How does a globular cluster die? Although this question sounds medical, it actually comes from astronomy. Globular clusters are remnants of the early days of the universe – anywhere from 100,000 to a million ancient stars, all spawned by a single cloud of gas, linked in a loose spherical shape. The Milky Way’s outer edge is ringed with about 150 of them – a large number considering how old they are -- hence the question about how they die. Palomar 5, located in the Serpent constellation (visible in the northern hemisphere), may provide the answer and it involves a mysterious process that doesn’t sound very pleasant – it may be consumed from within by a swarm of black holes.
“In this scenario, Palomar 5 formed with a ‘normal’ black hole mass fraction of a few per cent, but stars were lost at a higher rate than black holes, such that the black hole fraction gradually increased. This inflated the cluster, enhancing tidal stripping and tail formation.”
In a new study published in the journal Nature Astronomy, Professor Mark Gieles, an astronomer in the Institute of Cosmos Sciences at the University of Barcelona, and his team first describe the reasons why Palomar 5 caught their eyes – it began sparsely populated with stars, has very few black holes, and is distinguished by its unique “tidal tails” which span over 20 degrees across the sky. Astronomers believe these tails are made of stars the cluster has kicked out – not a good thing for Palomar 5 because that is slowly giving its black holes the upper hand. And when black holes get the upper hand in a low-mass globular cluster like Palomar 5, things can get weird … and deadly.
“This inflated the cluster, enhancing tidal stripping and tail formation. A billion years from now, the cluster will dissolve as a 100% black hole cluster. Initially denser clusters end up with lower black hole fractions, smaller sizes and no observable tails.”
The tail is the telltale sign of an eventual death-by-black-hole scenario for a globular cluster. The lack of a tail means stars have the upper hand and the gravity of the few black holes living amongst is too weak to throw them out, thus forming a tail. Palomar 5 began in a bad situation with a low density and lots of black holes. Not only does their gravity fling out stars, it also propels the black holes around Palomar 5 – causing them to become like galactic robot vacuum cleaners ridding the globular cluster of star dirt while bouncing around like a pool table break shot.
As usual, what’s bad for stars in globular clusters is good for astronomers.
“Black hole-dominated, extended star clusters are therefore the likely progenitors of the recently discovered thin stellar streams in the Galactic halo.”
The black holes are expected to eliminate the last of Palomar 5’s stars in about a billion years – a galactic blink of an eye. Astronomers using ever-stronger telescopes are finding more wispy tails on globular clusters, which means they face the same fate as Palomar 5.
Is a cluster of hungry black holes dangerous to others? Let’s just say it’s good that Palomar 5 is about 65,000 light-years from Earth.