Half a century ago, a nearby cluster of stars appeared to astronomers as a single glowing ball of gas.
As recently as 15 years ago, scientists realized it was in fact a cluster of stars but were convinced they all must have formed at the same time and with the same composition.
Now astronomers at Baltimore's Space Telescope Science Institute have found evidence that one cluster may actually be two, one a million years older than the other, in the process of merging. The clusters are 170,000 light years from Earth in an area known as the Tarantula Nebula.
The finding announced Thursday could help scientists better understand the environments in which stars form. It could also help them piece together what they are seeing occur in faraway galaxies, in which they can observe what the early development of the universe was like.
"We have sophisticated models, but they fail to explain many of the things we look at," said Elena Sabbi, the lead scientist in the study, published Aug. 1 in Astrophysical Journal Letters. With the understanding that some large star clusters may be a combination of smaller clusters, "you can also explain why these systems are so dynamic and evolved," she said.
Sabbi, with a group of collaborators from the telescope institute and in England and Spain, began exploring data from Hubble Space Telescope observations in November. They were comparing two sets of data collected years apart, looking for movement in stars that appear to have been ejected from clusters of stars.
In the process of studying those "runaway" stars, Sabbi noticed an unusual shape to the cluster, as well as a large number of stars within it. Instead of being spherical, it had a sort of tail emanating from it that contained stars older than the others. And instead of having just a few thousand stars, it had 10,000, she said.
Both characteristics contributed to the finding that the cluster may have been the product of an interaction between two star clusters.
The research is only a pit stop on the way to what scientists hope will be more findings related to the "runaway" stars. Jay Anderson, another scientist at the telescope institute, who did not work with Sabbi on the report, said learning more about those stars and about star cluster development and interaction is important in understanding more about what happens when stars are formed in the first place.
"When you see a cluster, you have to ask the question: Is this all the stars that formed together or just some small remnant of all the stars that formed together?" Anderson said. "We want to understand, how long do stars stay close to where they started?"
Star formation is a busy field of astronomy — scientists at the Massachusetts Institute of Technology announced Wednesday the discovery of a galaxy cluster 5.7 million light years away that contains a galaxy producing about 740 new stars per year, more than any other galaxy known to scientists.
Sabbi said studying the relatively nearby galaxy containing the star clusters she examined can also help teach lessons useful in exploring more distant bodies. The star-forming region the Tarantula Nebula, properly known as 30 Doradus, has long been a focus of study for scientists for that reason.
"It takes many billions of years for the light to come from faraway galaxies to us," Sabbi said. "We don't have the resolution to study details of those processes."
While Hubble was vital in Sabbi's research, the planned James Webb Space Telescope could help advance it even further, Anderson said. While Hubble's instruments observe space in wavelengths of near-ultraviolet light, light visible to humans and near-infrared light, Webb will observe infrared light.
That will allow it to look through much of the dust and gas that blocks Hubble from observing more stellar activity, he said. Webb is expected to launch around 2018.
"It will allow us to see even more stars than Hubble can see, and we'll be able to see the center of the galaxy better," Anderson said.