The supergiant galaxy known as M87 has hurled an entire star cluster out at more than three million kilometers per hour. The outcast cluster, made up of thousands of stars, is now doomed to drift through the void between the galaxies for all time.
"Astronomers have found runaway stars before, but this is the first time we've found a runaway star cluster," says Nelson Caldwell of the Harvard-Smithsonian Center for Astrophysics. Caldwell is lead author on the study, which will be published in The Astrophysical Journal Letters.
The speeding star cluster has been named HVGC-1, the initials standing for hypervelocity globular cluster. Relics of the early universe, such globular clusters usually contain thousands of stars crammed into a ball a few dozen light-years across. The Milky Way galaxy is home to about 150 globular clusters. The giant elliptical galaxy M87, in contrast, holds an estimated 15,000 star clusters.
The supergiant galaxy known as M87
M87 is a monstrous elliptical galaxy, home of several trillion stars, and at least one supermassive black hole. It weighs as much as 6 trillion Suns, making it one of the most massive galaxies in the nearby universe. It is located about 5.3 million light years away from Earth.
How did HVGC-1 get ejected at such a high speed? Astronomers aren't sure but say that one scenario depends on M87 having a pair of supermassive black holes at its core. The star cluster wandered too close to those black holes. Many of its outer stars were plucked off, but the dense core of the cluster remained intact. The two black holes then acted like a slingshot, flinging the cluster away at tremendous speed.
Having two supermassive black holes at its core must be the result of a long-ago collision between two galaxies, which merged to form a single giant galaxy. The same fate awaits our own Milky Way, which will collide with the Andromeda galaxy in a few billion years to create an ellilptical galaxy that astronomers have dubbed Milkomeda.
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