The closest black holes to Earth could be within the famous star cluster Haades
Evidence of black holes has been found at one-tenth the distance of the previous closest observation to Earth. Moreover, unlike previous discoveries, this is one where anyone with access to a fairly dark sky can stick out a finger and say: “They’re there.”
The Hyades Star Cluster is overshadowed in the public’s mind by the Pleiades, which moves across the sky during winter nights in the Northern Hemisphere. This is because the chandelier is not only brighter, but also more compact. The Hyades is an older cluster, having had time to disperse a bit, but at 153 light-years away it is also the closest star cluster to Earth.
The current record holder for closest black hole is Gaia BH1, about 1,600 light-years away. Reports of the closest candidates have been refuted. However, stellar black holes form from the collapse of much larger stars, whose short lives usually occur entirely within star clusters that have not had time to disperse. The Hyades is a logical place to look for closer examples.
Dr Stefano Torniamenti from the University of Padua led a team to measure the positions and movements of the Hyades stars to see if they were under the influence of invisible gravitational forces. “Our simulations cannot simultaneously match the mass and volume of the Hyades unless some black holes were present at the center of the cluster today (or until recently),” Tornementi said in a statement.
Star movements proved less useful than hoped. However, attempts to use computer models to replicate the distribution of stars around the center of the cluster produced the best match, with two or three black holes in the Hyades. Plausible matches were also found when black holes were assumed to have been ejected about 150 million years ago. This makes sense because the Hyades, which are thought to be 600-700 million years old, have already lost a significant portion of their stars, either due to random motions or an encounter with a nearby cluster.
Professor Mark Giles from the University of Barcelona said: “This observation helps us understand how the presence of black holes affects the evolution of star clusters and how star clusters in turn contribute to the sources of gravitational waves.”
This study was made possible by the exceptional accuracy with which Gaia was able to determine the location of stars, especially those as close (in galactic terms) as the Hyades.
Although the model suggests that almost all of the remaining black holes in the cluster should have been associated with stars, no candidate stars with strong evidence of orbits around heavier objects have been identified.
There are several hundred stars in the Hyades, with a total mass of about 400 times that of the Sun, but most of them require telescopes to see. However, the cluster’s brightest members form a distinct V-shape, complemented by the nearest and brightest star Aldebaran, which lies in the same direction.
Most searches for stellar-mass black holes (as opposed to the ultra-massive kind found at the center of galaxies) have focused on globular clusters in the Milky Way’s halo. Without this regular spherical shape, open assemblages such as Hyades are difficult to explore. However, they can be found much closer to Earth, opening up exciting opportunities for study if the hunt proves successful.
The study was published in the Monthly Notices of the Royal Astronomical Society.