The simple, four-star constellation in the southern sky known as Dorado, or Goldfish, was named by the German astronomer Johann Bayer because it looked to him like a Piscean backbone. But like all things in the Universe, the Dorado constellation is not as simple as it seems.
Buried 166,000 light years away in the belly of this stellar fish is our neighboring galaxy, the Large Magellanic Cloud. Now Hubble has helped astronomers at the Space Telescope Science Institute (STScI) and the European Space Agency find yet another layer of complexity by providing images of an unusual, glittering, double globular star cluster surrounded by a nebula that lies inside the Large Megallanic Cloud.
The clusters' lives revealed
The double cluster, named NGC 1850 and shown in the adjacent image, spans 96 light years. These unusual cosmic cohabitants are thought to be cousins of a sort.
The smaller group, much younger and hotter, sits 20 to 50 light years below and to the right of the center cluster. Nino Panagia, an astronomer at STScI, and his colleagues believe the larger cluster triggered the birth of the smaller cluster some 20 million years ago.
The estimated 10,000 blue and white stars in the large center cluster are 50 million years old and most have masses less than 10 solar masses. Larger stars were destroyed 20 million years ago in supernova explosions that compressed matter that eventually became the smaller stars, now just 4 million years old.
"The explosion started pushing matter around and compressing whatever was there," Panagia said, "and this compression helped more stars to form."
A filamentary nebula that surrounds NGC 1850 is evidence of this supernova. Although in this image the "explosion ghost" is shown only to the left of the main cluster, it actually surrounds both groups. The wispy gas is all that remains of the original stars.
"The compression of the preexisting gases triggered formation of the new stars," Panagia said. "This is reasonably obvious by the fact that the small cluster is plastered on the outside of that ball of gas."
Unique to our neighboring galaxy
The first Hubble observation of the double cluster was made in April 1994. The formation piqued the interest of STScI scientists because the stars surrounding NGC 1850 are a billion years old. More observations were taken in 1997 to help scientists understand this unique neighbor.
Globular star clusters as young and compact as NGC 1850 do not exist in the Milky Way, so to Panagia, the Hubble images provide an opportunity to learn more about star formation outside of our galaxy.
"The Small and Large Magellanic Cloud galaxies aren't as evolved as our own galaxy," he said. "They've taken a more leisurely pace to evolve, so they make a nice lab for studying the chemistry evolution in our galaxy, and all galaxies. The clusters are at least as interesting as [stars] here, but since they're different, they tell us more."
Panagia said he and his teammates will fill in the blanks of this double cluster's story by the end of this year.
"We're just analyzing the stars, but we want to characterize the gases, too. What we conclude with these observations will be the 3-Dstructure, the motion, chemical composition - it's never finished. There is always something to do," he said. "There is never enough known."
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