Globular Clusters and Black Holes

Keeping up with an interest amongst VFO readers, we turn now to a discussion on globular clusters.
These dense concentrations of about a million stars all in rapid orbit about each other had been notoriously uninteresting, that is until recently.

It seems that globular clusters may be the most efficient place to form a closely-attached pair of black holes called a "binary black hole". Let us see how this curious arrangement may have come about.

The standard lore had been that any black holes formed in a globular cluster would get thrown out on the grounds that they did not "mix" well with the other normal shining stars. The result would be a globular cluster devoid of black holes.

Recently, using computer simulations that assign more realistic masses to the black holes of 3 - 80 times the mass of the sun, multiple research groups are finding that dozens to hundreds of black holes are retained within the globular clusters.

And it gets even better. The concentration of stars plus black holes is much much higher in globular clusters than it is in the Milky Way and other galaxies. This means that there will be a correspondingly higher incidence of black holes encountering other black holes.

Most of the time, when a black hole approaches another black hole it will zoom right past it. One can invoke a "putting green" analogy here. For strong hits of a golf ball (black hole #1) towards the hole (black hole #2), the golf ball rolls right past the hole, and the same goes for weak hits. In fact, even when you think you applied just the right amount of force to the ball, there inevitably still are a few failures.

But if you work at it long enough, and/or if you have dozens to hundreds of friends all putting on
other greens at the same time, someone will hit the ball into the "gravitational" hole and become bound to it (until you pick the ball out of the hole).

Likewise, there is only a vanishingly small probably of a black hole becoming bound to another black hole. But there are dozens to hundreds of "putts" going on continuously for billions of years, occasionally there will be successes in making this holy grail of objects, the "binary black hole."

This is important as it is when binary black holes infall into each other that "ripples" in spacetime are produced which we can and do detect from Earth. For next time: how can we go out and observationally detect a binary black hole in a globular cluster?

Dr. Brenda Frye

About Dr. Brenda Frye

Brenda L. Frye is an observational cosmologist at the Department of Astronomy/Steward Observatory, University of Arizona. She earned her Ph. D. in Astrophysics from the University of California at Berkeley, assisted by a National Science Foundation Graduate Research Fellowship.

Her thesis work involved measuring the concentration of the total mass of visible plus dark matter in the fields of massive galaxy clusters, a program requiring the use of some of the largest telescopes in the world.

Moving a mile from her Ph. D. institution, she assumed a postdoctoral position with the Supernova Cosmology Project at Lawrence Berkeley National Laboratory under the direction of Professor Saul Permutter.

She then treked across the country to take a National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship and a Princeton Council on Sciences and Technology Fellowship both at Princeton University.

Moving further east, she became a Lecturer in Physics at Dublin City University in Dublin, Ireland, where a number of European collaborations were formed.

From there she crossed back across the pond to the west coast of the U. S. to become a tenure-track Assistant Professor of Physics at the University of San Francisco.

Her travels have now landed her at her Alma Mater in Tucson, where she teaches and does research. The aims of her research continue to be to use gravitational telescopes in space as 'lenses' to study the properties of dark matter and those of distant galaxies back to when the universe was <900 million years old.

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Globular Clusters and Black Holes — 1 Comment

  1. The expected LISA 2020’s launch with its 1.5 million mile triangular 1.5 million kilometer arms will be more sensitive to these universe gravitational ripples than earth based systems. The LISA Pathfinder has experimentally proven the concept as detailed at:
    This completed LISA Pathfinder experiment has observed some gas dynamics that give new meaning to the classical PV=nRT.
    Possibly there is an interpretation of the Ernst Mach conjecture. The data is being analyzed and will be reported in the journals.

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