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Star cluster not born of single boom

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.
This Hubble telescope image of a dense swarm of stars shows the central region of the globular cluster NGC 2808.
This Hubble telescope image of a dense swarm of stars shows the central region of the globular cluster NGC 2808.NASA, ESA, A. Sarajedini
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The long-held belief that globular star clusters form in a single boom has been challenged by new findings. 

Make it boom, boom, boom.

New observations by NASA's Hubble Space Telescope of the globular cluster NGC 2808 provides evidence that star birth occurred in three bouts, astronomers said today. Three generations of stars formed very early in the cluster's life, a discovery with deep cosmological implications.

Globular clusters are ancient. Those in our Milky Way Galaxy were formed as the galaxy formed, billions of years ago. The typically hundreds of thousands of stars swirl in a cluster, all held together by their collective 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..." said team member Luigi Bedin of the European Space Agency. "We were very surprised to find several distinct populations... All of the stars were born within 200 million years very early in the life of the 12.5-billion-year-old massive cluster."

Hubble's Advanced Camera for Surveys recorded the colors of the cluster stars, showing three distinct populations of slightly different colors, suggesting that successive generations contain a different elemental blend.

One assumption, said team member Ivan King of the University of Washington in Seattle, is 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."

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The star birth in general is a cyclic affair. Stars are born, and many die explosively, and then new starbirth is driven by shock waves from these exploding supernovas. Stellar winds from other giant stars compress the gas and help make new stars out of it, King explained. The first stars were pretty much all hydrogen, but then they forge heavier elements, and gas of newer stars is increasingly enriched in helium from previous generations.

Astronomers commonly assume that globular clusters produce only one stellar generation, because energy radiating from the first stars would clear most of the residual gas needed to make more stars. But a large cluster like NGC 2808, two to three times more massive than a typical globular cluster, may have enough gravity to retain that gas, enriched by helium from the first stars. NGC 2808 is one of the most massive globular clusters in our Milky Way Galaxy, containing more than a million stars.

A possible explanation for the multiple stellar populations is that NGC 2808 may not be a globular cluster. It may be a dwarf galaxy that was stripped of most of its material by our galaxy's gravitational effects.

Multiple stellar populations may be a typical occurrence in other massive clusters.

"This discovery shows that the study of stellar populations in globular clusters now opens up in a new direction," said King.