Astronomy: Black holes throw their weight around

The globular cluster of stars, Palomar 5, will turn into a collection of tens of black holes from which the stars have been ejected in a billion years, suggests a paper published in Nature Astronomy this week. These findings have implications not only for our understanding of the evolution of Palomar 5, but also for the fate of other clusters in the Milky Way.

Palomar 5 has two defining characteristics: it is one of the sparsest star clusters in the so-called Galactic halo — the collection of old stars that surrounds the Milky Way — and it has two long ‘tails’ streaming from it, composed of ejected stars. As these tails span more than 20 degrees across the sky, Palomar 5 is a valuable cluster for enriching our knowledge of tail formation.

Mark Gieles and colleagues performed star-by-star simulations to investigate the evolution of Palomar 5. These simulations reveal that the two distinguishing features of Palomar 5 — its structure and prominent tails — could be products of a population of stellar-mass black holes that constitute around 20% of the mass of the current cluster. In the scenario presented, this fraction of mass in black holes in Palomar 5 exceeded the critical threshold where stars become lost more quickly than black holes over time. Subsequently, the cluster inflated, boosting tail formation. In a billion years, the gravitational influence of the current black hole population will eject all of the stars in the cluster, leaving only black holes.

As originally denser clusters go on to be more compact, with a smaller proportion of black holes and no observable tails, the authors conclude that black hole rich, extended star clusters like Palomar 5 are thus the possible progenitors of the recently detected, thin streams of stars that have no cluster associated with them.