Seventy-five years after Edwin Hubble demonstrated that the universe extended beyond the Milky Way, three University of Washington astronomers using the telescope that bears his name have made some surprising discoveries about one object of his research.
UW astronomy professor Paul Hodge and graduate students Dan Zucker and Ted Wyder hope their findings will lead to greater understanding of the evolution of one of our neighboring galaxies.
Hubble used light-wave measurements to determine that there were galaxies beyond the Milky Way, a view not commonly held when he published his findings in 1925. His research included NGC 6822, or Barnard’s Galaxy, an irregular dwarf galaxy about 1.8 million light years from the Milky Way and perhaps one-tenth our galaxy’s size.
Within NGC 6822, Hubble found three star clusters, the remnants of massive star-forming events. The clusters, each containing 100,000 to 2 million stars, are very stable and are useful in determining the age of the galaxy. In this case, Hubble thought all three clusters were similar to the “globular” star clusters in the Milky Way, which astronomers now know to be the oldest objects in our galaxy.
However, using images taken last summer by the Hubble Space Telescope’s Wide Field Planetary Camera, Hodge, Zucker and Wyder found that the three clusters are of very different ages. The stars in a cluster called Hubble VII were formed about 15 billion years ago and are about the same age as the Milky Way and the universe itself.
A second cluster called Hubble VIII contains stars about 1.8 billion years old, while the third cluster, Hubble VI, has stars that are 100 million years old. In addition, the UW astronomers found that what Hubble believed to be a fourth star cluster in NGC 6822 actually is a large gas cloud.
The research, presented in a poster session at the American Astronomical Society winter meeting in Atlanta, shows that Barnard’s Galaxy is significantly different from the Milky Way.
“It’s as old as our galaxy, but it’s been active in creating new massive star clusters all along,” said Hodge. “Ours formed most of its big clusters in the first couple billion years after the big bang.”
NGC 6822 is among 30 galaxies that make up what is known as the local group, which includes the Milky Way and Andromeda. Hodge, Zucker and Wyder hope their observations of the star clusters lead to a greater understanding of how Barnard’s Galaxy evolved, and Wyder is using the research as the basis for his doctoral thesis.
At the time that Hubble published his findings, there raged a great debate about whether there were galaxies beyond the Milky Way. The most common view was that our galaxy made up the entire universe. Hubble’s work changed the nature of the debate.
“It was, in a sense, the first step out into the universe,” Hodge said.
During the AAS meeting, reporters may leave messages for Zucker and Wyder at the AAS press room, (404) 588-2908, (404) 588-2909, or (404) 588-2910, or at the Hyatt Regency Atlanta, (404) 577-1234.