of galaxies. They range from about five to tens of solar masses. The gravitational collapse of a star forms stellar-mass black holes. There are different sizes of black holes. One burning question is whether or not GCs host black holes, much like galaxies do. But the Hubble has studied them and removed at least some of the mystery. Globular Clusters are still mysterious objects. Globular clusters are located in the halo. GCs are older than the other type of cluster, the open clusters (OCs.) GC stars have lower metallicity than stars in OCs and are some of the oldest objects in the Universe.

In spiral galaxies like ours, they’re primarily found in the galactic halo. Ground-based telescopes have to deal with the light-distorting effects of the atmosphere, but the Hubble is above it all. But the Earth’s atmosphere didn’t impede Hubble’s observations. Ground-based telescopes struggle to resolve individual stars in GCs. Above the Earth’s atmosphere, the Hubble could get clear views of these objects. But the launch of the Hubble Space Telescope revolutionized the study of globular clusters. When telescopes came along, GCs looked like blobs, and some early astronomers thought they were comets. Early astronomers couldn’t discern GCs and thought they were individual stars.

New space telescope