Hubble finds multiple stellar generations in a globular cluster [heic0708]
2 May 2007Analysis of Hubble observations of the massive globular cluster NGC 2808 provides evidence that it has three generations of stars that formed early in the cluster's life. This is a major upset for conventional theories as astronomers have long thought that globular star clusters had only a single burst of star formation early in their lives and then settled down into a long, quiet middle age.
"We had never imagined that anything like this could happen," said Giampaolo Piotto of the University of Padua in Italy and leader of the team that made the discovery. "This is a complete shock."
Globular clusters are among the earliest settlers of our Milky Way Galaxy, born during our Galaxy's formation. A typical cluster consists of hundreds of thousands of stars held together by their mutual gravitation in a compact swarm.
"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 that they have co-evolved for thousands of millions of years," said team member Luigi Bedin of ESO in Garching, Germany, the European Space Agency, and Space Telescope Science Institute in Baltimore, USA. "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 and very early in the life of the 12.5 × 109 year-old massive cluster."
Finding multiple stellar populations in a globular cluster so close to home has deep cosmological implications, the researchers said. "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 the early Universe," Piotto explains.
"One assumption, although we have no direct proof," said team member Ivan King of the University of Washington in Seattle, USA, "is that the amount of helium increases with each generation of stars. The successively bluer colour of the stellar populations indicates that the amount of helium increases in each generation. Perhaps massive star clusters like NGC 2808 hold onto enough gas to ignite a rapid succession of stars."
The star birth would be driven by shock waves from supernovae and stellar winds from red giant stars, which compress the gas and make new stars, King explains. The gas would be increasingly enriched in helium from previous generations of stars more massive than the Sun.
Astronomers generally believed 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 is two to three times more massive than a typical globular cluster and may have sufficient gravity to hang onto enough gas, which is then enriched by helium from the first stars. Of the about 150 known globular clusters in our Milky Way Galaxy, NGC 2808 is one of the most massive, with a total mass of more than a million times the mass of the Sun.
Another possible explanation for the multiple stellar populations is that NGC 2808 may only be masquerading as a globular cluster. The stellar grouping may have been a dwarf galaxy that was stripped of most of its material due to gravitational capture by our Milky Way.
Omega Centauri, the first globular cluster Piotto's group found to have multiple generations of stars, is suspected to be the remnant core of a dwarf galaxy, Bedin said.
Although the astronomers have searched only two globular clusters for multiple stellar populations, they say this may be a typical occurrence in other massive clusters.
"No one would make the radical step of suggesting that previous work on other clusters is no longer valid," King said. "But this discovery shows that the study of stellar populations in globular clusters now opens up in a new direction."
The team plans to use ESO's Very Large Telescope in Chile to make spectroscopic observations of the chemical abundances in NGC 2808, which may offer further evidence that the stars were born at different times and yield clues to how they formed. They also will use Hubble to hunt for multiple generations of stars in about 10 more hefty globular clusters.
Notes for editors
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
These results are accepted for publication in the 20 May 2007 Astrophysical Journal Letters.
Image credit: NASA, ESA, G. Piotto (University of Padua, Italy) and A. Sarajedini (University of Florida, USA)
Acknowledgement: Davide de Martin (ESA/Hubble)
Team Members are: G. Piotto (University of Padua), L. R. Bedin (ESO), J. Anderson (Rice University), I. R. King (University of Washington), S. Cassisi (INAF-Astronomical Observatory of Collurania), A. P. Milone (University of Padua), S. Villanova (University of Padua), A. Pietrinferni (INAF-Astronomical Observatory of Collurania), A. Renzini (INAF-Astronomical Observatory of Padua)
The Hubblecast Video podcast episode "Hubble Finds Multiple Stellar 'Baby Booms' in a Globular Cluster" is available at: http://www.spacetelescope.org/videos/hubblecast.html (see related links on the right-hand side)
Animations and general Hubble Space Telescope background footage are available from: http://www.spacetelescope.org/bin/videos.pl? searchtype=news&string=heic0708 (see related links on the right-hand side)
Department of Astronomy, University of Padua
ESO, Garching, Germany
Lars Lindberg Christensen
Hubble/ESA, Garching, Germany
Space Telescope Science Institute, Baltimore, USA