ALBUQUERQUE NM - Astronomers have seen the first hard evidence of a cluster of stars being torn apart by the central mass of the Milky Way Galaxy. An event described as being so harsh that the cluster has lost more stars than it has retained due to the forces of gravity at work.
Two streams of stars have been tugged out from the so-called globular cluster over billions of years, leaving more stars in the leading and tailing streams than now remain in the cluster. By studying the shape of these two streams, researchers were able to calculate the odd path of the cluster as it careens around the galaxy.
"It's the first time we have actually managed to catch the Milky Way in the act of disrupting a globular cluster," said Eva Grebel of the Max Planck Institute for Astronomy in Germany.
Grebel and Michael Odenkirchen presented their results here today at the 200th meeting of the American Astronomical Society.
Globular clusters are relatively dense groups of stars that are typically at least 12 billion years old. They formed with the Milky Way but often orbit not in the main plane of the galaxy, where the Sun and most other stars reside, but in a vast and sparsely populated halo that surrounds the entire galaxy.
The newly studied cluster, called Palomar 5, is now calculated by the researchers to be at the furthest point in its orbit, high above the plane of the galaxy and 75,000 light-years from Earth. Its streams span more than 13,000 light-years in space. On the sky as seen from Earth, the structure occupies the space of 20 full moons. In about 100 million years it will plunge down through the galactic plane.
Meanwhile, the cluster is being torn apart by a process similar to what generates ocean tides on Earth, Odenkirchen explained. The Moon pulls the Earth more on the near side than the far side, but centrifugal force creates a bulge in the oceans on both sides of the planet.
Stars from Palomar 5 undergo similar tidal forces, Odenkirchen said, due to the relatively tremendous mass that exists at the center of the Milky Way. Stars on the near side of the cluster are pulled slightly closer to the center of the galaxy. They then have slightly less far to go to make their orbit around the galactic center, so they move ahead of the rest of the cluster.
Stars on the far side of the cluster are pulled away from the galactic center. Forced to trace a longer orbit around the galaxy, they then lag other stars on the cluster, creating a trailing stream.
Odenkirchen said the patchy nature of the two star streams indicate that the tidal disruptions must have been episodic in the past, rather than continuous. He said the data, collected as part of the Sloan Digital Sky Survey, represents the first observational proof that globular clusters are in fact torn apart in this manner, as astronomers had suspected.
The Milky Way is also known to disrupt other galaxies, called dwarf galaxies, generating similar star streams as a smaller galaxy -- which originated somewhere else -- is swallowed whole.
Eventually, studies of Palomar 5 and other globular clusters being torn apart should help astronomers improve estimates of unseen "dark matter" that is known to make up a healthy percentage of the Milky Way's mass budget, the researchers said.
advertisement
