OrbitalHub

NASA dicit:

Globular clusters are extremely dense stellar systems, which host stars that are closely packed together. These systems are also typically very old — the globular cluster at the focus of this study, NGC 6397, is almost as old as the universe itself. This cluster resides 7,800 light-years away, making it one of the closest globular clusters to Earth. Due to its very dense nucleus, it is known as a core-collapsed cluster.

At first, astronomers thought the globular cluster hosted an intermediate-mass black hole. These are the long-sought “missing link” between supermassive black holes (many millions of times our Sun’s mass) that lie at the cores of galaxies, and stellar-mass black holes (a few times our Sun’s mass) that form following the collapse of a single massive star. Their mere existence is hotly debated. Only a few candidates have been identified to date.

The researchers used previous estimates of the stars’ tiny proper motions (their apparent motions on the sky), which allow for determining their true velocities within the cluster. These precise measurements for stars in the cluster’s core could only be made with Hubble over several years of observation. The Hubble data were added to well-calibrated proper motion measurements provided by the European Space Agency’s Gaia space observatory which are less precise than Hubble’s observations in the core.

Video credit: NASA’s Goddard Space Flight Center/Paul Morris: Lead Producer/Music: “Glass Ships” by Chris Constantinou [PRS] and Paul Frazer [PRS] via Killer Tracks [BMI] and Universal Production Music/Visual Credits: Artist’s Impression of the Black Hole Concentration in NGC 6397/Video credit: ESA/Hubble, N. Bartmann/Callout of the Black Hole Concentration in NGC 6397/Video credit: ESA/Hubble, N. Bartmann/Artist Rendition of Gaia Spacecraft/Image credit: ESA, C. Carreau