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Hubble Observes a Giant Stellar Swarm

13 May 2007

Credit: NASA, ESA, A. Sarajedini (University of Florida) and G. Piotto (University of Padua [Padova])

Using NASA's Hubble Space Telescope (HST), a team of astronomers demonstrated that a massive globular cluster, known as NGC 2808, had multiple generations of stars rather than only a single, old boom of star formation.

A globular cluster is a spherical or an ellipsoidal stellar swarm, whose stars are tightly bound by immense gravity. A globular cluster may contain up to 1 million stars. Our Milky Way Galaxy has about 150 globular clusters, scattered throughout the galactic halo. These clusters orbit the center of the Galaxy. Other giant galaxies have thousands of globular clusters in their halos.

"We had never imagined that anything like this could happen," said Giampaolo Piotto of the University of Padova, Italy, and leader of the research team. "This is a complete shock."

Globular clusters are among the oldest objects in the Universe, believed to be nearly 13 billion years old.

"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. "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 galactic globular cluster 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.

NGC 2808 is one of the largest known globular clusters of our Galaxy. It contains about one million stars.

The astronomers applied Hubble's Advanced Camera for Surveys (ACS) to measure the brightness and color of NGC 2808's stars. With Hubble's superb resolution, the astronomers were able to discern the different stellar populations. Hubble's data revealed three distinct stellar populations, with each successive generation appearing slightly bluer. This color difference indicates a slightly different chemical composition of these stars.

"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." said team member Ivan King of the University of Washington, Seattle. "Perhaps massive star clusters like NGC 2808 hold onto enough gas to ignite a rapid succession of stars."

The vigorous star formation would be driven by shock waves from supernovae (exploding stars) and stellar winds from giant stars, which collide with the gas clouds, stimulating the birth of new stars, King explained. The gas would be increasingly enriched in helium, synthesized through nuclear reactions in the cores of massive, older stars.

It has been long believed that globular clusters undergo only one episode of star formation, because the prodigious radiation emitted from the first stars would wipe out most of the residual gas needed to make more stars. However, a gigantic cluster like NGC 2808, which is two to three times more massive than a typical globular cluster, may have enough gravity to hold that gas, which contains more helium, supplied by the first stars.

According to another explanation, NGC 2808 may be the residual core of a dwarf galaxy that was ripped apart by our Galaxy's formidable gravitational tug, not a true globular cluster.

Omega Centauri, another hefty globular cluster of our Galaxy, shows multiple stellar generations, according to Piotto's team, and is also believed to be the remains of a disrupted dwarf galaxy. The team concluded that multiple stellar generations may be a common feature of massive globular clusters.

The researchers plan to explore 10 more giant clusters, using Hubble.

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