The Milky Way, which is the name of the galaxy that contains our sun , is an average-sized galaxy, much like the earth is an average-sized planet. Like most if not all galaxies, as we are increasingly finding out, it contains a black hole at its' center, estimated to be about four million times the mass of the sun. M87, however, is an enormous elliptical galaxy, far bigger than the Milky Way, and is thought to have an enormous black black hole at its center five to eight billion times the mass of the sun. This black hole is so energetic that it ejects a powerful jet that can be seen by telescopes here on earth . And now it has apparently ejected an entire mini-star cluster right at us at a speed of more than two million miles per hour (see photo). The newly discovered cluster, which astronomers have named HVGC-1, is now on a journey to nowhere. Its fate: 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 is published in The Astrophysical Journal Letters.
The "HVGC" in HVGC-1 stands for hypervelocity globular cluster. Globular clusters are relics of the early universe. These groupings 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 thousands.
It took a stroke of luck to find HVGC-1. The discovery team has spent years studying the space around M87. They first sorted targets by color to separate stars and galaxies from globular clusters. Then they used the Hectospec instrument on the MMT Telescope in Arizona to examine hundreds of globular clusters in detail.
A computer automatically analyzed the data and calculated the speed of every cluster. Any oddities were examined by hand. Most of those turned out to be glitches, but HVGC-1 was different. Its surprisingly high velocity was real.
"We didn't expect to find anything moving that fast," says Jay Strader of Michigan State Univ., a co-author on the study.
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. (see last article) The star cluster evidently 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. In fact, HVGC-1 is moving so fast that it may well entirely escape the enormous gravitational pull of M87, if it hasn't done so already, and move out into intergalactic space in our general direction. However, even at a speed of two million miles per hour it will be quite some time until it reaches us, and by that time Darth Vader will probably have built another Death Star and blown us up.