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Galaxy Gave Star Cluster the Boot

The star cluster HVGC-1 had been part of the M87 galaxy, but now it's fleeing that galaxy at more than two million miles per hour. Clara Moskowitz reports


If galaxies were high school lunchrooms, the star cluster named HVGC-1 would be a social pariah. This unpopular cluster has been kicked out of its galaxy and forced to wander the cosmos alone.

HVGC-1 stands for hypervelocity globular cluster 1. Its thousands of stars had been part of the M87 galaxy, about 50 million light-years away. But now the cluster is fleeing that galaxy at more than two million miles per hour. Researchers reported the discovery in The Astrophysical Journal Letters. [Nelson Caldwell et al, A Globular Cluster Toward M87 with a Radial Velocity < -1000 km/s: The First Hypervelocity Cluster]

Astronomers are not sure why HVGC-1 was exiled from M87. One theory suggests that gravitational interactions with a pair of supermassive black holes at the galaxy’s center could have kicked out the cluster. Most galaxies contain a single giant black hole. If M87 is the product of a merger of two galaxies, it might host two central black holes in a binary system, researchers say.
In fact, that same double black hole future might await the Milky Way when we collide with our neighboring galaxy, Andromeda, in a few billion years. At which point, some star cluster may find itself ejected from the Milky Way, just like the ostracized HVGC-1.
—Clara Moskowitz
[The above text is a transcript of this podcast.]

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