May 3, 2007
Star birth goes boom, boom, boom: Three generations of stars form early in globular cluster’s life
Space Telescope Science Institute
Astronomers have long thought that globular star clusters had a single “baby boom” of stars early in their lives and then settled into a quiet existence.
New observations using NASA’s Hubble Space Telescope, however, show that idea might be too simple. The analysis of the massive globular cluster NGC 2808 provides evidence that star birth went “boom, boom, boom,” with three generations of stars forming very early in the cluster’s life.
“Instead of a single kind of star in this cluster there are three kinds, and that’s very unusual,” said Ivan King, a UW research professor in astronomy and a member of the team of scientists that made the discovery.
Team leader Giampaolo Piotto of the University of Padova in Italy added: “We had never imagined that anything like this could happen. This is a complete shock.”
Globular clusters are the homesteaders of the Milky Way, born during our galaxy’s formation. They are compact swarms of typically hundreds of thousands of stars held together by gravity.
“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,” said team member Luigi Bedin of the European Space Agency, the European Organization for Astronomical Research in the Southern Hemisphere, in Garching, Germany, and the Space Telescope Science Institute in Baltimore. “This is the cornerstone on which much of the study of stellar populations has been built. So we were very surprised to find in the massive cluster NGC 2808 several distinct populations of stars. All of the stars were born within 200 million years very early in the life of the 12.5-billion-year-old cluster.”
Finding the 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 our early universe,” Piotto explained.
The astronomers used Hubble’s Advanced Camera for Surveys to measure the brightness and color of the cluster stars. Hubble’s exquisite resolution allowed the astronomers to sort out the different stellar populations. The Hubble measurements showed three distinct populations, with each successive generation appearing slightly bluer. This color difference suggests that successive generations contain a slightly different mix of some chemical elements.
“One assumption, although we have no direct proof, is that the amount of helium differs,” King said. “The successively bluer color 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 multiple generations of stars, with enrichment of some elements.”
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, King explained. The gas would be increasingly enriched in helium from previous generations of stars more massive than the Sun.
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. Of the about 150 known globular clusters in the Milky Way, NGC 2808 is one of the most massive, containing more than 1 million stars. The cluster lies about 30,000 light years from Earth (a light year is about 5.88 trillion miles).
Another possible explanation for the multiple stellar populations is that NGC 2808 might 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 galaxy.
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 the European Southern Observatory’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.
The team’s results are to be published May 20 in Astrophysical Journal Letters.
For images and more information about stellar populations in globular cluster NGC 2808, visit http://hubblesite.org/news/2007/18.
The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. The Space Telescope Science Institute conducts Hubble science operations. The institute is operated for NASA by the Association of Universities for Research in Astronomy Inc. in Washington, D.C.
