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Astronomers Discover An Entire Cluster of Stars Tossed From Its Galaxy

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Scientists have seen hypervelocity stars before, but this is the first time they've ever observed an entire star cluster thrown from its host galaxy. Named HVGC-1, it's traveling at a rate of 2 million miles per hour — and its ultimate fate is to drift through the cosmic void for all time.

Top image: Artistic impression of HVGC-1. Credit: David A. Aguilar.

HVCG stands for hypervelocity globular cluster, and it's the first time a celestial object like this has ever been observed. Globular clusters are relics of the early universe, groupings which contain thousands of stars crammed into a tight ball just a few dozen light-years across. Astronomers have documented about 150 globular clusters in the Milky Way — but they've never seen anything quite like this before.


The discovery was made Nelson Caldwell of the Harvard-Smithsonian Center for Astrophysics and his team while studying the space around the supergiant elliptical galaxy M87. The astronomers used the Hectospec instrument on the MMT Telescope in Arizona to examine the area in detail.


The runaway star cluster HVGC-1 (circled) in a photo from the Canada-France-Hawaii Telescope (CFHT). The cluster is zooming toward us at a speed of more than two million miles per hour. Credit: CFHT.

As noted, HVGC-1 is travelling at a tremendous speed. According to Caldwell, a possible reason for this rapid ejection may be on account of M87 having a pair supermassive black holes at its core. As the star cluster got closer and closer to these black holes, its outer stars were plucked off while its dense core remained intact. Then, acting like a slingshot, the cluster was flung away at great speed. It's suspected that the cluster will eventually escape the gravitational confines of M87 and drift off into intergalactic space.


This paper is set to appear in an upcoming issue of The Astrophysical Journal Letters, but it's available here. Supplementary information via Harvard-Smithsonian Center for Astrophysics.