HUBBLE FINDS MULTIPLE STELLAR 'BABY BOOMS' IN A GLOBULAR CLUSTER
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 by NASA's Hubble Space Telescope, however, are showing that this idea
may be too simple. The Hubble 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.
"We had never imagined that anything like this could happen," said Giampaolo
Piotto of theUniversity of Padova in Italy and leader of the team that made the discovery.
"This is a complete shock."
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.
"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 (ESO), in Garching, Germany,
and the Space Telescope Science Institute in Baltimore, Md. "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."
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 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," said team member Ivan King of the
University of Washington in Seattle, "is that the successively bluer color of the stellar
populations indicates that the amount of helium increases with each generation of stars.
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 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 our Milky Way Galaxy, NGC 2808 is one of the most
massive, containing more than 1 million stars.
