This Hubble telescope image of a dense swarm of stars shows the central region of the globular cluster NGC 2808. Astronomers were surprised when Hubble spied three generations of cluster stars. The discovery is far different from the standard picture of a globular cluster. For decades, astronomers thought that cluster stars formed at the same time, in the same place, and from the same material, and have co-evolved for billions of years. Globular clusters are the homesteaders of our Milky Way Galaxy, born during our galaxy's formation. They are compact swarms of typically hundreds of thousands of stars held together by gravity. All the stars in NGC 2808 were born within 200 million years very early in the life of the 12.5-billion-year-old massive cluster. Of the about 150 known globular clusters in our Milky Way Galaxy, NGC 2808 is one of the most massive, containing more than 1 million stars. The sharp resolution of Hubble's Advanced Camera for Surveys measured the brightness and colors of the cluster stars to find the three stellar populations. The Hubble images were taken in May 2005 and in August and November 2006. Photo Credit: NASA, ESA, A. Sarajedini (University of Florida) and G. Piotto (University of Padova)Washington, May 3 (ANI): NASA’s Hubble Space Telescope has discovered what may be called a multiple ‘baby boom’ in a globular cluster.
For long, astronomers thought that globular star clusters had a single "baby boom" of stars early in their lives and then settled into a quiet existence.
But now, new observation of the massive globular cluster NGC 2808 has provided fresh evidence that nearly three generation of stars formed very early in the cluster’s life.
"We had never imagined that anything like this could happen. This is a complete shock,” said Giampaolo Piotto of the University of Padova in Italy and leader of the team that made the discovery.
"The standard picture of a globular cluster is that all of its stars formed at the same time, in the same place, and from the same material, and they have co-evolved for billions of years. This is the cornerstone on which much of the study of stellar populations has been built. So we were very surprised to find several distinct populations of stars in NGC 2808. All of the stars were born within 200 million years very early in the life of the 12.5-billion-year-old massive cluster,” said Luigi Bedin, team member and a research scientist with the ESA, the ESO and the Space Telescope Science Institute in Baltimore, US.
Astronomers commonly assume that globular clusters produce only one stellar generation, because the energy radiating from the first batch of stars would clear out most of the residual gas needed to make more stars.
But a hefty cluster like NGC 2808, which is two to three times more massive than a typical globular cluster, may have enough gravity to hang onto that gas, which has been enriched by helium from the first stars.
As such, it might hold enough gas to ignite a rapid succession of stars, said scientists.
“The star birth would be driven by shock waves from supernovae and stellar winds from giant stars, which compress the gas and make new stars The gas would be increasingly enriched in helium from previous generations of stars more massive than the Sun,” said team member Ivan King of the University of Washington in Seattle.
Researchers say finding multiple stellar populations in a globular cluster so close to the Milky Way Galaxy has deep cosmological implications as well.
"We need to do our best to solve the enigma of these multiple generations of stars found in these
Hubble observations so that we can understand how stars formed in distant galaxies in our early universe," Piotto said.
For their research, the scientists used the telescope’s Advanced Camera for Surveys to measure the brightness and color of the cluster stars.
Hubble's exquisite resolution allowed astronomers to sort out the different stellar populations. The measurements showed three distinct populations, with each successive generation appearing slightly bluer.
Researches believe this color difference suggested that successive generations contained a slightly different mix of some chemical elements.
"One assumption, although we have no direct proof is that the successively bluer color of the stellar populations indicates that the amount of helium increases with each generation of stars,” King said.
The team now plans to use the ESO's Very Large Telescope (VLT) in Chile to make spectroscopic observations of the chemical abundances in NGC 2808. They believe this might offer further evidence that the stars were born at different times and yield clues as to how they were formed.
The results are scheduled for publication in the Astrophysical Journal Letters. (ANI)