In April 1994, researchers, wanting harder evidence than theories and computer models, aimed the Hubble Space Telescope towards the Pegasus constellation and photographed the center of M15. In January 1996, the researchers, from the University of California, Fermi National Accelerator Laboratory, Pennsylvania State University and the Princeton Institute for Advanced Study, published the results. The pictures provided an extraordinary view of the heart of M15. Where before the best pictures had been blobs of bright light, now the astronomers could pick out more than 30,000 individual stars from the center. With the new set of pictures in hand, the astronomers are only one step away from knowing what force is jamming thousands of stars into M15.
The final step will be to snap a second set of Hubble pictures of the cluster core. Pinpointing individual stars from the first set of pictures and comparing them with a future set will allow scientists to say whether or not M15 harbors a black hole. Measuring the movement of the stars will indirectly lead to measurements of whatever is in the center. If a black hole lurks in the center, then gravity will speed up nearby stars. In contrast, a core collapse would have no effect on the speed of the stars. The speed of the stars cruising in the cluster center are known, will finally prove one of the two theories right.
Understanding the traffic patterns of the stars at the center of M15 will not only shed light on the behavior of globular clusters, but on some of the larger questions of the universe, too. Clusters are relics from the earliest days of our galaxy says Graeme Smith, of the University of California at Santa Cruz. The stars in the clusters all formed from the same cloud of gas and are pristine samples of the what the universe contained early on, and how stars change. By observing the stars in a cluster, researchers will be able to understand more about the evolution of stars and the formation of our galaxy.
Ruth Peterson of Lick Observatory says, "Globular clusters are like a giant puzzle. By going from step to step from what we know and measuring something new, like a clear image of the contents of the center and whether it contains unusual objects or even ones that are unseen altogether. It's like a detective story; one clue leads to another and that one to another when a few more suitable facts are found."