The research provides a portrait of suns hurtling into catastrophic crashes while others become trapped in inescapable orbital duets, one star sucking material from its companion.
"With a million stars concentrated in a space that would hold only one in most regions of the galaxy, these globular clusters force stars into interactions not seen anywhere else in the universe," says Adrienne Cool, assistant professor of astronomy at San Francisco State University and leader of the Hubble research team probing the star clusters. "The paired stars locked in orbit act like egg beaters keeping things stirred up deep within the cluster`s core and may prevent the core from collapsing into a black hole." Cool described the study's results this summer in a paper in the volume Stellar Evolution, published by Cambridge University Press.
Evidence of the dynamic star interactions comes from unusual wavelengths of light shining from faint stars Cool and her colleagues have been able to detect over the last three years. Most individual stars of this brightness appear red, but these are blue. The stars' brightness varies rapidly, and the stars emit an unusually large amount of ultraviolet light. All three traits are telltale signs that the source is not a typical star but two stars trapped in an orbiting dance.
In one kind of star pair Cool's team studied, a collapsed, incredibly dense, dying star called a white dwarf pulls material off a larger but lighter companion star in close orbit around it. The detected light varies as the dense white dwarf sucks huge globs of material away from the normal star.
"The light we are detecting is powered by gravitational energy, not the nuclear fusion that normally lights stars," Cool explains.
The phenomenon, called a cataclysmic variable, has been known for some time in the galaxy, but those seen before involve stars paired since birth. The pairs inside the dense globular cluster, by contrast, become locked together when two mature stars pass too closely in the night.
Cool's team studied one blue source of light never detected before. The light wasn't variable, and thus couldn't be a star pair. Faint blue, high in ultraviolet and unusually hot, this is the discovery that Cool and her colleagues think may be the exposed, extremely hot inner core of a star that has been stripped of its outer layers of gas by a passing star.
Globular clusters are the only regions of the galaxy yet studied where stellar collisions are likely to occur. The densely packed stars create far more stellar traffic than almost any other region in the galaxy -- or universe.
The distance between neighboring stars in a cluster is only 1/100 the typical distance between stars. Yet although stellar collisions may be spectacular, they are uncommon even here. Due to the vastness of space, collisions within a globular cluster occur once in every 10 to 100 million years. Only because clusters are more than 10 billion years old has there been time for hundreds of devastating collisions.
The faint stars which provided the study's key evidence can't be seen by ground-based telescopes, but the sharper focus of the orbiting Hubble telescope allowed Cool and her colleagues to detect them. The team included astronomy professors Jonathan Grindlay at Harvard, Charles Bailyn at Yale; and Haldan Cohn and Phyllis Lugger at Indiana University. Cool's paper describing the findings is titled "Binary Stars Below the Turnoff in Globular Cluster Color-Magnitude Diagrams."
Research on globular star clusters is increasing as astronomers gain access to highly sensitive instruments such as the Hubble. Earlier this year, Cool, Berkeley astronomer Ivan King and doctoral student Jay Anderson reported on efforts to count very faint stars in an attempt to gauge if a significant part of the missing mass of the universe might be held in such heavenly bodies. The answer appears to be "no."
"If you are looking for the missing mass, we now know you would probably have better luck looking somewhere else," Cool says.
More than 100 massive globular clusters reside within the halo of our galaxy, each containing about a million stars. A few can be seen with the naked eye as "fuzzy balls," says Cool. The cluster her group studied is called NGC 6397 and is in the direction of the constellation Ara in the southern hemisphere.
Our Milky Way's million-star globular clusters were all created when the galaxy was first forming, making their stars among the most ancient in the universe, Cool says.
"These are the oldest stars we have been able to age with any certainty," she adds.
The research is supported by NASA and the Space Telescope Science Institute.